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Skylark in Go: Language definition

Skylark is a dialect of Python intended for use as a configuration language. A Skylark interpreter is typically embedded within a larger application, and this application may define additional domain-specific functions and data types beyond those provided by the core language. For example, Skylark is embedded within (and was originally developed for) the Bazel build tool, and Bazel's build language is based on Skylark.

This document describes the Go implementation of Skylark at github.com/google/skylark. The language it defines is similar but not identical to the Java-based implementation used by Bazel. We identify places where their behaviors differ, and an appendix provides a summary of those differences. We plan to converge both implementations on a single specification in early 2018.

This document is maintained by Alan Donovan adonovan@google.com. It was influenced by the Python specification, Copyright 1990–2017, Python Software Foundation, and the Go specification, Copyright 2009–2017, The Go Authors.

Skylark was designed and implemented in Java by Laurent Le Brun, Dmitry Lomov, Jon Brandvin, and Damien Martin-Guillerez, standing on the shoulders of the Python community. The Go implementation was written by Alan Donovan and Jay Conrod; its scanner was derived from one written by Russ Cox.

The name "Skylark" is a code name of the Bazel project. We plan to rename the language before the end of 2017 to reflect its applicability to projects unrelated to Bazel.

Overview

Skylark is an untyped dynamic language with high-level data types, first-class functions with lexical scope, and automatic memory management or garbage collection.

Skylark is strongly influenced by Python, and is almost a subset of that language. In particular, its data types and syntax for statements and expressions will be very familiar to any Python programmer. However, Skylark is intended not for writing applications but for expressing configuration: its programs are short-lived and have no external side effects and their main result is structured data or side effects on the host application. As a result, Skylark has no need for classes, exceptions, reflection, concurrency, and other such features of Python.

Contents

Lexical elements

A Skylark program consists of one or more modules. Each module is defined by a single UTF-8-encoded text file.

A complete grammar of Skylark can be found in grammar.txt. That grammar is presented piecemeal throughout this document in boxes such as this one, which explains the notation:

Grammar notation

- lowercase and 'quoted' items are lexical tokens.
- Capitalized names denote grammar productions.
- (...) implies grouping.
- x | y means either x or y.
- [x] means x is optional.
- {x} means x is repeated zero or more times.
- The end of each declaration is marked with a period.

The contents of a Skylark file are broken into a sequence of tokens of five kinds: white space, punctuation, keywords, identifiers, and literals. Each token is formed from the longest sequence of characters that would form a valid token of each kind.

File = {Statement | newline} eof .

White space consists of spaces (U+0020), tabs (U+0009), carriage returns (U+000D), and newlines (U+000A). Within a line, white space has no effect other than to delimit the previous token, but newlines, and spaces at the start of a line, are significant tokens.

Comments: A hash character (#) appearing outside of a string literal marks the start of a comment; the comment extends to the end of the line, not including the newline character. Comments are treated like other white space.

Punctuation: The following punctuation characters or sequences of characters are tokens:

+    -    *    /    //   %    =
+=   -=   *=   /=   //=  %=   ==   !=
^    <    >    <<   >>   &    |
^=   <=   >=   <<=  >>=  &=   |=
.    ,    ;    :    ~    **
(    )    [    ]    {    }

Keywords: The following tokens are keywords and may not be used as identifiers:

and            else           load
break          for            not
continue       if             or
def            in             pass
elif           lambda         return

The tokens below also may not be used as identifiers although they do not appear in the grammar; they are reserved as possible future keywords:

as             import
assert         is
class          nonlocal
del            raise
except         try
finally        while
from           with
global         yield

Implementation note: The Go implementation permits assert to be used as an identifier, and this feature is widely used in its tests.

Implementation note: The Java implementation does not recognize the following tokens: &, &=, |=, <<, >>, <<=, >>=, ^, ^=, ~.

Identifiers: an identifier is a sequence of Unicode letters, decimal digits, and underscores (_), not starting with a digit. Identifiers are used as names for values.

Examples:

None    True    len
x       index   starts_with     arg0

Literals: literals are tokens that denote specific values. Skylark has string, integer, and floating-point literals.

0                               # int
123                             # decimal int
0x7f                            # hexadecimal int
0755                            # octal int
0b1011                          # binary int

0.0     0.       .0             # float
1e10    1e+10    1e-10
1.1e10  1.1e+10  1.1e-10

"hello"      'hello'            # string
'''hello'''  """hello"""        # triple-quoted string
r'hello'     r"hello"           # raw string literal

Integer and floating-point literal tokens are defined by the following grammar:

int         = decimal_lit | octal_lit | hex_lit | binary_lit .
decimal_lit = ('1' … '9') {decimal_digit} .
octal_lit   = '0' {octal_digit} .
            | '0' ('o'|'O') octal_digit {octal_digit} .
hex_lit     = '0' ('x'|'X') hex_digit {hex_digit} .
binary_lit  = '0' ('b'|'B') binary_digit {binary_digit} .

float     = decimals '.' [decimals] [exponent]
          | decimals exponent
          | '.' decimals [exponent]
          .
decimals  = decimal_digit {decimal_digit} .
exponent  = ('e'|'E') ['+'|'-'] decimals .

decimal_digit = '0' … '9' .
octal_digit   = '0' … '7' .
hex_digit     = '0' … '9' | 'A' … 'F' | 'a' … 'f' .
binary_digit  = '0' | '1' .

TODO: define string_lit, indent, outdent, semicolon, newline, eof

Data types

These are the main data types built in to the interpreter:

NoneType                     # the type of None
bool                         # True or False
int                          # a signed integer of arbitrary magnitude
float                        # an IEEE 754 double-precision floating point number
string                       # a byte string
list                         # a fixed-length sequence of values
tuple                        # a fixed-length sequence of values, unmodifiable
dict                         # a mapping from values to values
set                          # a set of values
function                     # a function implemented in Skylark
builtin_function_or_method   # a function or method implemented by the interpreter or host application

Some functions, such as the iteration methods of string, or the range function, return instances of special-purpose types that don't appear in this list. Additional data types may be defined by the host application into which the interpreter is embedded, and those data types may participate in basic operations of the language such as arithmetic, comparison, indexing, and function calls.

Some operations can be applied to any Skylark value. For example, every value has a type string that can be obtained with the expression type(x), and any value may be converted to a string using the expression str(x), or to a Boolean truth value using the expression bool(x). Other operations apply only to certain types. For example, the indexing operation a[i] works only with strings, lists, and tuples, and any application-defined types that are indexable. The value concepts section explains the groupings of types by the operators they support.

None

None is a distinguished value used to indicate the absence of any other value. For example, the result of a call to a function that contains no return statement is None.

None is equal only to itself. Its type is "NoneType". The truth value of None is False.

Booleans

There are two Boolean values, True and False, representing the truth or falsehood of a predicate. The type of a Boolean is "bool".

Boolean values are typically used as conditions in if-statements, although any Skylark value used as a condition is implicitly interpreted as a Boolean. For example, the values None, 0, 0.0, and the empty sequences "", (), [], and {} have a truth value of False, whereas non-zero numbers and non-empty sequences have a truth value of True. Application-defined types determine their own truth value. Any value may be explicitly converted to a Boolean using the built-in bool function.

1 + 1 == 2                              # True
2 + 2 == 5                              # False

if 1 + 1:
        print("True")
else:
        print("False")

Integers

The Skylark integer type represents integers. Its type is "int".

Integers may be positive or negative, and arbitrarily large. Integer arithmetic is exact. Integers are totally ordered; comparisons follow mathematical tradition.

The + and - operators perform addition and subtraction, respectively. The * operator performs multiplication.

The // and % operations on integers compute floored division and remainder of floored division, respectively. If the signs of the operands differ, the sign of the remainder x % y matches that of the dividend, x. For all finite x and y (y ≠ 0), (x // y) * y + (x % y) == x. The / operator implements real division, and yields a float result even when its operands are both of type int.

Integers, including negative values, may be interpreted as bit vectors. The |, &, and ^ operators implement bitwise OR, AND, and XOR, respectively. The unary ~ operator yields the bitwise inversion of its integer argument. The << and >> operators shift the first argument to the left or right by the number of bits given by the second argument. (These features are not part of the Java implementation.)

Any bool, number, or string may be interpreted as an integer by using the int built-in function.

An integer used in a Boolean context is considered true if it is non-zero.

100 // 5 * 9 + 32               # 212
3 // 2                          # 1
3 / 2                           # 1.5
111111111 * 111111111           # 12345678987654321
"0x%x" % (0x1234 & 0xf00f)      # "0x1004"
int("ffff", 16)                 # 65535, 0xffff

Implementation note: In the Go implementation of Skylark, integer representation and arithmetic is exact, motivated by the need for lossless manipulation of protocol messages which may contain signed and unsigned 64-bit integers. The Java implementation currently supports only signed 32-bit integers.

The Go implementation of the Skylark REPL requires the -bitwise flag to enable support for &, |, ^, ~, <<, and >> operations. The Java implementation does not support ^, ~, <<, and >> operations.

Floating-point numbers

The Skylark floating-point data type represents an IEEE 754 double-precision floating-point number. Its type is "float".

Arithmetic on floats using the +, -, *, /, //, and % operators follows the IEE 754 standard. However, computing the division or remainder of division by zero is a dynamic error.

An arithmetic operation applied to a mixture of float and int operands works as if the int operand is first converted to a float. For example, 3.141 + 1 is equivalent to 3.141 + float(1). There are two floating-point division operators: x / y yields the floating-point quotient of x and y, whereas x // y yields floor(x / y), that is, the largest integer value not greater than x / y. Although the resulting number is integral, it is represented as a float if either operand is a float.

The infinite float values +Inf and -Inf represent numbers greater/less than all finite float values.

The non-finite NaN value represents the result of dubious operations such as Inf/Inf. A NaN value compares neither less than, nor greater than, nor equal to any value, including itself.

All floats other than NaN are totally ordered, so they may be compared using operators such as == and <.

Any bool, number, or string may be interpreted as a floating-point number by using the float built-in function.

A float used in a Boolean context is considered true if it is non-zero.

1.23e45 * 1.23e45                               # 1.5129e+90
1.111111111111111 * 1.111111111111111           # 1.23457
3.0 / 2                                         # 1.5
3 / 2.0                                         # 1.5
float(3) / 2                                    # 1.5
3.0 // 2.0                                      # 1

Implementation note: The Go implementation of Skylark supports floating-point numbers as an optional feature, motivated by the need for lossless manipulation of protocol messages. The Go implementation of the Skylark REPL requires the -fp flag to enable support for floating-point literals, the float built-in function, and the real division operator /. The Java implementation does not yet support floating-point numbers.

Strings

A string represents an immutable sequence of bytes. The type of a string is "string".

Strings can represent arbitrary binary data, including zero bytes, but most strings contain text, encoded by convention using UTF-8.

The built-in len function returns the number of bytes in a string.

Strings may be concatenated with the + operator.

The substring expression s[i:j] returns the substring of s from index i up to index j. The index expression s[i] returns the 1-byte substring s[i:i+1].

Strings are hashable, and thus may be used as keys in a dictionary.

Strings are totally ordered lexicographically, so strings may be compared using operators such as == and <.

Strings are not iterable sequences, so they cannot be used as the operand of a for-loop, list comprehension, or any other operation than requires an iterable sequence. To obtain a view of a string as an iterable sequence of numeric byte values, 1-byte substrings, numeric Unicode code points, or 1-code point substrings, you must explicitly call one of its four methods: elems, elem_ords, codepoints, or codepoint_ords.

Any value may formatted as a string using the str or repr built-in functions, the str % tuple operator, or the str.format method.

A string used in a Boolean context is considered true if it is non-empty.

Strings have several built-in methods:

Implementation note: The type of a string element varies across implementations. There is agreement that byte strings, with text conventionally encoded using UTF-8, is the ideal choice, but the Java implementation treats strings as sequences of UTF-16 codes and changing it appears intractible; see Google Issue b/36360490.

Implementation note: The Java implementation does not consistently treat strings as iterable; see testdata/string.sky in the test suite and Google Issue b/34385336 for further details.

Lists

A list is a mutable sequence of values. The type of a list is "list".

Lists are indexable sequences: the elements of a list may be iterated over by for-loops, list comprehensions, and various built-in functions.

List may be constructed using bracketed list notation:

[]              # an empty list
[1]             # a 1-element list
[1, 2]          # a 2-element list

Lists can also be constructed from any iterable sequence by using the built-in list function.

The built-in len function applied to a list returns the number of elements. The index expression list[i] returns the element at index i, and the slice expression list[i:j] returns a new list consisting of the elements at indices from i to j.

List elements may be added using the append or extend methods, removed using the remove method, or reordered by assignments such as list[i] = list[j].

The concatenation operation x + y yields a new list containing all the elements of the two lists x and y.

For most types, x += y is equivalent to x = x + y, except that it evaluates x only once, that is, it allocates a new list to hold the concatenation of x and y. However, if x refers to a list, the statement does not allocate a new list but instead mutates the original list in place, similar to x.extend(y).

Lists are not hashable, so may not be used in the keys of a dictionary.

A list used in a Boolean context is considered true if it is non-empty.

A list comprehension creates a new list whose elements are the result of some expression applied to each element of another sequence.

[x*x for x in [1, 2, 3, 4]]      # [1, 4, 9, 16]

A list value has these methods:

Tuples

A tuple is an immutable sequence of values. The type of a tuple is "tuple".

Tuples are constructed using parenthesized list notation:

()                      # the empty tuple
(1,)                    # a 1-tuple
(1, 2)                  # a 2-tuple ("pair")
(1, 2, 3)               # a 3-tuple

Observe that for the 1-tuple, the trailing comma is necessary to distinguish it from the parenthesized expression (1). 1-tuples are seldom used.

Skylark, unlike Python, does not permit a trailing comma to appear in an unparenthesized tuple expression:

for k, v, in dict.items(): pass                 # syntax error at 'in'
_ = [(v, k) for k, v, in dict.items()]          # syntax error at 'in'
f = lambda a, b, : None                         # syntax error at ':'

sorted(3, 1, 4, 1,)                             # ok
[1, 2, 3, ]                                     # ok
{1: 2, 3:4, }                                   # ok

Any iterable sequence may be converted to a tuple by using the built-in tuple function.

Like lists, tuples are indexed sequences, so they may be indexed and sliced. The index expression tuple[i] returns the tuple element at index i, and the slice expression tuple[i:j] returns a sub-sequence of a tuple.

Tuples are iterable sequences, so they may be used as the operand of a for-loop, a list comprehension, or various built-in functions.

Unlike lists, tuples cannot be modified. However, the mutable elements of a tuple may be modified.

Tuples are hashable (assuming their elements are hashable), so they may be used as keys of a dictionary.

Tuples may be concatenated using the + operator.

A tuple used in a Boolean context is considered true if it is non-empty.

Dictionaries

A dictionary is a mutable mapping from keys to values. The type of a dictionary is "dict".

Dictionaries provide constant-time operations to insert an element, to look up the value for a key, or to remove an element. Dictionaries are implemented using hash tables, so keys must be hashable. Hashable values include None, Booleans, numbers, and strings, and tuples composed from hashable values. Most mutable values, such as lists, dictionaries, and sets, are not hashable, even when frozen. Attempting to use a non-hashable value as a key in a dictionary results in a dynamic error, as does passing one to the built-in hash function.

A dictionary expression specifies a dictionary as a set of key/value pairs enclosed in braces:

coins = {
  "penny": 1,
  "nickel": 5,
  "dime": 10,
  "quarter": 25,
}

The expression d[k], where d is a dictionary and k is a key, retrieves the value associated with the key. If the dictionary contains no such item, the operation fails:

coins["penny"]          # 1
coins["dime"]           # 10
coins["silver dollar"]  # error: key not found

The number of items in a dictionary d is given by len(d). A key/value item may be added to a dictionary, or updated if the key is already present, by using d[k] on the left side of an assignment:

len(coins)				# 4
coins["shilling"] = 20
len(coins)				# 5, item was inserted
coins["shilling"] = 5
len(coins)				# 5, existing item was updated

A dictionary can also be constructed using a dictionary comprehension, which evaluates a pair of expressions, the key and the value, for every element of another iterable such as a list. This example builds a mapping from each word to its length in bytes:

words = ["able", "baker", "charlie"]
{x: len(x) for x in words}	# {"charlie": 7, "baker": 5, "able": 4}

Dictionaries are iterable sequences, so they may be used as the operand of a for-loop, a list comprehension, or various built-in functions. Iteration yields the dictionary's keys in the order in which they were inserted; updating the value associated with an existing key does not affect the iteration order.

x = dict([("a", 1), ("b", 2)])          # {"a": 1, "b": 2}
x.update([("a", 3), ("c", 4)])          # {"a": 3, "b": 2, "c": 4}
for name in coins:
  print(name, coins[name])	# prints "quarter 25", "dime 10", ...

Like all mutable values in Skylark, a dictionary can be frozen, and once frozen, all subsequent operations that attempt to update it will fail.

A dictionary used in a Boolean context is considered true if it is non-empty.

The binary + operation may be applied to two dictionaries. It yields a new dictionary whose elements are the union of the two operands. If a key is present in both operands, the result contains the value from the right operand. Note: this feature is deprecated. Use the dict.update method instead.

Dictionaries may be compared for equality using == and !=. Two dictionaries compare equal if they contain the same number of items and each key/value item (k, v) found in one dictionary is also present in the other. Dictionaries are not ordered; it is an error to compare two dictionaries with <.

A dictionary value has these methods:

Sets

A set is a mutable set of values. The type of a set is "set".

Like dictionaries, sets are implemented using hash tables, so the elements of a set must be hashable.

Sets may be compared for equality or inequality using == and !=. Two sets compare equal if they contain the same elements.

Sets are iterable sequences, so they may be used as the operand of a for-loop, a list comprehension, or various built-in functions. Iteration yields the set's elements in the order in which they were inserted.

The binary | and & operators compute union and intersection when applied to sets. The right operand of the | operator may be any iterable value. The binary in operator performs a set membership test when its right operand is a set.

The binary ^ operator performs symmetric difference of two sets.

Sets are instantiated by calling the built-in set function, which returns a set containing all the elements of its optional argument, which must be an iterable sequence. Sets have no literal syntax.

The only method of a set is union, which is equivalent to the | operator.

A set used in a Boolean context is considered true if it is non-empty.

Implementation note: The Go implementation of the Skylark REPL requires the -set flag to enable support for sets and the -bitwise flag to enable support for the &, |, and ^ operators. The Java implementation does not support sets.

Functions

A function value represents a function defined in Skylark. Its type is "function". A function value used in a Boolean context is always considered true.

Functions defined by a def statement are named; functions defined by a lambda expression are anonymous.

Function definitions may be nested, and an inner function may refer to a local variable of an outer function.

A function definition defines zero or more named parameters. Skylark has a rich mechanism for passing arguments to functions.

The example below shows a definition and call of a function of two required parameters, x and y.

def idiv(x, y):
  return x // y

idiv(6, 3)		# 2

A call may provide arguments to function parameters either by position, as in the example above, or by name, as in first two calls below, or by a mixture of the two forms, as in the third call below. All the positional arguments must precede all the named arguments. Named arguments may improve clarity, especially in functions of several parameters.

idiv(x=6, y=3)		# 2
idiv(y=3, x=6)		# 2

idiv(6, y=3)		# 2

Optional parameters: A parameter declaration may specify a default value using name=value syntax; such a parameter is optional. The default value expression is evaluated during execution of the def statement or evaluation of the lambda expression, and the default value forms part of the function value. All optional parameters must follow all non-optional parameters. A function call may omit arguments for any suffix of the optional parameters; the effective values of those arguments are supplied by the function's parameter defaults.

def f(x, y=3):
  return x, y

f(1, 2)	# (1, 2)
f(1)	# (1, 3)

If a function parameter's default value is a mutable expression, modifications to the value during one call may be observed by subsequent calls. Beware of this when using lists or dicts as default values. If the function becomes frozen, its parameters' default values become frozen too.

# module a.sky
def f(x, list=[]):
  list.append(x)
  return list

f(4, [1,2,3])           # [1, 2, 3, 4]
f(1)                    # [1]
f(2)                    # [1, 2], not [2]!

# module b.sky
load("a.sky", "f")
f(3)                    # error: cannot append to frozen list

Variadic functions: Some functions allow callers to provide an arbitrary number of arguments. After all required and optional parameters, a function definition may specify a variadic arguments or varargs parameter, indicated by a star preceding the parameter name: *args. Any surplus positional arguments provided by the caller are formed into a tuple and assigned to the args parameter.

def f(x, y, *args):
  return x, y, args

f(1, 2)                 # (1, 2, ())
f(1, 2, 3, 4)           # (1, 2, (3, 4))

Keyword-variadic functions: Some functions allow callers to provide an arbitrary sequence of name=value keyword arguments. A function definition may include a final keyworded arguments or kwargs parameter, indicated by a double-star preceding the parameter name: **kwargs. Any surplus named arguments that do not correspond to named parameters are collected in a new dictionary and assigned to the kwargs parameter:

def f(x, y, **kwargs):
  return x, y, kwargs

f(1, 2)                 # (1, 2, {})
f(x=2, y=1)             # (2, 1, {})
f(x=2, y=1, z=3)        # (2, 1, {"z": 3})

It is a static error if any two parameters of a function have the same name.

Just as a function definition may accept an arbitrary number of positional or keyworded arguments, a function call may provide an arbitrary number of positional or keyworded arguments supplied by a list or dictionary:

def f(a, b, c=5):
  return a * b + c

f(*[2, 3])              # 11
f(*[2, 3, 7])           # 13
f(*[2])                 # error: f takes at least 2 arguments (1 given)

f(**dict(b=3, a=2))             # 11
f(**dict(c=7, a=2, b=3))        # 13
f(**dict(a=2))                  # error: f takes at least 2 arguments (1 given)
f(**dict(d=4))                  # error: f got unexpected keyword argument "d"

Once the parameters have been successfully bound to the arguments supplied by the call, the sequence of statements that comprise the function body is executed.

A function call completes normally after the execution of either a return statement, or of the last statement in the function body. The result of the function call is the value of the return statement's operand, or None if the return statement had no operand or if the function completeted without executing a return statement.

def f(x):
  if x == 0:
    return
  if x < 0:
    return -x
  print(x)

f(1)            # returns None after printing "1"
f(0)            # returns None without printing
f(-1)           # returns 1 without printing

It is a dynamic error for a function to call itself or another function value with the same declaration.

def fib(x):
  if x < 2:
    return x
  return fib(x-2) + fib(x-1)	# dynamic error: function fib called recursively

fib(5)

This rule, combined with the invariant that all loops are iterations over finite sequences, implies that Skylark programs are not Turing-complete.

Built-in functions

A built-in function is a function or method implemented in Go by the interpreter or the application into which the interpreter is embedded.

The type of a built-in function is "builtin_function_or_method". Implementation note: The Java implementation of type(x) returns "function" for all functions, whether built in or defined in Skylark, even though applications distinguish these two types.

A built-in function value used in a Boolean context is always considered true.

Many built-in functions are predeclared in the environment (see Name Resolution). Some built-in functions such as len are universal, that is, available to all Skylark programs. The host application may predeclare additional built-in functions in the environment of a specific module.

Except where noted, built-in functions accept only positional arguments. The parameter names serve merely as documentation.

Name binding and variables

After a Skylark file is parsed, but before its execution begins, the Skylark interpreter checks statically that the program is well formed. For example, break and continue statements may appear only within a loop; if, for, and return statements may appear only within a function; and load statements may appear only outside any function.

Name resolution is the static checking process that resolves names to variable bindings. During execution, names refer to variables. Statically, names denote places in the code where variables are created; these places are called bindings. A name may denote different bindings at different places in the program. The region of text in which a particular name refers to the same binding is called that binding's scope.

Four Skylark constructs bind names, as illustrated in the example below: load statements (a and b), def statements (c), function parameters (d), and assignments (e, h, including the augmented assignment e += h). Variables may be assigned or re-assigned explicitly (e, h), or implicitly, as in a for-loop (f) or comprehension (g, i).

load("lib.sky", "a", b="B")

def c(d):
  e = 0
  for f in d:
     print([True for g in f])
     e += 1

h = [2*i for i in a]

The environment of a Skylark program is structured as a tree of lexical blocks, each of which may contain name bindings. The tree of blocks is parallel to the syntax tree. Blocks are of four kinds.

At the root of the tree is the predeclared block, which binds several names implicitly. The set of predeclared names includes the universal constant values None, True, and False, and various built-in functions such as len and list; these functions are immutable and stateless. An application may pre-declare additional names to provide domain-specific functions to that file, for example. These additional functions may have side effects on the application. Skylark programs cannot change the set of predeclared bindings or assign new values to them.

Nested beneath the predeclared block is the module block, which contains the bindings of the current file. Bindings in the module block (such as a, b, c, and h in the example) are called global. The module block is empty at the start of the file and is populated by top-level binding statements.

A module block contains a function block for each top-level function, and a comprehension block for each top-level comprehension. Bindings inside either of these kinds of block are called local. Additional functions and comprehensions, and their blocks, may be nested in any order, to any depth.

If name is bound anywhere within a block, all uses of the name within the block are treated as references to that binding, even uses that appear before the binding. This is true even in the module block, unlike Python. The binding of y on the last line of the example below makes y local to the function hello, so the use of y in the print statement also refers to the local y, even though it appears earlier.

y = "goodbye"

def hello():
  for x in (1, 2):
    if x == 2:
      print(y) # prints "hello"
    if x == 1:
      y = "hello"

It is a dynamic error to evaluate a reference to a local variable before it has been bound:

def f():
  print(x)              # dynamic error: local variable x referenced before assignment
  x = "hello"

The same is true for global variables:

print(x)                # dynamic error: global variable x referenced before assignment
x = "hello"

It is a static error to bind a global variable already explicitly bound in the file:

x = 1
x = 2                   # static error: cannot reassign global x declared on line 1

If a name was pre-bound by the application, the Skylark program may explicitly bind it, but only once.

Implementation note: An augmented assignment statement such as x += 1 is considered a binding of x. However, because of the special behavior of += for lists, which acts like a non-binding reference, the Go implementation suppresses the "cannot reassign" error for all augmented assigments at toplevel, whereas the Java implementation reports the error even when the statement would apply += to a list.

A function may refer to variables defined in an enclosing function. In this example, the inner function f refers to a variable x that is local to the outer function squarer. x is a free variable of f. The function value (f) created by a def statement holds a reference to each of its free variables so it may use them even after the enclosing function has returned.

def squarer():
    x = [0]
    def f():
      x[0] += 1
      return x[0]*x[0]
    return f

sq = squarer()
print(sq(), sq(), sq(), sq()) # "1 4 9 16"

An inner function cannot assign to a variable bound in an enclosing function, because the assignment would bind the variable in the inner function. In the example below, the x += 1 statement binds x within f, hiding the outer x. Execution fails because the inner x has not been assigned before the attempt to increment it.

def squarer():
    x = 0
    def f():
      x += 1            # dynamic error: local variable x referenced before assignment
      return x*x
    return f

sq = squarer()

(Skylark has no equivalent of Python's nonlocal or global declarations, but as the first version of squarer showed, this omission can be worked around by using a list of a single element.)

A name appearing after a dot, such as split in get_filename().split('/'), is not resolved statically. The dot expression .split is a dynamic operation on the value returned by get_filename().

Value concepts {#value-concepts}

Skylark has eleven core data types. An application that embeds the Skylark intepreter may define additional types that behave like Skylark values. All values, whether core or application-defined, implement a few basic behaviors:

str(x)		-- return a string representation of x
type(x)		-- return a string describing the type of x
bool(x)		-- convert x to a Boolean truth value
hash(x)		-- return a hash code for x

Identity and mutation

Skylark is an imperative language: programs consist of sequences of statements executed for their side effects. For example, an assignment statement updates the value held by a variable, and calls to some built-in functions such as print change the state of the application that embeds the interpreter.

Values of some data types, such as NoneType, bool, int, float, and string, are immutable; they can never change. Immutable values have no notion of identity: it is impossible for a Skylark program to tell whether two integers, for instance, are represented by the same object; it can tell only whether they are equal.

Values of other data types, such as list, dict, and set, are mutable: they may be modified by a statement such as a[i] = 0 or items.clear(). Although tuple and function values are not directly mutable, they may refer to mutable values indirectly, so for this reason we consider them mutable too. Skylark values of these types are actually references to variables.

Copying a reference to a variable, using an assignment statement for instance, creates an alias for the variable, and the effects of operations applied to the variable through one alias are visible through all others.

x = []                          # x refers to a new empty list variable
y = x                           # y becomes an alias for x
x.append(1)                     # changes the variable referred to by x
print(y)                        # "[1]"; y observes the mutation

Skylark uses call-by-value parameter passing: in a function call, argument values are assigned to function parameters as if by assignment statements. If the values are references, the caller and callee may refer to the same variables, so if the called function changes the variable referred to by a parameter, the effect may also be observed by the caller:

def f(y):
    y.append(1)                 # changes the variable referred to by x

x = []                          # x refers to a new empty list variable
f(x)                            # f's parameter y becomes an alias for x
print(x)                        # "[1]"; x observes the mutation

As in all imperative languages, understanding aliasing, the relationship between reference values and the variables to which they refer, is crucial to writing correct programs.

Freezing a value

Skylark has a feature unusual among imperative programming languages: a mutable value may be frozen so that all subsequent attempts to mutate it fail with a dynamic error; the value, and all other values reachable from it, become immutable.

Immediately after execution of a Skylark module, all values in its top-level environment are frozen. Because all the global variables of an initialized Skylark module are immutable, the module may be published to and used by other threads in a parallel program without the need for locks. For example, the Bazel build system loads and executes BUILD and .bzl files in parallel, and two modules being executed concurrently may freely access variables or call functions from a third without the possibility of a race condition.

Hashing

The dict and set data types are implemented using hash tables, so only hashable values are suitable as keys of a dict or elements of a set. Attempting to use a non-hashable value as the key in a hash table, or as the operand of the hash built-in function, results in a dynamic error.

The hash of a value is an unspecified integer chosen so that two equal values have the same hash, in other words, x == y => hash(x) == hash(y). A hashable value has the same hash throughout its lifetime.

Values of the types NoneType, bool, int, float, and string, which are all immutable, are hashable.

Values of mutable types such as list, dict, and set are not hashable. These values remain unhashable even if they have become immutable due to freezing.

A tuple value is hashable only if all its elements are hashable. Thus ("localhost", 80) is hashable but ([127, 0, 0, 1], 80) is not.

Values of the types function and builtin_function_or_method are also hashable. Although functions are not necessarily immutable, as they may be closures that refer to mutable variables, instances of these types are compared by reference identity (see Comparisons), so their hash values are derived from their identity.

Sequence types

Many Skylark data types represent a sequence of values: lists, tuples, and sets are sequences of arbitrary values, and in many contexts dictionaries act like a sequence of their keys.

We can classify different kinds of sequence types based on the operations they support. Each is listed below using the name of its corresponding interface in the interpreter's Go API.

  • Iterable: an iterable value lets us process each of its elements in a fixed order. Examples: dict, set, list, tuple, but not string.
  • Sequence: a sequence of known length lets us know how many elements it contains without processing them. Examples: dict, set, list, tuple, but not string.
  • Indexable: an indexed type has a fixed length and provides efficient random access to its elements, which are identified by integer indices. Examples: string, tuple, and list.
  • SetIndexable: a settable indexed type additionally allows us to modify the element at a given integer index. Example: list.
  • Mapping: a mapping is an association of keys to values. Example: dict.

Although all of Skylark's core data types for sequences implement at least the Sequence contract, it's possible for an application that embeds the Skylark interpreter to define additional data types representing sequences of unknown length that implement only the Iterable contract.

Strings are not iterable, though they do support the len(s) and s[i] operations. Skylark deviates from Python here to avoid common pitfall in which a string is used by mistake where a list containing a single string was intended, resulting in its interpretation as a sequence of bytes.

Most Skylark operators and built-in functions that need a sequence of values will accept any iterable.

It is a dynamic error to mutate a sequence such as a list, set, or dictionary while iterating over it.

def increment_values(dict):
  for k in dict:
    dict[k] += 1			# error: cannot insert into hash table during iteration

dict = {"one": 1, "two": 2}
increment_values(dict)

Indexing

Many Skylark operators and functions require an index operand i, such as a[i] or list.insert(i, x). Others require two indices i and j that indicate the start and end of a sub-sequence, such as a[i:j], list.index(x, i, j), or string.find(x, i, j). All such operations follow similar conventions, described here.

Indexing in Skylark is zero-based. The first element of a string or list has index 0, the next 1, and so on. The last element of a sequence of length n has index n-1.

"hello"[0]			# "h"
"hello"[4]			# "o"
"hello"[5]			# error: index out of range

For sub-sequence operations that require two indices, the first is inclusive and the second exclusive. Thus a[i:j] indicates the sequence starting with element i up to but not including element j. The length of this sub-sequence is j-i. This convention is known as half-open indexing.

"hello"[1:4]			# "ell"

Either or both of the index operands may be omitted. If omitted, the first is treated equivalent to 0 and the second is equivalent to the length of the sequence:

"hello"[1:]                     # "ello"
"hello"[:4]                     # "hell"

It is permissible to supply a negative integer to an indexing operation. The effective index is computed from the supplied value by the following two-step procedure. First, if the value is negative, the length of the sequence is added to it. This provides a convenient way to address the final elements of the sequence:

"hello"[-1]                     # "o",  like "hello"[4]
"hello"[-3:-1]                  # "ll", like "hello"[2:4]

Second, for sub-sequence operations, if the value is still negative, it is replaced by zero, or if it is greater than the length n of the sequence, it is replaced by n. In effect, the index is "truncated" to the nearest value in the range [0:n].

"hello"[-1000:+1000]		# "hello"

This truncation step does not apply to indices of individual elements:

"hello"[-6]		# error: index out of range
"hello"[-5]		# "h"
"hello"[4]		# "o"
"hello"[5]		# error: index out of range

Expressions

An expression specifies the computation of a value.

The Skylark grammar defines several categories of expression. An operand is an expression consisting of a single token (such as an identifier or a literal), or a bracketed expression. Operands are self-delimiting. An operand may be followed by any number of dot, call, or slice suffixes, to form a primary expression. In some places in the Skylark grammar where an expression is expected, it is legal to provide a comma-separated list of expressions denoting a tuple. The grammar uses Expression where a multiple-component expression is allowed, and Test where it accepts an expression of only a single component.

Expression = Test {',' Test} .

Test = LambdaExpr | IfExpr | PrimaryExpr | UnaryExpr | BinaryExpr .

PrimaryExpr = Operand
            | PrimaryExpr DotSuffix
            | PrimaryExpr CallSuffix
            | PrimaryExpr SliceSuffix
            .

Operand = identifier
        | int | float | string
        | ListExpr | ListComp
        | DictExpr | DictComp
        | '(' [Expression] [,] ')'
        | ('-' | '+') PrimaryExpr
        .

DotSuffix   = '.' identifier .
CallSuffix  = '(' [Arguments [',']] ')' .
SliceSuffix = '[' [Expression] [':' Test [':' Test]] ']' .

# A CallSuffix does not allow a trailing comma
# if the last argument is '*' Test or '**' Test.

TODO: resolve position of +x, -x, and 'not x' in grammar: Operand or UnaryExpr?

Identifiers

Primary = identifier

An identifier is a name that identifies a value.

Lookup of locals and globals may fail if not yet defined.

Literals

Skylark supports string literals of three different kinds:

Primary = int | float | string

Evaluation of a literal yields a value of the given type (string, int, or float) with the given value. See [Literals](#lexical elements) for details.

Parenthesized expressions

Primary = '(' [Expression] ')'

A single expression enclosed in parentheses yields the result of that expression. Explicit parentheses may be used for clarity, or to override the default association of subexpressions.

1 + 2 * 3 + 4                   # 11
(1 + 2) * (3 + 4)               # 21

If the parentheses are empty, or contain a single expression followed by a comma, or contain two or more expressions, the expression yields a tuple.

()                              # (), the empty tuple
(1,)                            # (1,), a tuple of length 1
(1, 2)                          # (1, 2), a 2-tuple or pair
(1, 2, 3)                       # (1, 2, 3), a 3-tuple or triple

In some contexts, such as a return or assignment statement or the operand of a for statement, a tuple may be expressed without parentheses.

x, y = 1, 2

return 1, 2

for x in 1, 2:
   print(x)

Skylark (like Python 3) does not accept an unparenthesized tuple expression as the operand of a list comprehension:

[2*x for x in 1, 2, 3]	       	# parse error: unexpected ','

Dictionary expressions

A dictionary expression is a comma-separated list of colon-separated key/value expression pairs, enclosed in curly brackets, and it yields a new dictionary object. An optional comma may follow the final pair.

DictExpr = '{' [Entries [',']] '}' .
Entries  = Entry {',' Entry} .
Entry    = Test ':' Test .

Examples:

{}
{"one": 1}
{"one": 1, "two": 2,}

The key and value expressions are evaluated in left-to-right order. Evaluation fails if the same key is used multiple times.

Only hashable values may be used as the keys of a dictionary. This includes all built-in types except dictionaries, sets, and lists; a tuple is hashable only if its elements are hashable.

List expressions

A list expression is a comma-separated list of element expressions, enclosed in square brackets, and it yields a new list object. An optional comma may follow the last element expression.

ListExpr = '[' [Expression [',']] ']' .

Element expressions are evaluated in left-to-right order.

Examples:

[]                      # [], empty list
[1]                     # [1], a 1-element list
[1, 2, 3,]              # [1, 2, 3], a 3-element list

Unary operators

There are three unary operators, all appearing before their operand: +, -, ~, and not.

UnaryExpr = '+' PrimaryExpr
          | '-' PrimaryExpr
          | '~' PrimaryExpr
          | 'not' Test
          .
+ number        unary positive          (int, float)
- number        unary negation          (int, float)
~ number        unary bitwise inversion (int)
not x           logical negation        (any type)

The + and - operators may be applied to any number (int or float) and return the number unchanged. Unary + is never necessary in a correct program, but may serve as an assertion that its operand is a number, or as documentation.

if x > 0:
	return +1
else if x < 0:
	return -1
else:
	return 0

The not operator returns the negation of the truth value of its operand.

not True                        # False
not False                       # True
not [1, 2, 3]                   # False
not ""                          # True
not 0                           # True

The ~ operator yields the bitwise inversion of its integer argument. The bitwise inversion of x is defined as -(x+1).

~1                              # -2
~-1                             # 0
~0                              # -1

Implementation note: The parser in the Java implementation of Skylark does not accept unary + and ~ expressions.

Binary operators

Skylark has the following binary operators, arranged in order of increasing precedence:

or
and
not
==   !=   <    >   <=   >=   in   not in
|
^
&
<<   >>
-    +
*    /    //   %

Comparison operators, in, and not in are non-associative, so the parser will not accept 0 <= i < n. All other binary operators of equal precedence associate to the left.

BinaryExpr = Test {Binop Test} .

Binop = 'or'
      | 'and'
      | 'not'
      | '==' | '!=' | '<' | '>' | '<=' | '>=' | 'in' | 'not' 'in'
      | '|'
      | '^'
      | '&'
      | '-' | '+'
      | '*' | '%' | '/' | '//'
      | '<<' | '>>'
      .

or and and

The or and and operators yield, respectively, the logical disjunction and conjunction of their arguments, which need not be Booleans. The expression x or y yields the value of x if its truth value is True, or the value of y otherwise.

False or False		# False
False or True		# True
True  or False		# True
True  or True		# True

0 or "hello"		# "hello"
1 or "hello"		# 1

Similarly, x and y yields the value of x if its truth value is False, or the value of y otherwise.

False and False		# False
False and True		# False
True  and False		# False
True  and True		# True

0 and "hello"		# 0
1 and "hello"		# "hello"

These operators use "short circuit" evaluation, so the second expression is not evaluated if the value of the first expression has already determined the result, allowing constructions like these:

len(x) > 0 and x[0] == 1		# x[0] is not evaluated if x is empty
x and x[0] == 1
len(x) == 0 or x[0] == ""
not x or not x[0]

Comparisons

The == operator reports whether its operands are equal; the != operator is its negation.

The operators <, >, <=, and >= perform an ordered comparison of their operands. It is an error to apply these operators to operands of unequal type, unless one of the operands is an int and the other is a float. Of the built-in types, only the following support ordered comparison, using the ordering relation shown:

NoneType        # None <= None
bool            # False < True
int             # mathematical
float           # as defined by IEEE 754
string          # lexicographical
tuple           # lexicographical
list            # lexicographical

Comparison of floating point values follows the IEEE 754 standard, which breaks several mathematical identities. For example, if x is a NaN value, the comparisons x < y, x == y, and x > y all yield false for all values of y.

Applications may define additional types that support ordered comparison.

The remaining built-in types support only equality comparisons. Values of type dict or set compare equal if their elements compare equal, and values of type function or builtin_function_or_method are equal only to themselves.

dict                            # equal contents
set                             # equal contents
function                        # identity
builtin_function_or_method      # identity

Arithmetic operations

The following table summarizes the binary arithmetic operations available for built-in types:

Arithmetic (int or float; result has type float unless both operands have type int)
   number + number              # addition
   number - number              # subtraction
   number * number              # multiplication
   number / number              # real division  (result is always a float)
   number // number             # floored division
   number % number              # remainder of floored division
   number ^ number              # bitwise XOR
   number << number             # bitwise left shift
   number >> number             # bitwise right shift

Concatenation
   string + string
     list + list
    tuple + tuple
     dict + dict                # (deprecated)

Repetition (string/list/tuple)
      int * sequence
 sequence * int

String interpolation
   string % any                 # see String Interpolation

Sets
      int | int                 # bitwise union (OR)
      set | set                 # set union
      int & int                 # bitwise intersection (AND)
      set & set                 # set intersection
      set ^ set                 # set symmetric difference

The operands of the arithmetic operators +, -, *, //, and % must both be numbers (int or float) but need not have the same type. The type of the result has type int only if both operands have that type. The result of real division / always has type float.

The + operator may be applied to non-numeric operands of the same type, such as two lists, two tuples, or two strings, in which case it computes the concatenation of the two operands and yields a new value of the same type.

"Hello, " + "world"		# "Hello, world"
(1, 2) + (3, 4)			# (1, 2, 3, 4)
[1, 2] + [3, 4]			# [1, 2, 3, 4]

The x + y operation is deprecated for dict operands; see Google Issue b/31994014. Use the dict·update method instead:

# z = x + y
z = dict(x)
z.update(y)

The * operator may be applied to an integer n and a value of type string, list, or tuple, in which case it yields a new value of the same sequence type consisting of n repetitions of the original sequence. The order of the operands is immaterial. Negative values of n behave like zero.

'mur' * 2               # 'murmur'
3 * range(3)            # [0, 1, 2, 0, 1, 2, 0, 1, 2]

Applications may define additional types that support any subset of these operators.

The & operator requires two operands of the same type, either int or set. For integers, it yields the bitwise intersection (AND) of its operands. For sets, it yields a new set containing the intersection of the elements of the operand sets, preserving the element order of the left operand.

The | operator likewise computes bitwise or set unions. The result of set | set is a new set whose elements are the union of the operands, preserving the order of the elements of the operands, left before right.

The ^ operator accepts operands of either int or set type. For integers, it yields the bitwise XOR (exclusive OR) of its operands. For sets, it yields a new set containing elements of either first or second operand but not both (symmetric difference).

The << and >> operators require operands of int type both. They shift the first operand to the left or right by the number of bits given by the second operand. It is a dynamic error if the second operand is negative. Implementations may impose a limit on the second operand of a left shift.

0x12345678 & 0xFF               # 0x00000078
0x12345678 | 0xFF               # 0x123456FF
0b01011101 ^ 0b110101101        # 0b111110000
0b01011101 >> 2                 # 0b010111
0b01011101 << 2                 # 0b0101110100

set([1, 2]) & set([2, 3])       # set([2])
set([1, 2]) | set([2, 3])       # set([1, 2, 3])
set([1, 2]) ^ set([2, 3])       # set([1, 3])

Implementation note: The Go implementation of the Skylark REPL requires the -set flag to enable support for sets. The Java implementation does not support sets, nor recognize & as a token, nor support int | int.

Membership tests

      any in     sequence		(list, tuple, dict, set, string)
      any not in sequence

The in operator reports whether its first operand is a member of its second operand, which must be a list, tuple, dict, set, or string. The not in operator is its negation. Both return a Boolean.

The meaning of membership varies by the type of the second operand: the members of a list, tuple, or set are its elements; the members of a dict are its keys; the members of a string are all its substrings.

1 in [1, 2, 3]                  # True
4 in (1, 2, 3)                  # False
4 not in set([1, 2, 3])         # True

d = {"one": 1, "two": 2}
"one" in d                      # True
"three" in d                    # False
1 in d                          # False
[] in d				# False

"nasty" in "dynasty"            # True
"a" in "banana"                 # True
"f" not in "way"                # True

String interpolation

The expression format % args performs string interpolation, a simple form of template expansion. The format string is interpreted as a sequence of literal portions and conversions. Each conversion, which starts with a % character, is replaced by its corresponding value from args. The characters following % in each conversion determine which argument it uses and how to convert it to a string.

Each % character marks the start of a conversion specifier, unless it is immediately followed by another %, in which case both characters together denote a literal percent sign.

If the "%" is immediately followed by "(key)", the parenthesized substring specifies the key of the args dictionary whose corresponding value is the operand to convert. Otherwise, the conversion's operand is the next element of args, which must be a tuple with exactly one component per conversion, unless the format string contains only a single conversion, in which case args itself is its operand.

Skylark does not support the flag, width, and padding specifiers supported by Python's % and other variants of C's printf.

After the optional (key) comes a single letter indicating what operand types are valid and how to convert the operand x to a string:

%       none            literal percent sign
s       any             as if by str(x)
r       any             as if by repr(x)
d       number          signed integer decimal
i       number          signed integer decimal
o       number          signed octal
x       number          signed hexadecimal, lowercase
X       number          signed hexadecimal, uppercase
e       number          float exponential format, lowercase
E       number          float exponential format, uppercase
f       number          float decimal format, lowercase
F       number          float decimal format, uppercase
g       number          like %e for large exponents, %f otherwise
G       number          like %E for large exponents, %F otherwise
c       string          x (string must encode a single Unicode code point)
        int             as if by chr(x)

It is an error if the argument does not have the type required by the conversion specifier. A Boolean argument is not considered a number.

Examples:

"Hello %s, your score is %d" % ("Bob", 75)      # "Hello Bob, your score is 75"

"%d %o %x %c" % (65, 65, 65, 65)                # "65 101 41 A" (decimal, octal, hexadecimal, Unicode)

"%(greeting)s, %(audience)s" % dict(            # "Hello, world"
  greeting="Hello",
  audience="world",
)

"rate = %g%% APR" % 3.5                         # "rate = 3.5% APR"

One subtlety: to use a tuple as the operand of a conversion in format string containing only a single conversion, you must wrap the tuple in a singleton tuple:

"coordinates=%s" % (40.741491, -74.003680)	# error: too many arguments for format string
"coordinates=%s" % ((40.741491, -74.003680),)	# "coordinates=(40.741491, -74.003680)"

TODO: specify %e and %f more precisely.

Conditional expressions

A conditional expression has the form a if cond else b. It first evaluates the condition cond. If it's true, it evaluates a and yields its value; otherwise it yields the value of b.

IfExpr = Test 'if' Test 'else' Test .

Example:

"yes" if enabled else "no"

Comprehensions

A comprehension constructs new list or dictionary value by looping over one or more iterables and evaluating a body expression that produces successive elements of the result.

A list comprehension consists of a single expression followed by one or more clauses, the first of which must be a for clause. Each for clause resembles a for statement, and specifies an iterable operand and a set of variables to be assigned by successive values of the iterable. An if cause resembles an if statement, and specifies a condition that must be met for the body expression to be evaluated. A sequence of for and if clauses acts like a nested sequence of for and if statements.

ListComp = '[' Test {CompClause} ']'.
DictComp = '{' Entry {CompClause} '}' .

CompClause = 'for' LoopVariables 'in' Test
           | 'if' Test .

LoopVariables = PrimaryExpr {',' PrimaryExpr} .

Examples:

[x*x for x in range(5)]                 # [0, 1, 4, 9, 16]
[x*x for x in range(5) if x%2 == 0]     # [0, 4, 16]
[(x, y) for x in range(5)
        if x%2 == 0
        for y in range(5)
        if y > x]                       # [(0, 1), (0, 2), (0, 3), (0, 4), (2, 3), (2, 4)]

A dict comprehension resembles a list comprehension, but its body is a pair of expressions, key: value, separated by a colon, and its result is a dictionary containing the key/value pairs for which the body expression was evaluated. Evaluation fails if the value of any key is unhashable.

As with a for loop, the loop variables may exploit compound assignment:

[x*y+z for (x, y), z in [((2, 3), 5), (("o", 2), "!")]]         # [11, 'oo!']

Skylark, following Python 3, does not accept an unparenthesized tuple or lambda expression as the operand of a for clause:

[x*x for x in 1, 2, 3]		# parse error: unexpected comma
[x*x for x in lambda: 0]	# parse error: unexpected lambda

Comprehensions in Skylark, again following Python 3, define a new lexical block, so assignments to loop variables have no effect on variables of the same name in an enclosing block:

x = 1
_ = [x for x in [2]]            # new variable x is local to the comprehension
print(x)                        # 1

The operand of a comprehension's first clause (always a for) is resolved in the lexical block enclosing the comprehension. In the examples below, identifiers referring to the outer variable named x have been distinguished by subscript.

x₀ = (1, 2, 3)
[x*x for x in x₀]               # [1, 4, 9]
[x*x for x in x₀ if x%2 == 0]   # [4]

All subsequent for and if expressions are resolved within the comprehension's lexical block, as in this rather obscure example:

x₀ = ([1, 2], [3, 4], [5, 6])
[x*x for x in x₀ for x in x if x%2 == 0]     # [4, 16, 36]

which would be more clearly rewritten as:

x = ([1, 2], [3, 4], [5, 6])
[z*z for y in x for z in y if z%2 == 0]     # [4, 16, 36]

Function and method calls

CallSuffix = '(' [Arguments] ')' .

Arguments = Argument {',' Argument} .
Argument  = Test | identifier '=' Test | '*' Test | '**' Test .

A value f of type function or builtin_function_or_method may be called using the expression f(...). Applications may define additional types whose values may be called in the same way.

A method call such as filename.endswith(".sky") is the composition of two operations, m = filename.endswith and m(".sky"). The first, a dot operation, yields a bound method, a function value that pairs a receiver value (the filename string) with a choice of method (string·endswith).

Only built-in or application-defined types may have methods.

See Functions for an explanation of function parameter passing.

Dot expressions

A dot expression x.f selects the attribute f (a field or method) of the value x.

Fields are possessed by none of the main Skylark data types, but some application-defined types have them. Methods belong to the built-in types string, list, dict, and set, and to many application-defined types.

DotSuffix = '.' identifier .

A dot expression fails if the value does not have an attribute of the specified name.

Use the built-in function hasattr(x, "f") to ascertain whether a value has a specific attribute, or dir(x) to enumerate all its attributes. The getattr(x, "f") function can be used to select an attribute when the name "f" is not known statically.

A dot expression that selects a method typically appears within a call expression, as in these examples:

["able", "baker", "charlie"].index("baker")     # 1
"banana".count("a")                             # 3
"banana".reverse()                              # error: string has no .reverse field or method

But when not called immediately, the dot expression evaluates to a bound method, that is, a method coupled to a specific receiver value. A bound method can be called like an ordinary function, without a receiver argument:

f = "banana".count
f                                               # <built-in method count of string value>
f("a")                                          # 3
f("n")                                          # 2

Implementation note: The Java implementation does not currently allow a method to be selected but not immediately called. See Google Issue b/21392896.

Index expressions

An index expression a[i] yields the ith element of an indexable type such as a string, tuple, or list. The index i must be an int value in the range -ni < n, where n is len(a); any other index results in an error.

SliceSuffix = '[' [Expression] [':' Test [':' Test]] ']' .

A valid negative index i behaves like the non-negative index n+i, allowing for convenient indexing relative to the end of the sequence.

"abc"[0]                        # "a"
"abc"[1]                        # "b"
"abc"[-1]                       # "c"

("zero", "one", "two")[0]       # "zero"
("zero", "one", "two")[1]       # "one"
("zero", "one", "two")[-1]      # "two"

An index expression d[key] may also be applied to a dictionary d, to obtain the value associated with the specified key. It is an error if the dictionary contains no such key.

An index expression appearing on the left side of an assignment causes the specified list or dictionary element to be updated:

a = range(3)            # a == [0, 1, 2]
a[2] = 7                # a == [0, 1, 7]

coins["suzie b"] = 100

It is a dynamic error to attempt to update an element of an immutable type, such as a tuple or string, or a frozen value of a mutable type.

Slice expressions

A slice expression a[start:stop:stride] yields a new value containing a sub-sequence of a, which must be a string, tuple, or list.

SliceSuffix = '[' [Expression] [':' Test [':' Test]] ']' .

Each of the start, stop, and stride operands is optional; if present, and not None, each must be an integer. The stride value defaults to 1. If the stride is not specified, the colon preceding it may be omitted too. It is an error to specify a stride of zero.

Conceptually, these operands specify a sequence of values i starting at start and successively adding stride until i reaches or passes stop. The result consists of the concatenation of values of a[i] for which i is valid.`

The effective start and stop indices are computed from the three operands as follows. Let n be the length of the sequence.

If the stride is positive: If the start operand was omitted, it defaults to -infinity. If the end operand was omitted, it defaults to +infinity. For either operand, if a negative value was supplied, n is added to it. The start and end values are then "clamped" to the nearest value in the range 0 to n, inclusive.

If the stride is negative: If the start operand was omitted, it defaults to +infinity. If the end operand was omitted, it defaults to -infinity. For either operand, if a negative value was supplied, n is added to it. The start and end values are then "clamped" to the nearest value in the range -1 to n-1, inclusive.

"abc"[1:]               # "bc"  (remove first element)
"abc"[:-1]              # "ab"  (remove last element)
"abc"[1:-1]             # "b"   (remove first and last element)
"banana"[1::2]          # "aaa" (select alternate elements starting at index 1)
"banana"[4::-2]         # "nnb" (select alternate elements in reverse, starting at index 4)

Unlike Python, Skylark does not allow a slice expression on the left side of an assignment.

Slicing a tuple or string may be more efficient than slicing a list because tuples and strings are immutable, so the result of the operation can share the underlying representation of the original operand (when the stride is 1). By contrast, slicing a list requires the creation of a new list and copying of the necessary elements.

Lambda expressions

A lambda expression yields a new function value.

LambdaExpr = 'lambda' [Parameters] ':' Test .

Parameters = Parameter {',' Parameter} .
Parameter  = identifier
           | identifier '=' Test
           | '*' identifier
           | '**' identifier
           .

Syntactically, a lambda expression consists of the keyword lambda, followed by a parameter list like that of a def statement but unparenthesized, then a colon :, and a single expression, the function body.

Example:

def map(f, list):
    return [f(x) for x in list]

map(lambda x: 2*x, range(3))    # [2, 4, 6]

As with functions created by a def statement, a lambda function captures the syntax of its body, the default values of any optional parameters, the value of each free variable appearing in its body, and the global dictionary of the current module.

The name of a function created by a lambda expression is "lambda".

The two statements below are essentially equivalent, but that the function created by the def statement is named twice and the function created by the lambda expression is called lambda.

def twice(x):
   return x * 2

twice = lambda(x): x * 2

Implementation note: The Go implementation of the Skylark REPL requires the -lambda flag to enable support for lambda expressions. The Java implementation does not support them. See Google Issue b/36358844.

Statements

Statement  = DefStmt | IfStmt | ForStmt | SimpleStmt .
SimpleStmt = SmallStmt {';' SmallStmt} [';'] '\n' .
SmallStmt  = ReturnStmt
           | BreakStmt | ContinueStmt | PassStmt
           | AssignStmt
           | ExprStmt
           | LoadStmt
           .

Pass statements

A pass statement does nothing. Use a pass statement when the syntax requires a statement but no behavior is required, such as the body of a function that does nothing.

PassStmt = 'pass' .

Example:

def noop():
   pass

def list_to_dict(items):
  # Convert list of tuples to dict
  m = {}
  for k, m[k] in items:
    pass
  return m

Assignments

An assignment statement has the form lhs = rhs. It evaluates the expression on the right-hand side then assigns its value (or values) to the variable (or variables) on the left-hand side.

AssignStmt = Expression '=' Expression .

The expression on the left-hand side is called a target. The simplest target is the name of a variable, but a target may also have the form of an index expression, to update the element of a list or dictionary, or a dot expression, to update the field of an object:

k = 1
a[i] = v
m.f = ""

Compound targets may consist of a comma-separated list of subtargets, optionally surrounded by parentheses or square brackets, and targets may be nested arbitarily in this way. An assignment to a compound target checks that the right-hand value is a sequence with the same number of elements as the target. Each element of the sequence is then assigned to the corresponding element of the target, recursively applying the same logic. It is a static error if the sequence is empty.

pi, e = 3.141, 2.718
(x, y) = f()
[zero, one, two] = range(3)

[(a, b), (c, d)] = ("ab", "cd")

The same process for assigning a value to a target expression is used in for loops and in comprehensions.

Implementation note: In the Java implementation, targets cannot be dot expressions.

Augmented assignments

An augmented assignment, which has the form lhs op= rhs updates the variable lhs by applying a binary arithmetic operator op (one of +, -, *, /, //, %, &, |, ^, <<, >>) to the previous value of lhs and the value of rhs.

AssignStmt = Expression ('=' | '+=' | '-=' | '*=' | '/=' | '//=' | '%=' | '&=' | '|=' | '^=' | '<<=' | '>>=') Expression .

The left-hand side must be a simple target: a name, an index expression, or a dot expression.

x -= 1
x.filename += ".sky"
a[index()] *= 2

Any subexpressions in the target on the left-hand side are evaluated exactly once, before the evaluation of rhs. The first two assignments above are thus equivalent to:

x = x - 1
x.filename = x.filename + ".sky"

and the third assignment is similar in effect to the following two statements but does not declare a new temporary variable i:

i = index()
a[i] = a[i] * 2

Function definitions

A def statement creates a named function and assigns it to a variable.

DefStmt = 'def' identifier '(' [Parameters [',']] ')' ':' Suite .

Example:

def twice(x):
    return x * 2

str(twice)              # "<function f>"
twice(2)                # 4
twice("two")            # "twotwo"

The function's name is preceded by the def keyword and followed by the parameter list (which is enclosed in parentheses), a colon, and then an indented block of statements which form the body of the function.

The parameter list is a comma-separated list whose elements are of four kinds. First come zero or more required parameters, which are simple identifiers; all calls must provide an argument value for these parameters.

The required parameters are followed by zero or more optional parameters, of the form name=expression. The expression specifies the default value for the parameter for use in calls that do not provide an argument value for it.

The required parameters are optionally followed by a single parameter name preceded by a *. This is the called the varargs parameter, and it accumulates surplus positional arguments specified by a call.

Finally, there may be an optional parameter name preceded by **. This is called the keyword arguments parameter, and accumulates in a dictionary any surplus name=value arguments that do not match a prior parameter.

Here are some example parameter lists:

def f(): pass
def f(a, b, c): pass
def f(a, b, c=1): pass
def f(a, b, c=1, *args): pass
def f(a, b, c=1, *args, **kwargs): pass
def f(**kwargs): pass

Execution of a def statement creates a new function object. The function object contains: the syntax of the function body; the default value for each optional parameter; the value of each free variable referenced within the function body; and the global dictionary of the current module.

Implementation note: The Go implementation of the Skylark REPL requires the -nesteddef flag to enable support for nested def statements. The Java implementation does not permit a def expression to be nested within the body of another function.

Return statements

A return statement ends the execution of a function and returns a value to the caller of the function.

ReturnStmt = 'return' [Expression] .

A return statement may have zero, one, or more result expressions separated by commas. With no expressions, the function has the result None. With a single expression, the function's result is the value of that expression. With multiple expressions, the function's result is a tuple.

return                  # returns None
return 1                # returns 1
return 1, 2             # returns (1, 2)

Expression statements

An expression statement evaluates an expression and discards its result.

ExprStmt = Expression .

Any expression may be used as a statement, but an expression statement is most often used to call a function for its side effects.

list.append(1)

If statements

An if statement evaluates an expression (the condition), then, if the truth value of the condition is True, executes a list of statements.

IfStmt = 'if' Test ':' Suite {'elif' Test ':' Suite} ['else' ':' Suite] .

Example:

if score >= 100:
    print("You win!")
    return

An if statement may have an else block defining a second list of statements to be executed if the condition is false.

if score >= 100:
        print("You win!")
        return
else:
        print("Keep trying...")
        continue

It is common for the else block to contain another if statement. To avoid increasing the nesting depth unnecessarily, the else and following if may be combined as elif:

if x > 0:
        result = +1
elif x < 0:
        result = -1
else:
        result = 0

An if statement is permitted only within a function definition. An if statement at top level results in a static error.

For loops

A for loop evaluates its operand, which must be an iterable value. Then, for each element of the iterable's sequence, the loop assigns the successive element values to one or more variables and executes a list of statements, the loop body.

ForStmt = 'for' LoopVariables 'in' Expression ':' Suite .

Example:

for x in range(10):
   print(10)

The assignment of each value to the loop variables follows the same rules as an ordinary assignment. In this example, two-element lists are repeatedly assigned to the pair of variables (a, i):

for a, i in [["a", 1], ["b", 2], ["c", 3]]:
  print(a, i)                          # prints "a 1", "b 2", "c 3"

Because Skylark loops always iterate over a finite sequence, they are guaranteed to terminate, unlike loops in most languages which can execute an arbitrary and perhaps unbounded number of iterations.

Within the body of a for loop, break and continue statements may be used to stop the execution of the loop or advance to the next iteration.

In Skylark, a for loop is permitted only within a function definition. A for loop at top level results in a static error.

Break and Continue

The break and continue statements terminate the current iteration of a for loop. Whereas the continue statement resumes the loop at the next iteration, a break statement terminates the entire loop.

BreakStmt    = 'break' .
ContinueStmt = 'continue' .

Example:

for x in range(10):
    if x%2 == 1:
        continue        # skip odd numbers
    if x > 7:
        break           # stop at 8
    print(x)            # prints "0", "2", "4", "6"

Both statements affect only the innermost lexically enclosing loop. It is a static error to use a break or continue statement outside a loop.

Load statements

The load statement loads another Skylark module, extracts one or more values from it, and binds them to names in the current module.

Syntactically, a load statement looks like a function call load(...).

LoadStmt = 'load' '(' string {',' [identifier '='] string} [','] ')' .

A load statement requires at least two "arguments". The first must be a literal string; it identifies the module to load. Its interpretation is determined by the application into which the Skylark interpreter is embedded, and is not specified here.

During execution, the application determines what action to take for a load statement. A typical implementation locates and executes a Skylark file, populating a cache of files executed so far to avoid duplicate work, to obtain a module, which is a mapping from global names to values.

The remaining arguments are a mixture of literal strings, such as "x", or named literal strings, such as y="x".

The literal string ("x"), which must denote a valid identifier not starting with _, specifies the name to extract from the loaded module. In effect, names starting with _ are not exported. The name (y) specifies the local name; if no name is given, the local name matches the quoted name.

load("module.sky", "x", "y", "z")       # assigns x, y, and z
load("module.sky", "x", y2="y", "z")    # assigns x, y2, and z

A load statement within a function is a static error.

Module execution

Each Skylark file defines a module, which is a mapping from the names of global variables to their values. When a Skylark file is executed, whether directly by the application or indirectly through a load statement, a new Skylark thread is created, and this thread executes all the top-level statements in the file. Because if-statements and for-loops cannot appear outside of a function, control flows from top to bottom.

If execution reaches the end of the file, module initialization is successful. At that point, the value of each of the module's global variables is frozen, rendering subsequent mutation impossible. The module is then ready for use by another Skylark thread, such as one executing a load statement. Such threads may access values or call functions defined in the loaded module.

A Skylark thread may carry state on behalf of the application into which it is embedded, and application-defined functions may behave differently depending on this thread state. Because module initialization always occurs in a new thread, thread state is never carried from a higher-level module into a lower-level one. The initialization behavior of a module is thus independent of whichever module triggered its initialization.

If a Skylark thread encounters an error, execution stops and the error is reported to the application, along with a backtrace showing the stack of active function calls at the time of the error. If an error occurs during initialization of a Skylark module, any active load statements waiting for initialization of the module also fail.

Skylark provides no mechanism by which errors can be handled within the language.

Built-in constants and functions

The outermost block of the Skylark environment is known as the "predeclared" block. It defines a number of fundamental values and functions needed by all Skylark programs, such as None, True, False, and len, and possibly additional application-specific names.

These names are not reserved words so Skylark programs are free to redefine them in a smaller block such as a function body or even at the top level of a module. However, doing so may be confusing to the reader. Nonetheless, this rule permits names to be added to the predeclared block in later versions of the language (or application-specific dialect) without breaking existing programs.

None

None is the distinguished value of the type NoneType.

True and False

True and False are the two values of type bool.

any

any(x) returns True if any element of the iterable sequence x is true. If the iterable is empty, it returns False.

all

all(x) returns False if any element of the iterable sequence x is false. If the iterable is empty, it returns True.

bool

bool(x) interprets x as a Boolean value---True or False. With no argument, bool() returns False.

chr

chr(i) returns a string that encodes the single Unicode code point whose value is specified by the integer i. chr fails unless 0 ≤ i ≤ 0x10FFFF.

Example:

chr(65)                         # "A",
chr(1049)                       # "Й", CYRILLIC CAPITAL LETTER SHORT I
chr(0x1F63F)                    # "😿", CRYING CAT FACE

See also: ord.

Implementation note: chr is not provided by the Java implementation.

dict

dict creates a dictionary. It accepts up to one positional argument, which is interpreted as an iterable of two-element sequences (pairs), each specifying a key/value pair in the resulting dictionary.

dict also accepts any number of keyword arguments, each of which specifies a key/value pair in the resulting dictionary; each keyword is treated as a string.

dict()                          # {}, empty dictionary
dict([(1, 2), (3, 4)])          # {1: 2, 3: 4}
dict([(1, 2), ["a", "b"]])      # {1: 2, "a": "b"}
dict(one=1, two=2)              # {"one": 1, "two", 1}
dict([(1, 2)], x=3)             # {1: 2, "x": 3}

With no arguments, dict() returns a new empty dictionary.

dict(x) where x is a dictionary returns a new copy of x.

dir

dir(x) returns a list of the names of the attributes (fields and methods) of its operand. The attributes of a value x are the names f such that x.f is a valid expression.

For example,

dir("hello")                    # ['capitalize', 'count', ...], the methods of a string

Several types known to the interpreter, such as list, string, and dict, have methods, but none have fields. However, an application may define types with fields that may be read or set by statements such as these:

y = x.f
x.f = y

enumerate

enumerate(x) returns a list of (index, value) pairs, each containing successive values of the iterable sequence xand the index of the value within the sequence.

The optional second parameter, start, specifies an integer value to add to each index.

enumerate(["zero", "one", "two"])               # [(0, "zero"), (1, "one"), (2, "two")]
enumerate(["one", "two"], 1)                    # [(1, "one"), (2, "two")]

float

float(x) interprets its argument as a floating-point number.

If x is a float, the result is x. if x is an int, the result is the nearest floating point value to x. If x is a string, the string is interpreted as a floating-point literal. With no arguments, float() returns 0.0.

Implementation note: Floating-point numbers are an optional feature. The Go implementation of the Skylark REPL requires the -fp flag to enable support for floating-point literals, the float built-in function, and the real division operator /. The Java implementation does not yet support floating-point numbers.

getattr

getattr(x, name) returns the value of the attribute (field or method) of x named name. It is a dynamic error if x has no such attribute.

getattr(x, "f") is equivalent to x.f.

getattr("banana", "split")("a")	       # ["b", "n", "n", ""], equivalent to "banana".split("a")

hasattr

hasattr(x, name) reports whether x has an attribute (field or method) named name.

hash

hash(x) returns an integer hash value for x such that x == y implies hash(x) == hash(y).

hash fails if x, or any value upon which its hash depends, is unhashable.

Implementation note: the Java implementation of the hash function accepts only strings.

int

int(x[, base]) interprets its argument as an integer.

If x is an int, the result is x. If x is a float, the result is the integer value nearest to x, truncating towards zero; it is an error if x is not finite (NaN, +Inf, -Inf). If x is a bool, the result is 0 for False or 1 for True.

If x is a string, it is interpreted as a sequence of digits in the specified base, decimal by default. If base is zero, x is interpreted like an integer literal, the base being inferred from an optional base marker such as 0b, 0o, or 0x preceding the first digit. Irrespective of base, the string may start with an optional + or - sign indicating the sign of the result.

len

len(x) returns the number of elements in its argument.

It is a dynamic error if its argument is not a sequence.

list

list constructs a list.

list(x) returns a new list containing the elements of the iterable sequence x.

With no argument, list() returns a new empty list.

max

max(x) returns the greatest element in the iterable sequence x.

It is an error if any element does not support ordered comparison, or if the sequence is empty.

The optional named parameter key specifies a function to be applied to each element prior to comparison.

max([3, 1, 4, 1, 5, 9])                         # 9
max("two", "three", "four")                     # "two", the lexicographically greatest
max("two", "three", "four", key=len)            # "three", the longest

min

min(x) returns the least element in the iterable sequence x.

It is an error if any element does not support ordered comparison, or if the sequence is empty.

min([3, 1, 4, 1, 5, 9])                         # 1
min("two", "three", "four")                     # "four", the lexicographically least
min("two", "three", "four", key=len)            # "two", the shortest

ord

ord(s) returns the integer value of the sole Unicode code point encoded by the string s.

If s does not encode exactly one Unicode code point, ord fails. Each invalid code within the string is treated as if it encodes the Unicode replacement character, U+FFFD.

Example:

ord("A")				# 65
ord("Й")				# 1049
ord("😿")					# 0x1F63F
ord("Й"[1:])				# 0xFFFD (Unicode replacement character)

See also: chr.

Implementation note: ord is not provided by the Java implementation.

print

print(*args, **kwargs) prints its arguments, followed by a newline. Arguments are formatted as if by str(x) and separated with a space. Keyword arguments are preceded by their name.

Example:

print(1, "hi", x=3)	# "1 hi x=3\n"

Typically the formatted string is printed to the standard error file, but the exact behavior is a property of the Skylark thread and is determined by the host application.

range

range returns an immutable sequence of integers defined by the specified interval and stride.

range(stop)                             # equivalent to range(0, stop)
range(start, stop)                      # equivalent to range(start, stop, 1)
range(start, stop, step)

range requires between one and three integer arguments. With one argument, range(stop) returns the ascending sequence of non-negative integers less than stop. With two arguments, range(start, stop) returns only integers not less than start.

With three arguments, range(start, stop, step) returns integers formed by successively adding step to start until the value meets or passes stop. A call to range fails if the value of step is zero.

A call to range does not materialize the entire sequence, but returns a fixed-size value of type "range" that represents the parameters that define the sequence. The range value is iterable and may be indexed efficiently.

list(range(10))                         # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
list(range(3, 10))                      # [3, 4, 5, 6, 7, 8, 9]
list(range(3, 10, 2))                   # [3, 5, 7, 9]
list(range(10, 3, -2))                  # [10, 8, 6, 4]

The len function applied to a range value returns its length. The truth value of a range value is True if its length is non-zero.

Range values are comparable: two range values compare equal if they denote the same sequence of integers, even if they were created using different parameters.

Range values are not hashable.

The str function applied to a range value yields a string of the form range(10), range(1, 10), or range(1, 10, 2).

The x in y operator, where y is a range, reports whether x is equal to some member of the sequence y; the operation fails unless x is a number.

repr

repr(x) formats its argument as a string.

All strings in the result are double-quoted.

repr(1)                 # '1'
repr("x")               # '"x"'
repr([1, "x"])          # '[1, "x"]'

reversed

reversed(x) returns a new list containing the elements of the iterable sequence x in reverse order.

reversed(range(5))                              # [4, 3, 2, 1, 0]
reversed("stressed".codepoints())               # ["d", "e", "s", "s", "e", "r", "t", "s"]
reversed({"one": 1, "two": 2}.keys())           # ["two", "one"]

set

set(x) returns a new set containing the elements of the iterable x. With no argument, set() returns a new empty set.

set([3, 1, 4, 1, 5, 9])         # set([3, 1, 4, 5, 9])

Implementation note: Sets are an optional feature of the Go implementation of Skylark.

sorted

sorted(x) returns a new list containing the elements of the iterable sequence x, in sorted order. The sort algorithm is stable.

The optional named parameter reverse, if true, causes sorted to return results in reverse sorted order.

The optional named parameter key specifies a function of one argument to apply to obtain the value's sort key. The default behavior is the identity function.

sorted(set("harbors".codepoints()))                             # ['a', 'b', 'h', 'o', 'r', 's']
sorted([3, 1, 4, 1, 5, 9])                                      # [1, 1, 3, 4, 5, 9]
sorted([3, 1, 4, 1, 5, 9], reverse=True)                        # [9, 5, 4, 3, 1, 1]

sorted(["two", "three", "four"], key=len)                       # ["two", "four", "three"], shortest to longest
sorted(["two", "three", "four"], key=len, reverse=True)         # ["three", "four", "two"], longest to shortest

Implementation note: The Java implementation does not support the key, and reverse parameters.

str

str(x) formats its argument as a string.

If x is a string, the result is x (without quotation). All other strings, such as elements of a list of strings, are double-quoted.

str(1)                          # '1'
str("x")                        # 'x'
str([1, "x"])                   # '[1, "x"]'

tuple

tuple(x) returns a tuple containing the elements of the iterable x.

With no arguments, tuple() returns the empty tuple.

type

type(x) returns a string describing the type of its operand.

type(None)              # "NoneType"
type(0)                 # "int"
type(0.0)               # "float"

zip

zip() returns a new list of n-tuples formed from corresponding elements of each of the n iterable sequences provided as arguments to zip. That is, the first tuple contains the first element of each of the sequences, the second element contains the second element of each of the sequences, and so on. The result list is only as long as the shortest of the input sequences.

zip()                                   # []
zip(range(5))                           # [(0,), (1,), (2,), (3,), (4,)]
zip(range(5), "abc")                    # [(0, "a"), (1, "b"), (2, "c")]

Built-in methods

This section lists the methods of built-in types. Methods are selected using dot expressions. For example, strings have a count method that counts occurrences of a substring; "banana".count("a") yields 3.

As with built-in functions, built-in methods accept only positional arguments except where noted. The parameter names serve merely as documentation.

dict·clear

D.clear() removes all the entries of dictionary D and returns None. It fails if the dictionary is frozen or if there are active iterators.

x = {"one": 1, "two": 2}
x.clear()                               # None
print(x)                                # {}

Implementation note: dict·clear is not provided by the Java implementation.

dict·get

D.get(key[, default]) returns the dictionary value corresponding to the given key. If the dictionary contains no such value, get returns None, or the value of the optional default parameter if present.

get fails if key is unhashable, or the dictionary is frozen or has active iterators.

x = {"one": 1, "two": 2}
x.get("one")                            # 1
x.get("three")                          # None
x.get("three", 0)                       # 0

dict·items

D.items() returns a new list of key/value pairs, one per element in dictionary D, in the same order as they would be returned by a for loop.

x = {"one": 1, "two": 2}
x.items()                               # [("one", 1), ("two", 2)]

dict·keys

D.keys() returns a new list containing the keys of dictionary D, in the same order as they would be returned by a for loop.

x = {"one": 1, "two": 2}
x.keys()                               # ["one", "two"]

dict·pop

D.pop(key[, default]) returns the value corresponding to the specified key, and removes it from the dictionary. If the dictionary contains no such value, and the optional default parameter is present, pop returns that value; otherwise, it fails.

pop fails if key is unhashable, or the dictionary is frozen or has active iterators.

x = {"one": 1, "two": 2}
x.pop("one")                            # 1
x                                       # {"two": 2}
x.pop("three", 0)                       # 0
x.pop("four")                           # error: missing key

dict·popitem

D.popitem() returns the first key/value pair, removing it from the dictionary.

popitem fails if the dictionary is empty, frozen, or has active iterators.

x = {"one": 1, "two": 2}
x.popitem()                             # ("one", 1)
x.popitem()                             # ("two", 2)
x.popitem()                             # error: empty dict

dict·setdefault

D.setdefault(key[, default]) returns the dictionary value corresponding to the given key. If the dictionary contains no such value, setdefault, like get, returns None or the value of the optional default parameter if present; setdefault additionally inserts the new key/value entry into the dictionary.

setdefault fails if the key is unhashable, or if the dictionary is frozen or has active iterators.

x = {"one": 1, "two": 2}
x.setdefault("one")                     # 1
x.setdefault("three", 0)                # 0
x                                       # {"one": 1, "two": 2, "three": 0}
x.setdefault("four")                    # None
x                                       # {"one": 1, "two": 2, "three": None}

dict·update

D.update([pairs][, name=value[, ...]) makes a sequence of key/value insertions into dictionary D, then returns None.

If the positional argument pairs is present, it must be None, another dict, or some other iterable. If it is another dict, then its key/value pairs are inserted into D. If it is an iterable, it must provide a sequence of pairs (or other iterables of length 2), each of which is treated as a key/value pair to be inserted into D.

For each name=value argument present, the name is converted to a string and used as the key for an insertion into D, with its corresponding value being value.

update fails if the dictionary is frozen or has active iterators.

x = {}
x.update([("a", 1), ("b", 2)], c=3)
x.update({"d": 4})
x.update(e=5)
x                                       # {"a": 1, "b": "2", "c": 3, "d": 4, "e": 5}

dict·values

D.values() returns a new list containing the dictionary's values, in the same order as they would be returned by a for loop over the dictionary.

x = {"one": 1, "two": 2}
x.values()                              # [1, 2]

list·append

L.append(x) appends x to the list L, and returns None.

append fails if the list is frozen or has active iterators.

x = []
x.append(1)                             # None
x.append(2)                             # None
x.append(3)                             # None
x                                       # [1, 2, 3]

list·clear

L.clear() removes all the elements of the list L and returns None. It fails if the list is frozen or if there are active iterators.

x = [1, 2, 3]
x.clear()                               # None
x                                       # []

list·extend

L.extend(x) appends the elements of x, which must be iterable, to the list L, and returns None.

extend fails if x is not iterable, or if the list L is frozen or has active iterators.

x = []
x.extend([1, 2, 3])                     # None
x.extend(["foo"])                       # None
x                                       # [1, 2, 3, "foo"]

list·index

L.insert(x[, start[, end]]) finds x within the list L and returns its index.

The optional start and end parameters restrict the portion of list L that is inspected. If provided and not None, they must be list indices of type int. If an index is negative, len(L) is effectively added to it, then if the index is outside the range [0:len(L)], the nearest value within that range is used; see Indexing.

insert fails if x is not found in L, or if start or end is not a valid index (int or None).

x = list("banana".codepoints())
x.index("a")                            # 1 (bAnana)
x.index("a", 2)                         # 3 (banAna)
x.index("a", -2)                        # 5 (bananA)

list·insert

L.insert(i, x) inserts the value x in the list L at index i, moving higher-numbered elements along by one. It returns None.

As usual, the index i must be an int. If its value is negative, the length of the list is added, then its value is clamped to the nearest value in the range [0:len(L)] to yield the effective index.

insert fails if the list is frozen or has active iterators.

x = ["b", "c", "e"]
x.insert(0, "a")                        # None
x.insert(-1, "d")                       # None
x                                       # ["a", "b", "c", "d", "e"]

list·pop

L.pop([index]) removes and returns the last element of the list L, or, if the optional index is provided, at that index.

insert fails if the index is negative or not less than the length of the list, of if the list is frozen or has active iterators.

x = [1, 2, 3]
x.pop()                                 # 3
x.pop()                                 # 2
x                                       # [1]

list·remove

L.remove(x) removes the first occurrence of the value x from the list L, and returns None.

remove fails if the list does not contain x, is frozen, or has active iterators.

x = [1, 2, 3, 2]
x.remove(2)                             # None (x == [1, 3, 2])
x.remove(2)                             # None (x == [1, 3])
x.remove(2)                             # error: element not found

set·union

S.union(iterable) returns a new set into which have been inserted all the elements of set S and all the elements of the argument, which must be iterable.

union fails if any element of the iterable is not hashable.

x = set([1, 2])
y = set([2, 3])
x.union(y)                              # set([1, 2, 3])

string·elem_ords

S.elem_ords() returns an iterable value containing the sequence of numeric bytes values in the string S.

To materialize the entire sequence of bytes, apply list(...) to the result.

Example:

list("Hello, 世界".elem_ords())        # [72, 101, 108, 108, 111, 44, 32, 228, 184, 150, 231, 149, 140]

See also: string·elems.

Implementation note: elem_ords is not provided by the Java implementation.

string·capitalize

S.capitalize() returns a copy of string S with all Unicode letters that begin words changed to their title case.

"hello, world!".capitalize()		# "Hello, World!"

string·codepoint_ords

S.codepoint_ords() returns an iterable value containing the sequence of integer Unicode code points encoded by the string S. Each invalid code within the string is treated as if it encodes the Unicode replacement character, U+FFFD.

By returning an iterable, not a list, the cost of decoding the string is deferred until actually needed; apply list(...) to the result to materialize the entire sequence.

Example:

list("Hello, 世界".codepoint_ords())        # [72, 101, 108, 108, 111, 44, 32, 19990, 30028]

for cp in "Hello, 世界".codepoint_ords():
   print(chr(cp))  # prints 'H', 'e', 'l', 'l', 'o', ',', ' ', '世', '界'

See also: string·codepoints.

Implementation note: codepoint_ords is not provided by the Java implementation.

string·count

S.count(sub[, start[, end]]) returns the number of occcurences of sub within the string S, or, if the optional substring indices start and end are provided, within the designated substring of S. They are interpreted according to Skylark's indexing conventions.

"hello, world!".count("o")              # 2
"hello, world!".count("o", 7, 12)       # 1  (in "world")

string·endswith

S.endswith(suffix[, start[, end]]) reports whether the string S[start:end] has the specified suffix.

"filename.sky".endswith(".sky")         # True

The suffix argument may be a tuple of strings, in which case the function reports whether any one of them is a suffix.

'foo.cc'.endswith(('.cc', '.h'))         # True

string·find

S.find(sub[, start[, end]]) returns the index of the first occurrence of the substring sub within S.

If either or both of start or end are specified, they specify a subrange of S to which the search should be restricted. They are interpreted according to Skylark's indexing conventions.

If no occurrence is found, found returns -1.

"bonbon".find("on")             # 1
"bonbon".find("on", 2)          # 4
"bonbon".find("on", 2, 5)       # -1

string·format

S.format(*args, **kwargs) returns a version of the format string S in which bracketed portions {...} are replaced by arguments from args and kwargs.

Within the format string, a pair of braces {{ or }} is treated as a literal open or close brace. Each unpaired open brace must be matched by a close brace }. The optional text between corresponding open and close braces specifies which argument to use and how to format it, and consists of three components, all optional: a field name, a conversion preceded by '!', and a format specifier preceded by ':'.

{field}
{field:spec}
{field!conv}
{field!conv:spec}

The field name may be either a decimal number or a keyword. A number is interpreted as the index of a positional argument; a keyword specifies the value of a keyword argument. If all the numeric field names form the sequence 0, 1, 2, and so on, they may be omitted and those values will be implied; however, the explicit and implicit forms may not be mixed.

The conversion specifies how to convert an argument value x to a string. It may be either !r, which converts the value using repr(x), or !s, which converts the value using str(x) and is the default.

The format specifier, after a colon, specifies field width, alignment, padding, and numeric precision. Currently it must be empty, but it is reserved for future use.

"a{x}b{y}c{}".format(1, x=2, y=3)               # "a2b3c1"
"a{}b{}c".format(1, 2)                          # "a1b2c"
"({1}, {0})".format("zero", "one")              # "(one, zero)"
"Is {0!r} {0!s}?".format('heterological')       # 'is "heterological" heterological?'

string·index

S.index(sub[, start[, end]]) returns the index of the first occurrence of the substring sub within S, like S.find, except that if the substring is not found, the operation fails.

"bonbon".index("on")             # 1
"bonbon".index("on", 2)          # 4
"bonbon".index("on", 2, 5)       # error: substring not found  (in "nbo")

string·isalnum

S.isalnum() reports whether the string S is non-empty and consists only Unicode letters and digits.

"base64".isalnum()              # True
"Catch-22".isalnum()            # False

string·isalpha

S.isalpha() reports whether the string S is non-empty and consists only of Unicode letters.

"ABC".isalpha()                 # True
"Catch-22".isalpha()            # False
"".isalpha()                    # False

string·isdigit

S.isdigit() reports whether the string S is non-empty and consists only of Unicode digits.

"123".isdigit()                 # True
"Catch-22".isdigit()            # False
"".isdigit()                    # False

string·islower

S.islower() reports whether the string S contains at least one cased Unicode letter, and all such letters are lowercase.

"hello, world".islower()        # True
"Catch-22".islower()            # False
"123".islower()                 # False

string·isspace

S.isspace() reports whether the string S is non-empty and consists only of Unicode spaces.

"    ".isspace()                # True
"\r\t\n".isspace()              # True
"".isspace()                    # False

string·istitle

S.istitle() reports whether the string S contains at least one cased Unicode letter, and all such letters that begin a word are in title case.

"Hello, World!".istitle()       # True
"Catch-22".istitle()            # True
"HAL-9000".istitle()            # False
"123".istitle()                 # False

string·isupper

S.isupper() reports whether the string S contains at least one cased Unicode letter, and all such letters are uppercase.

"HAL-9000".isupper()            # True
"Catch-22".isupper()            # False
"123".isupper()                 # False

string·join

S.join(iterable) returns the string formed by concatenating each element of its argument, with a copy of the string S between successive elements. The argument must be an iterable whose elements are strings.

", ".join(["one", "two", "three"])      # "one, two, three"
"a".join("ctmrn".codepoints())          # "catamaran"

string·lower

S.lower() returns a copy of the string S with letters converted to lowercase.

"Hello, World!".lower()                 # "hello, world!"

string·lstrip

S.lstrip() returns a copy of the string S with leading whitespace removed.

"  hello  ".lstrip()                    # "  hello"

string·partition

S.partition(x) splits string S into three parts and returns them as a tuple: the portion before the first occurrence of string x, x itself, and the portion following it. If S does not contain x, partition returns (S, "", "").

partition fails if x is not a string, or is the empty string.

"one/two/three".partition("/")		# ("one", "/", "two/three")

string·replace

S.replace(old, new[, count]) returns a copy of string S with all occurrences of substring old replaced by new. If the optional argument count, which must be an int, is non-negative, it specifies a maximum number of occurrences to replace.

"banana".replace("a", "o")		# "bonono"
"banana".replace("a", "o", 2)		# "bonona"

string·rfind

S.rfind(sub[, start[, end]]) returns the index of the substring sub within S, like S.find, except that rfind returns the index of the substring's last occurrence.

"bonbon".rfind("on")             # 4
"bonbon".rfind("on", None, 5)    # 1
"bonbon".rfind("on", 2, 5)       # -1

string·rindex

S.rindex(sub[, start[, end]]) returns the index of the substring sub within S, like S.index, except that rindex returns the index of the substring's last occurrence.

"bonbon".rindex("on")             # 4
"bonbon".rindex("on", None, 5)    # 1                           (in "bonbo")
"bonbon".rindex("on", 2, 5)       # error: substring not found  (in "nbo")

string·rpartition

S.rpartition(x) is like partition, but splits S at the last occurrence of x.

"one/two/three".partition("/")		# ("one/two", "/", "three")

string·rsplit

S.rsplit([sep[, maxsplit]]) splits a string into substrings like S.split, except that when a maximum number of splits is specified, rsplit chooses the rightmost splits.

"banana".rsplit("n")                         # ["ba", "a", "a"]
"banana".rsplit("n", 1)                      # ["bana", "a"]
"one two  three".rsplit(None, 1)             # ["one two", "three"]

string·rstrip

S.rstrip() returns a copy of the string S with trailing whitespace removed.

"  hello  ".rstrip()                    # "hello  "

string·split

S.split([sep [, maxsplit]]) returns the list of substrings of S, splitting at occurrences of the delimiter string sep.

Consecutive occurrences of sep are considered to delimit empty strings, so 'food'.split('o') returns ['f', '', 'd']. Splitting an empty string with a specified separator returns ['']. If sep is the empty string, split fails.

If sep is not specified or is None, split uses a different algorithm: it removes all leading spaces from S (or trailing spaces in the case of rsplit), then splits the string around each consecutive non-empty sequence of Unicode white space characters.

If S consists only of white space, split returns the empty list.

If maxsplit is given and non-negative, it specifies a maximum number of splits.

"one two  three".split()                    # ["one", "two", "three"]
"one two  three".split(" ")                 # ["one", "two", "", "three"]
"one two  three".split(None, 1)             # ["one", "two  three"]
"banana".split("n")                         # ["ba", "a", "a"]
"banana".split("n", 1)                      # ["ba", "ana"]

string·elems

S.elems() returns an iterable value containing successive 1-byte substrings of S. To materialize the entire sequence, apply list(...) to the result.

Example:

list('Hello, 世界'.elems())  # ["H", "e", "l", "l", "o", ",", " ", "\xe4", "\xb8", "\x96", "\xe7", "\x95", "\x8c"]

See also: string·elem_ords.

string·codepoints

S.codepoints() returns an iterable value containing the sequence of substrings of S that each encode a single Unicode code point. Each invalid code within the string is treated as if it encodes the Unicode replacement character, U+FFFD.

By returning an iterable, not a list, the cost of decoding the string is deferred until actually needed; apply list(...) to the result to materialize the entire sequence.

Example:

list('Hello, 世界'.codepoints())  # ['H', 'e', 'l', 'l', 'o', ',', ' ', '世', '界']

for cp in 'Hello, 世界'.codepoints():
   print(cp)  # prints 'H', 'e', 'l', 'l', 'o', ',', ' ', '世', '界'

See also: string·codepoint_ords.

Implementation note: codepoints is not provided by the Java implementation.

string·splitlines

S.splitlines([keepends]) returns a list whose elements are the successive lines of S, that is, the strings formed by splitting S at line terminators (currently assumed to be a single newline, \n, regardless of platform).

The optional argument, keepends, is interpreted as a Boolean. If true, line terminators are preserved in the result, though the final element does not necessarily end with a line terminator.

"one\n\ntwo".splitlines()       # ["one", "", "two"]
"one\n\ntwo".splitlines(True)   # ["one\n", "\n", "two"]

string·startswith

S.startswith(prefix[, start[, end]]) reports whether the string S[start:end] has the specified prefix.

"filename.sky".startswith("filename")         # True

The prefix argument may be a tuple of strings, in which case the function reports whether any one of them is a prefix.

'abc'.startswith(('a', 'A'))                  # True
'ABC'.startswith(('a', 'A'))                  # True
'def'.startswith(('a', 'A'))                  # False

string·strip

S.strip() returns a copy of the string S with leading and trailing whitespace removed.

"  hello  ".strip()                     # "hello"

string·title

S.title() returns a copy of the string S with letters converted to titlecase.

Letters are converted to uppercase at the start of words, lowercase elsewhere.

"hElLo, WoRlD!".title()                 # "Hello, World!"

string·upper

S.upper() returns a copy of the string S with letters converted to uppercase.

"Hello, World!".upper()                 # "HELLO, WORLD!"

Dialect differences

The list below summarizes features of the Go implementation that are known to differ from the Java implementation of Skylark used by Bazel. Some of these features may be controlled by global options to allow applications to mimic the Bazel dialect more closely. Our goal is eventually to eliminate all such differences on a case-by-case basis.

  • Integers are represented with infinite precision.
  • Integer arithmetic is exact.
  • Integers support bitwise operators &, |, <<, >>, ^, ~, and their assignment forms.
  • Floating-point literals are supported (option: -float).
  • The float built-in function is provided (option: -float).
  • Real division using float / float is supported (option: -float).
  • def statements may be nested (option: -nesteddef).
  • lambda expressions are supported (option: -lambda).
  • String elements are bytes.
  • Non-ASCII strings are encoded using UTF-8.
  • Strings have the additional methods elem_ords, codepoint_ords, and codepoints.
  • The chr and ord built-in functions are supported.
  • The set built-in function is provided (option: -set).
  • set & set and set | set compute set intersection and union, respectively.
  • x += y rebindings are permitted at top level.
  • assert is a valid identifier.
  • The parser accepts unary + expressions.
  • A method call x.f() may be separated into two steps: y = x.f; y().
  • Dot expressions may appear on the left side of an assignment: x.f = 1.
  • hash accepts operands besides strings.
  • sorted accepts the additional parameters key and reverse.
  • The dict type has a clear method.
  • type(x) returns "builtin_function_or_method" for built-in functions.