ast
Martin v. Löwis <martin@v.loewis.de>
Georg Brandl <georg@python.org>
import ast
Source code: Lib/ast.py
The ast
module helps Python applications to process trees of the Python abstract syntax grammar. The abstract syntax itself might change with each Python release; this module helps to find out programmatically what the current grammar looks like.
An abstract syntax tree can be generated by passing ast.PyCF_ONLY_AST
as a flag to the compile
built-in function, or using the parse
helper provided in this module. The result will be a tree of objects whose classes all inherit from ast.AST
. An abstract syntax tree can be compiled into a Python code object using the built-in compile
function.
The abstract grammar is currently defined as follows:
../../Parser/Python.asdl
This is the base of all AST node classes. The actual node classes are derived from the Parser/Python.asdl
file, which is reproduced below <abstract-grammar>
. They are defined in the _ast
C module and re-exported in ast
.
There is one class defined for each left-hand side symbol in the abstract grammar (for example, ast.stmt
or ast.expr
). In addition, there is one class defined for each constructor on the right-hand side; these classes inherit from the classes for the left-hand side trees. For example, ast.BinOp
inherits from ast.expr
. For production rules with alternatives (aka "sums"), the left-hand side class is abstract: only instances of specific constructor nodes are ever created.
single: ? (question mark); in AST grammar
single: * (asterisk); in AST grammar
_fields
Each concrete class has an attribute _fields
which gives the names of all child nodes.
Each instance of a concrete class has one attribute for each child node, of the type as defined in the grammar. For example, ast.BinOp
instances have an attribute left
of type ast.expr
.
If these attributes are marked as optional in the grammar (using a question mark), the value might be None
. If the attributes can have zero-or-more values (marked with an asterisk), the values are represented as Python lists. All possible attributes must be present and have valid values when compiling an AST with compile
.
lineno col_offset end_lineno end_col_offset
Instances of ast.expr
and ast.stmt
subclasses have lineno
, col_offset
, end_lineno
, and end_col_offset
attributes. The lineno
and end_lineno
are the first and last line numbers of source text span (1-indexed so the first line is line 1) and the col_offset
and end_col_offset
are the corresponding UTF-8 byte offsets of the first and last tokens that generated the node. The UTF-8 offset is recorded because the parser uses UTF-8 internally.
Note that the end positions are not required by the compiler and are therefore optional. The end offset is after the last symbol, for example one can get the source segment of a one-line expression node using source_line[node.col_offset : node.end_col_offset]
.
The constructor of a class ast.T
parses its arguments as follows:
- If there are positional arguments, there must be as many as there are items in
T._fields
; they will be assigned as attributes of these names. - If there are keyword arguments, they will set the attributes of the same names to the given values.
For example, to create and populate an ast.UnaryOp
node, you could use :
node = ast.UnaryOp()
node.op = ast.USub()
node.operand = ast.Constant()
node.operand.value = 5
node.operand.lineno = 0
node.operand.col_offset = 0
node.lineno = 0
node.col_offset = 0
or the more compact :
node = ast.UnaryOp(ast.USub(), ast.Constant(5, lineno=0, col_offset=0),
lineno=0, col_offset=0)
3.8
Class ast.Constant
is now used for all constants.
3.9
Simple indices are represented by their value, extended slices are represented as tuples.
3.8
Old classes ast.Num
, ast.Str
, ast.Bytes
, ast.NameConstant
and ast.Ellipsis
are still available, but they will be removed in future Python releases. In the meantime, instantiating them will return an instance of a different class.
3.9
Old classes ast.Index
and ast.ExtSlice
are still available, but they will be removed in future Python releases. In the meantime, instantiating them will return an instance of a different class.
Note
The descriptions of the specific node classes displayed here were initially adapted from the fantastic Green Tree Snakes project and all its contributors.
A constant value. The value
attribute of the Constant
literal contains the Python object it represents. The values represented can be simple types such as a number, string or None
, but also immutable container types (tuples and frozensets) if all of their elements are constant.
>>> print(ast.dump(ast.parse('123', mode='eval'), indent=4)) Expression( body=Constant(value=123))
Node representing a single formatting field in an f-string. If the string contains a single formatting field and nothing else the node can be isolated otherwise it appears in JoinedStr
.
value
is any expression node (such as a literal, a variable, or a function call).conversion
is an integer:- -1: no formatting
- 115:
!s
string formatting - 114:
!r
repr formatting - 97:
!a
ascii formatting
format_spec
is aJoinedStr
node representing the formatting of the value, orNone
if no format was specified. Bothconversion
andformat_spec
can be set at the same time.
An f-string, comprising a series of FormattedValue
and Constant
nodes.
>>> print(ast.dump(ast.parse('f"sin({a}) is {sin(a):.3}"', mode='eval'), indent=4)) Expression( body=JoinedStr( values=[ Constant(value='sin('), FormattedValue( value=Name(id='a', ctx=Load()), conversion=-1), Constant(value=') is '), FormattedValue( value=Call( func=Name(id='sin', ctx=Load()), args=[ Name(id='a', ctx=Load())], keywords=[]), conversion=-1, format_spec=JoinedStr( values=[ Constant(value='.3')]))]))
A list or tuple. elts
holds a list of nodes representing the elements. ctx
is Store
if the container is an assignment target (i.e. (x,y)=something
), and Load
otherwise.
>>> print(ast.dump(ast.parse('[1, 2, 3]', mode='eval'), indent=4)) Expression( body=List( elts=[ Constant(value=1), Constant(value=2), Constant(value=3)], ctx=Load())) >>> print(ast.dump(ast.parse('(1, 2, 3)', mode='eval'), indent=4)) Expression( body=Tuple( elts=[ Constant(value=1), Constant(value=2), Constant(value=3)], ctx=Load()))
A set. elts
holds a list of nodes representing the set's elements.
>>> print(ast.dump(ast.parse('{1, 2, 3}', mode='eval'), indent=4)) Expression( body=Set( elts=[ Constant(value=1), Constant(value=2), Constant(value=3)]))
A dictionary. keys
and values
hold lists of nodes representing the keys and the values respectively, in matching order (what would be returned when calling dictionary.keys()
and dictionary.values()
).
When doing dictionary unpacking using dictionary literals the expression to be expanded goes in the values
list, with a None
at the corresponding position in keys
.
>>> print(ast.dump(ast.parse('{"a":1, **d}', mode='eval'), indent=4)) Expression( body=Dict( keys=[ Constant(value='a'), None], values=[ Constant(value=1), Name(id='d', ctx=Load())]))
A variable name. id
holds the name as a string, and ctx
is one of the following types.
Variable references can be used to load the value of a variable, to assign a new value to it, or to delete it. Variable references are given a context to distinguish these cases.
>>> print(ast.dump(ast.parse('a'), indent=4)) Module( body=[ Expr( value=Name(id='a', ctx=Load()))], type_ignores=[])
>>> print(ast.dump(ast.parse('a = 1'), indent=4)) Module( body=[ Assign( targets=[ Name(id='a', ctx=Store())], value=Constant(value=1))], type_ignores=[])
>>> print(ast.dump(ast.parse('del a'), indent=4)) Module( body=[ Delete( targets=[ Name(id='a', ctx=Del())])], type_ignores=[])
A *var
variable reference. value
holds the variable, typically a Name
node. This type must be used when building a Call
node with *args
.
>>> print(ast.dump(ast.parse('a, *b = it'), indent=4)) Module( body=[ Assign( targets=[ Tuple( elts=[ Name(id='a', ctx=Store()), Starred( value=Name(id='b', ctx=Store()), ctx=Store())], ctx=Store())], value=Name(id='it', ctx=Load()))], type_ignores=[])
When an expression, such as a function call, appears as a statement by itself with its return value not used or stored, it is wrapped in this container. value
holds one of the other nodes in this section, a Constant
, a Name
, a Lambda
, a Yield
or YieldFrom
node.
>>> print(ast.dump(ast.parse('-a'), indent=4)) Module( body=[ Expr( value=UnaryOp( op=USub(), operand=Name(id='a', ctx=Load())))], type_ignores=[])
A unary operation. op
is the operator, and operand
any expression node.
Unary operator tokens. Not
is the not
keyword, Invert
is the ~
operator.
>>> print(ast.dump(ast.parse('not x', mode='eval'), indent=4)) Expression( body=UnaryOp( op=Not(), operand=Name(id='x', ctx=Load())))
A binary operation (like addition or division). op
is the operator, and left
and right
are any expression nodes.
>>> print(ast.dump(ast.parse('x + y', mode='eval'), indent=4)) Expression( body=BinOp( left=Name(id='x', ctx=Load()), op=Add(), right=Name(id='y', ctx=Load())))
Binary operator tokens.
A boolean operation, 'or' or 'and'. op
is Or
or And
. values
are the values involved. Consecutive operations with the same operator, such as a or b or c
, are collapsed into one node with several values.
This doesn't include not
, which is a UnaryOp
.
>>> print(ast.dump(ast.parse('x or y', mode='eval'), indent=4)) Expression( body=BoolOp( op=Or(), values=[ Name(id='x', ctx=Load()), Name(id='y', ctx=Load())]))
Boolean operator tokens.
A comparison of two or more values. left
is the first value in the comparison, ops
the list of operators, and comparators
the list of values after the first element in the comparison.
>>> print(ast.dump(ast.parse('1 <= a < 10', mode='eval'), indent=4)) Expression( body=Compare( left=Constant(value=1), ops=[ LtE(), Lt()], comparators=[ Name(id='a', ctx=Load()), Constant(value=10)]))
Comparison operator tokens.
A function call. func
is the function, which will often be a Name
or Attribute
object. Of the arguments:
args
holds a list of the arguments passed by position.keywords
holds a list ofkeyword
objects representing arguments passed by keyword.
When creating a Call
node, args
and keywords
are required, but they can be empty lists. starargs
and kwargs
are optional.
>>> print(ast.dump(ast.parse('func(a, b=c, d,*e)', mode='eval'), indent=4)) Expression( body=Call( func=Name(id='func', ctx=Load()), args=[ Name(id='a', ctx=Load()), Starred( value=Name(id='d', ctx=Load()), ctx=Load())], keywords=[ keyword( arg='b', value=Name(id='c', ctx=Load())), keyword( value=Name(id='e', ctx=Load()))]))
A keyword argument to a function call or class definition. arg
is a raw string of the parameter name, value
is a node to pass in.
An expression such as a if b else c
. Each field holds a single node, so in the following example, all three are Name
nodes.
>>> print(ast.dump(ast.parse('a if b else c', mode='eval'), indent=4)) Expression( body=IfExp( test=Name(id='b', ctx=Load()), body=Name(id='a', ctx=Load()), orelse=Name(id='c', ctx=Load())))
Attribute access, e.g. d.keys
. value
is a node, typically a Name
. attr
is a bare string giving the name of the attribute, and ctx
is Load
, Store
or Del
according to how the attribute is acted on.
>>> print(ast.dump(ast.parse('snake.colour', mode='eval'), indent=4)) Expression( body=Attribute( value=Name(id='snake', ctx=Load()), attr='colour', ctx=Load()))
A named expression. This AST node is produced by the assignment expressions operator (also known as the walrus operator). As opposed to the Assign
node in which the first argument can be multiple nodes, in this case both target
and value
must be single nodes.
>>> print(ast.dump(ast.parse('(x := 4)', mode='eval'), indent=4)) Expression( body=NamedExpr( target=Name(id='x', ctx=Store()), value=Constant(value=4)))
A subscript, such as l[1]
. value
is the subscripted object (usually sequence or mapping). slice
is an index, slice or key. It can be a Tuple
and contain a Slice
. ctx
is Load
, Store
or Del
according to the action performed with the subscript.
>>> print(ast.dump(ast.parse('l[1:2, 3]', mode='eval'), indent=4)) Expression( body=Subscript( value=Name(id='l', ctx=Load()), slice=Tuple( elts=[ Slice( lower=Constant(value=1), upper=Constant(value=2)), Constant(value=3)], ctx=Load()), ctx=Load()))
Regular slicing (on the form lower:upper
or lower:upper:step
). Can occur only inside the slice field of Subscript
, either directly or as an element of Tuple
.
>>> print(ast.dump(ast.parse('l[1:2]', mode='eval'), indent=4)) Expression( body=Subscript( value=Name(id='l', ctx=Load()), slice=Slice( lower=Constant(value=1), upper=Constant(value=2)), ctx=Load()))
List and set comprehensions, generator expressions, and dictionary comprehensions. elt
(or key
and value
) is a single node representing the part that will be evaluated for each item.
generators
is a list of comprehension
nodes.
>>> print(ast.dump(ast.parse('[x for x in numbers]', mode='eval'), indent=4)) Expression( body=ListComp( elt=Name(id='x', ctx=Load()), generators=[ comprehension( target=Name(id='x', ctx=Store()), iter=Name(id='numbers', ctx=Load()), ifs=[], is_async=0)])) >>> print(ast.dump(ast.parse('{x: x**2 for x in numbers}', mode='eval'), indent=4)) Expression( body=DictComp( key=Name(id='x', ctx=Load()), value=BinOp( left=Name(id='x', ctx=Load()), op=Pow(), right=Constant(value=2)), generators=[ comprehension( target=Name(id='x', ctx=Store()), iter=Name(id='numbers', ctx=Load()), ifs=[], is_async=0)])) >>> print(ast.dump(ast.parse('{x for x in numbers}', mode='eval'), indent=4)) Expression( body=SetComp( elt=Name(id='x', ctx=Load()), generators=[ comprehension( target=Name(id='x', ctx=Store()), iter=Name(id='numbers', ctx=Load()), ifs=[], is_async=0)]))
One for
clause in a comprehension. target
is the reference to use for each element - typically a Name
or Tuple
node. iter
is the object to iterate over. ifs
is a list of test expressions: each for
clause can have multiple ifs
.
is_async
indicates a comprehension is asynchronous (using an async for
instead of for
). The value is an integer (0 or 1).
>>> print(ast.dump(ast.parse('[ord(c) for line in file for c in line]', mode='eval'), ... indent=4)) # Multiple comprehensions in one. Expression( body=ListComp( elt=Call( func=Name(id='ord', ctx=Load()), args=[ Name(id='c', ctx=Load())], keywords=[]), generators=[ comprehension( target=Name(id='line', ctx=Store()), iter=Name(id='file', ctx=Load()), ifs=[], is_async=0), comprehension( target=Name(id='c', ctx=Store()), iter=Name(id='line', ctx=Load()), ifs=[], is_async=0)]))
>>> print(ast.dump(ast.parse('(n**2 for n in it if n>5 if n<10)', mode='eval'), ... indent=4)) # generator comprehension Expression( body=GeneratorExp( elt=BinOp( left=Name(id='n', ctx=Load()), op=Pow(), right=Constant(value=2)), generators=[ comprehension( target=Name(id='n', ctx=Store()), iter=Name(id='it', ctx=Load()), ifs=[ Compare( left=Name(id='n', ctx=Load()), ops=[ Gt()], comparators=[ Constant(value=5)]), Compare( left=Name(id='n', ctx=Load()), ops=[ Lt()], comparators=[ Constant(value=10)])], is_async=0)]))
>>> print(ast.dump(ast.parse('[i async for i in soc]', mode='eval'), ... indent=4)) # Async comprehension Expression( body=ListComp( elt=Name(id='i', ctx=Load()), generators=[ comprehension( target=Name(id='i', ctx=Store()), iter=Name(id='soc', ctx=Load()), ifs=[], is_async=1)]))
An assignment. targets
is a list of nodes, and value
is a single node.
Multiple nodes in targets
represents assigning the same value to each. Unpacking is represented by putting a Tuple
or List
within targets
.
type_comment
type_comment
is an optional string with the type annotation as a comment.
>>> print(ast.dump(ast.parse('a = b = 1'), indent=4)) # Multiple assignment Module( body=[ Assign( targets=[ Name(id='a', ctx=Store()), Name(id='b', ctx=Store())], value=Constant(value=1))], type_ignores=[])
>>> print(ast.dump(ast.parse('a,b = c'), indent=4)) # Unpacking Module( body=[ Assign( targets=[ Tuple( elts=[ Name(id='a', ctx=Store()), Name(id='b', ctx=Store())], ctx=Store())], value=Name(id='c', ctx=Load()))], type_ignores=[])
An assignment with a type annotation. target
is a single node and can be a Name
, a Attribute
or a Subscript
. annotation
is the annotation, such as a Constant
or Name
node. value
is a single optional node. simple
is a boolean integer set to True for a Name
node in target
that do not appear in between parenthesis and are hence pure names and not expressions.
>>> print(ast.dump(ast.parse('c: int'), indent=4)) Module( body=[ AnnAssign( target=Name(id='c', ctx=Store()), annotation=Name(id='int', ctx=Load()), simple=1)], type_ignores=[])
>>> print(ast.dump(ast.parse('(a): int = 1'), indent=4)) # Annotation with parenthesis Module( body=[ AnnAssign( target=Name(id='a', ctx=Store()), annotation=Name(id='int', ctx=Load()), value=Constant(value=1), simple=0)], type_ignores=[])
>>> print(ast.dump(ast.parse('a.b: int'), indent=4)) # Attribute annotation Module( body=[ AnnAssign( target=Attribute( value=Name(id='a', ctx=Load()), attr='b', ctx=Store()), annotation=Name(id='int', ctx=Load()), simple=0)], type_ignores=[])
>>> print(ast.dump(ast.parse('a[1]: int'), indent=4)) # Subscript annotation Module( body=[ AnnAssign( target=Subscript( value=Name(id='a', ctx=Load()), slice=Constant(value=1), ctx=Store()), annotation=Name(id='int', ctx=Load()), simple=0)], type_ignores=[])
Augmented assignment, such as a += 1
. In the following example, target
is a Name
node for x
(with the Store
context), op
is Add
, and value
is a Constant
with value for 1.
The target
attribute cannot be of class Tuple
or List
, unlike the targets of Assign
.
>>> print(ast.dump(ast.parse('x += 2'), indent=4)) Module( body=[ AugAssign( target=Name(id='x', ctx=Store()), op=Add(), value=Constant(value=2))], type_ignores=[])
A raise
statement. exc
is the exception object to be raised, normally a Call
or Name
, or None
for a standalone raise
. cause
is the optional part for y
in raise x from y
.
>>> print(ast.dump(ast.parse('raise x from y'), indent=4)) Module( body=[ Raise( exc=Name(id='x', ctx=Load()), cause=Name(id='y', ctx=Load()))], type_ignores=[])
An assertion. test
holds the condition, such as a Compare
node. msg
holds the failure message.
>>> print(ast.dump(ast.parse('assert x,y'), indent=4)) Module( body=[ Assert( test=Name(id='x', ctx=Load()), msg=Name(id='y', ctx=Load()))], type_ignores=[])
Represents a del
statement. targets
is a list of nodes, such as Name
, Attribute
or Subscript
nodes.
>>> print(ast.dump(ast.parse('del x,y,z'), indent=4)) Module( body=[ Delete( targets=[ Name(id='x', ctx=Del()), Name(id='y', ctx=Del()), Name(id='z', ctx=Del())])], type_ignores=[])
A pass
statement.
>>> print(ast.dump(ast.parse('pass'), indent=4)) Module( body=[ Pass()], type_ignores=[])
Other statements which are only applicable inside functions or loops are described in other sections.
An import statement. names
is a list of alias
nodes.
>>> print(ast.dump(ast.parse('import x,y,z'), indent=4)) Module( body=[ Import( names=[ alias(name='x'), alias(name='y'), alias(name='z')])], type_ignores=[])
Represents from x import y
. module
is a raw string of the 'from' name, without any leading dots, or None
for statements such as from . import foo
. level
is an integer holding the level of the relative import (0 means absolute import).
>>> print(ast.dump(ast.parse('from y import x,y,z'), indent=4)) Module( body=[ ImportFrom( module='y', names=[ alias(name='x'), alias(name='y'), alias(name='z')], level=0)], type_ignores=[])
Both parameters are raw strings of the names. asname
can be None
if the regular name is to be used.
>>> print(ast.dump(ast.parse('from ..foo.bar import a as b, c'), indent=4)) Module( body=[ ImportFrom( module='foo.bar', names=[ alias(name='a', asname='b'), alias(name='c')], level=2)], type_ignores=[])
Note
Optional clauses such as else
are stored as an empty list if they're not present.
An if
statement. test
holds a single node, such as a Compare
node. body
and orelse
each hold a list of nodes.
elif
clauses don't have a special representation in the AST, but rather appear as extra If
nodes within the orelse
section of the previous one.
>>> print(ast.dump(ast.parse(""" ... if x: ... ... ... elif y: ... ... ... else: ... ... ... """), indent=4)) Module( body=[ If( test=Name(id='x', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ If( test=Name(id='y', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A for
loop. target
holds the variable(s) the loop assigns to, as a single Name
, Tuple
or List
node. iter
holds the item to be looped over, again as a single node. body
and orelse
contain lists of nodes to execute. Those in orelse
are executed if the loop finishes normally, rather than via a break
statement.
type_comment
type_comment
is an optional string with the type annotation as a comment.
>>> print(ast.dump(ast.parse(""" ... for x in y: ... ... ... else: ... ... ... """), indent=4)) Module( body=[ For( target=Name(id='x', ctx=Store()), iter=Name(id='y', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ Expr( value=Constant(value=Ellipsis))])], type_ignores=[])
A while
loop. test
holds the condition, such as a Compare
node.
>> print(ast.dump(ast.parse(""" ... while x: ... ... ... else: ... ... ... """), indent=4)) Module( body=[ While( test=Name(id='x', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))], orelse=[ Expr( value=Constant(value=Ellipsis))])], type_ignores=[])
The break
and continue
statements.
>>> print(ast.dump(ast.parse("""... for a in b: ... if a > 5: ... break ... else: ... continue ... ... """), indent=4)) Module( body=[ For( target=Name(id='a', ctx=Store()), iter=Name(id='b', ctx=Load()), body=[ If( test=Compare( left=Name(id='a', ctx=Load()), ops=[ Gt()], comparators=[ Constant(value=5)]), body=[ Break()], orelse=[ Continue()])], orelse=[])], type_ignores=[])
try
blocks. All attributes are list of nodes to execute, except for handlers
, which is a list of ExceptHandler
nodes.
>>> print(ast.dump(ast.parse(""" ... try: ... ... ... except Exception: ... ... ... except OtherException as e: ... ... ... else: ... ... ... finally: ... ... ... """), indent=4)) Module( body=[ Try( body=[ Expr( value=Constant(value=Ellipsis))], handlers=[ ExceptHandler( type=Name(id='Exception', ctx=Load()), body=[ Expr( value=Constant(value=Ellipsis))]), ExceptHandler( type=Name(id='OtherException', ctx=Load()), name='e', body=[ Expr( value=Constant(value=Ellipsis))])], orelse=[ Expr( value=Constant(value=Ellipsis))], finalbody=[ Expr( value=Constant(value=Ellipsis))])], type_ignores=[])
A single except
clause. type
is the exception type it will match, typically a Name
node (or None
for a catch-all except:
clause). name
is a raw string for the name to hold the exception, or None
if the clause doesn't have as foo
. body
is a list of nodes.
>>> print(ast.dump(ast.parse("""... try: ... a + 1 ... except TypeError: ... pass ... """), indent=4)) Module( body=[ Try( body=[ Expr( value=BinOp( left=Name(id='a', ctx=Load()), op=Add(), right=Constant(value=1)))], handlers=[ ExceptHandler( type=Name(id='TypeError', ctx=Load()), body=[ Pass()])], orelse=[], finalbody=[])], type_ignores=[])
A with
block. items
is a list of withitem
nodes representing the context managers, and body
is the indented block inside the context.
type_comment
type_comment
is an optional string with the type annotation as a comment.
A single context manager in a with
block. context_expr
is the context manager, often a Call
node. optional_vars
is a Name
, Tuple
or List
for the as foo
part, or None
if that isn't used.
>>> print(ast.dump(ast.parse("""... with a as b, c as d: ... something(b, d) ... """), indent=4)) Module( body=[ With( items=[ withitem( context_expr=Name(id='a', ctx=Load()), optional_vars=Name(id='b', ctx=Store())), withitem( context_expr=Name(id='c', ctx=Load()), optional_vars=Name(id='d', ctx=Store()))], body=[ Expr( value=Call( func=Name(id='something', ctx=Load()), args=[ Name(id='b', ctx=Load()), Name(id='d', ctx=Load())], keywords=[]))])], type_ignores=[])
A match
statement. subject
holds the subject of the match (the object that is being matched against the cases) and cases
contains an iterable of match_case
nodes with the different cases.
A single case pattern in a match
statement. pattern
contains the match pattern that the subject will be matched against. Note that the AST
nodes produced for patterns differ from those produced for expressions, even when they share the same syntax.
The guard
attribute contains an expression that will be evaluated if the pattern matches the subject.
body
contains a list of nodes to execute if the pattern matches and the result of evaluating the guard expression is true.
>>> print(ast.dump(ast.parse(""" ... match x: ... case [x] if x>0: ... ... ... case tuple(): ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSequence( patterns=[ MatchAs(name='x')]), guard=Compare( left=Name(id='x', ctx=Load()), ops=[ Gt()], comparators=[ Constant(value=0)]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchClass( cls=Name(id='tuple', ctx=Load()), patterns=[], kwd_attrs=[], kwd_patterns=[]), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match literal or value pattern that compares by equality. value
is an expression node. Permitted value nodes are restricted as described in the match statement documentation. This pattern succeeds if the match subject is equal to the evaluated value.
>>> print(ast.dump(ast.parse(""" ... match x: ... case "Relevant": ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchValue( value=Constant(value='Relevant')), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match literal pattern that compares by identity. value
is the singleton to be compared against: None
, True
, or False
. This pattern succeeds if the match subject is the given constant.
>>> print(ast.dump(ast.parse(""" ... match x: ... case None: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSingleton(value=None), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match sequence pattern. patterns
contains the patterns to be matched against the subject elements if the subject is a sequence. Matches a variable length sequence if one of the subpatterns is a MatchStar
node, otherwise matches a fixed length sequence.
>>> print(ast.dump(ast.parse(""" ... match x: ... case [1, 2]: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSequence( patterns=[ MatchValue( value=Constant(value=1)), MatchValue( value=Constant(value=2))]), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
Matches the rest of the sequence in a variable length match sequence pattern. If name
is not None
, a list containing the remaining sequence elements is bound to that name if the overall sequence pattern is successful.
>>> print(ast.dump(ast.parse(""" ... match x: ... case [1, 2, rest]: ... ... ... case [_]: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchSequence( patterns=[ MatchValue( value=Constant(value=1)), MatchValue( value=Constant(value=2)), MatchStar(name='rest')]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchSequence( patterns=[ MatchStar()]), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match mapping pattern. keys
is a sequence of expression nodes. patterns
is a corresponding sequence of pattern nodes. rest
is an optional name that can be specified to capture the remaining mapping elements. Permitted key expressions are restricted as described in the match statement documentation.
This pattern succeeds if the subject is a mapping, all evaluated key expressions are present in the mapping, and the value corresponding to each key matches the corresponding subpattern. If rest
is not None
, a dict containing the remaining mapping elements is bound to that name if the overall mapping pattern is successful.
>>> print(ast.dump(ast.parse(""" ... match x: ... case {1: _, 2: _}: ... ... ... case {**rest}: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchMapping( keys=[ Constant(value=1), Constant(value=2)], patterns=[ MatchAs(), MatchAs()]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchMapping(keys=[], patterns=[], rest='rest'), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match class pattern. cls
is an expression giving the nominal class to be matched. patterns
is a sequence of pattern nodes to be matched against the class defined sequence of pattern matching attributes. kwd_attrs
is a sequence of additional attributes to be matched (specified as keyword arguments in the class pattern), kwd_patterns
are the corresponding patterns (specified as keyword values in the class pattern).
This pattern succeeds if the subject is an instance of the nominated class, all positional patterns match the corresponding class-defined attributes, and any specified keyword attributes match their corresponding pattern.
Note: classes may define a property that returns self in order to match a pattern node against the instance being matched. Several builtin types are also matched that way, as described in the match statement documentation.
>>> print(ast.dump(ast.parse(""" ... match x: ... case Point2D(0, 0): ... ... ... case Point3D(x=0, y=0, z=0): ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchClass( cls=Name(id='Point2D', ctx=Load()), patterns=[ MatchValue( value=Constant(value=0)), MatchValue( value=Constant(value=0))], kwd_attrs=[], kwd_patterns=[]), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchClass( cls=Name(id='Point3D', ctx=Load()), patterns=[], kwd_attrs=[ 'x', 'y', 'z'], kwd_patterns=[ MatchValue( value=Constant(value=0)), MatchValue( value=Constant(value=0)), MatchValue( value=Constant(value=0))]), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match "as-pattern", capture pattern or wildcard pattern. pattern
contains the match pattern that the subject will be matched against. If the pattern is None
, the node represents a capture pattern (i.e a bare name) and will always succeed.
The name
attribute contains the name that will be bound if the pattern is successful. If name
is None
, pattern
must also be None
and the node represents the wildcard pattern.
>>> print(ast.dump(ast.parse(""" ... match x: ... case [x] as y: ... ... ... case _: ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchAs( pattern=MatchSequence( patterns=[ MatchAs(name='x')]), name='y'), body=[ Expr( value=Constant(value=Ellipsis))]), match_case( pattern=MatchAs(), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A match "or-pattern". An or-pattern matches each of its subpatterns in turn to the subject, until one succeeds. The or-pattern is then deemed to succeed. If none of the subpatterns succeed the or-pattern fails. The patterns
attribute contains a list of match pattern nodes that will be matched against the subject.
>>> print(ast.dump(ast.parse(""" ... match x: ... case [x] | (y): ... ... ... """), indent=4)) Module( body=[ Match( subject=Name(id='x', ctx=Load()), cases=[ match_case( pattern=MatchOr( patterns=[ MatchSequence( patterns=[ MatchAs(name='x')]), MatchAs(name='y')]), body=[ Expr( value=Constant(value=Ellipsis))])])], type_ignores=[])
A function definition.
name
is a raw string of the function name.args
is anarguments
node.body
is the list of nodes inside the function.decorator_list
is the list of decorators to be applied, stored outermost first (i.e. the first in the list will be applied last).returns
is the return annotation.
type_comment
type_comment
is an optional string with the type annotation as a comment.
lambda
is a minimal function definition that can be used inside an expression. Unlike FunctionDef
, body
holds a single node.
>>> print(ast.dump(ast.parse('lambda x,y: ...'), indent=4)) Module( body=[ Expr( value=Lambda( args=arguments( posonlyargs=[], args=[ arg(arg='x'), arg(arg='y')], kwonlyargs=[], kw_defaults=[], defaults=[]), body=Constant(value=Ellipsis)))], type_ignores=[])
The arguments for a function.
posonlyargs
,args
andkwonlyargs
are lists ofarg
nodes.vararg
andkwarg
are singlearg
nodes, referring to the*args, **kwargs
parameters.kw_defaults
is a list of default values for keyword-only arguments. If one isNone
, the corresponding argument is required.defaults
is a list of default values for arguments that can be passed positionally. If there are fewer defaults, they correspond to the last n arguments.
A single argument in a list. arg
is a raw string of the argument name, annotation
is its annotation, such as a Str
or Name
node.
type_comment
type_comment
is an optional string with the type annotation as a comment
>>> print(ast.dump(ast.parse("""... @decorator1 ... @decorator2 ... def f(a: 'annotation', b=1, c=2, d, e, f=3,*g) -> 'return annotation': ... pass ... """), indent=4)) Module( body=[ FunctionDef( name='f', args=arguments( posonlyargs=[], args=[ arg( arg='a', annotation=Constant(value='annotation')), arg(arg='b'), arg(arg='c')], vararg=arg(arg='d'), kwonlyargs=[ arg(arg='e'), arg(arg='f')], kw_defaults=[ None, Constant(value=3)], kwarg=arg(arg='g'), defaults=[ Constant(value=1), Constant(value=2)]), body=[ Pass()], decorator_list=[ Name(id='decorator1', ctx=Load()), Name(id='decorator2', ctx=Load())], returns=Constant(value='return annotation'))], type_ignores=[])
A return
statement.
>>> print(ast.dump(ast.parse('return 4'), indent=4)) Module( body=[ Return( value=Constant(value=4))], type_ignores=[])
A yield
or yield from
expression. Because these are expressions, they must be wrapped in a Expr
node if the value sent back is not used.
>>> print(ast.dump(ast.parse('yield x'), indent=4)) Module( body=[ Expr( value=Yield( value=Name(id='x', ctx=Load())))], type_ignores=[])
>>> print(ast.dump(ast.parse('yield from x'), indent=4)) Module( body=[ Expr( value=YieldFrom( value=Name(id='x', ctx=Load())))], type_ignores=[])
global
and nonlocal
statements. names
is a list of raw strings.
>>> print(ast.dump(ast.parse('global x,y,z'), indent=4)) Module( body=[ Global( names=[ 'x', 'y', 'z'])], type_ignores=[])
>>> print(ast.dump(ast.parse('nonlocal x,y,z'), indent=4)) Module( body=[ Nonlocal( names=[ 'x', 'y', 'z'])], type_ignores=[])
A class definition.
name
is a raw string for the class namebases
is a list of nodes for explicitly specified base classes.keywords
is a list ofkeyword
nodes, principally for 'metaclass'. Other keywords will be passed to the metaclass, as per PEP-3115.starargs
andkwargs
are each a single node, as in a function call. starargs will be expanded to join the list of base classes, and kwargs will be passed to the metaclass.body
is a list of nodes representing the code within the class definition.decorator_list
is a list of nodes, as inFunctionDef
.
>>> print(ast.dump(ast.parse("""... @decorator1 ... @decorator2 ... class Foo(base1, base2, metaclass=meta): ... pass ... """), indent=4)) Module( body=[ ClassDef( name='Foo', bases=[ Name(id='base1', ctx=Load()), Name(id='base2', ctx=Load())], keywords=[ keyword( arg='metaclass', value=Name(id='meta', ctx=Load()))], body=[ Pass()], decorator_list=[ Name(id='decorator1', ctx=Load()), Name(id='decorator2', ctx=Load())])], type_ignores=[])
An async def
function definition. Has the same fields as FunctionDef
.
An await
expression. value
is what it waits for. Only valid in the body of an AsyncFunctionDef
.
>>> print(ast.dump(ast.parse("""... async def f(): ... await other_func() ... """), indent=4)) Module( body=[ AsyncFunctionDef( name='f', args=arguments( posonlyargs=[], args=[], kwonlyargs=[], kw_defaults=[], defaults=[]), body=[ Expr( value=Await( value=Call( func=Name(id='other_func', ctx=Load()), args=[], keywords=[])))], decorator_list=[])], type_ignores=[])
async for
loops and async with
context managers. They have the same fields as For
and With
, respectively. Only valid in the body of an AsyncFunctionDef
.
Note
When a string is parsed by ast.parse
, operator nodes (subclasses of ast.operator
, ast.unaryop
, ast.cmpop
, ast.boolop
and ast.expr_context
) on the returned tree will be singletons. Changes to one will be reflected in all other occurrences of the same value (e.g. ast.Add
).
Apart from the node classes, the ast
module defines these utility functions and classes for traversing abstract syntax trees:
parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None)
Parse the source into an AST node. Equivalent to compile(source, filename, mode, ast.PyCF_ONLY_AST)
.
If type_comments=True
is given, the parser is modified to check and return type comments as specified by 484
and 526
. This is equivalent to adding ast.PyCF_TYPE_COMMENTS
to the flags passed to compile()
. This will report syntax errors for misplaced type comments. Without this flag, type comments will be ignored, and the type_comment
field on selected AST nodes will always be None
. In addition, the locations of # type: ignore
comments will be returned as the type_ignores
attribute of Module
(otherwise it is always an empty list).
In addition, if mode
is 'func_type'
, the input syntax is modified to correspond to 484
"signature type comments", e.g. (str, int) -> List[str]
.
Also, setting feature_version
to a tuple (major, minor)
will attempt to parse using that Python version's grammar. Currently major
must equal to 3
. For example, setting feature_version=(3, 4)
will allow the use of async
and await
as variable names. The lowest supported version is (3, 4)
; the highest is sys.version_info[0:2]
.
If source contains a null character ('0'), ValueError
is raised.
Warning
Note that succesfully parsing souce code into an AST object doesn't guarantee that the source code provided is valid Python code that can be executed as the compilation step can raise further
SyntaxError
exceptions. For instance, the sourcereturn 42
generates a valid AST node for a return statement, but it cannot be compiled alone (it needs to be inside a function node).In particular,
ast.parse
won't do any scoping checks, which the compilation step does.
Warning
It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python's AST compiler.
3.8 Added type_comments
, mode='func_type'
and feature_version
.
unparse(ast_obj)
Unparse an ast.AST
object and generate a string with code that would produce an equivalent ast.AST
object if parsed back with ast.parse
.
Warning
The produced code string will not necessarily be equal to the original code that generated the ast.AST
object (without any compiler optimizations, such as constant tuples/frozensets).
Warning
Trying to unparse a highly complex expression would result with RecursionError
.
3.9
literal_eval(node_or_string)
Safely evaluate an expression node or a string containing a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans, None
and Ellipsis
.
This can be used for safely evaluating strings containing Python values from untrusted sources without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.
Warning
It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python's AST compiler.
It can raise ValueError
, TypeError
, SyntaxError
, MemoryError
and RecursionError
depending on the malformed input.
3.2 Now allows bytes and set literals.
3.9 Now supports creating empty sets with 'set()'
.
3.10 For string inputs, leading spaces and tabs are now stripped.
get_docstring(node, clean=True)
Return the docstring of the given node (which must be a FunctionDef
, AsyncFunctionDef
, ClassDef
, or Module
node), or None
if it has no docstring. If clean is true, clean up the docstring's indentation with inspect.cleandoc
.
3.5 AsyncFunctionDef
is now supported.
get_source_segment(source, node, *, padded=False)
Get source code segment of the source that generated node. If some location information (lineno
, end_lineno
, col_offset
, or end_col_offset
) is missing, return None
.
If padded is True
, the first line of a multi-line statement will be padded with spaces to match its original position.
3.8
fix_missing_locations(node)
When you compile a node tree with compile
, the compiler expects lineno
and col_offset
attributes for every node that supports them. This is rather tedious to fill in for generated nodes, so this helper adds these attributes recursively where not already set, by setting them to the values of the parent node. It works recursively starting at node.
increment_lineno(node, n=1)
Increment the line number and end line number of each node in the tree starting at node by n. This is useful to "move code" to a different location in a file.
copy_location(new_node, old_node)
Copy source location (lineno
, col_offset
, end_lineno
, and end_col_offset
) from old_node to new_node if possible, and return new_node.
iter_fields(node)
Yield a tuple of (fieldname, value)
for each field in node._fields
that is present on node.
iter_child_nodes(node)
Yield all direct child nodes of node, that is, all fields that are nodes and all items of fields that are lists of nodes.
walk(node)
Recursively yield all descendant nodes in the tree starting at node (including node itself), in no specified order. This is useful if you only want to modify nodes in place and don't care about the context.
A node visitor base class that walks the abstract syntax tree and calls a visitor function for every node found. This function may return a value which is forwarded by the visit
method.
This class is meant to be subclassed, with the subclass adding visitor methods.
visit(node)
Visit a node. The default implementation calls the method called self.visit_{classname}
where classname is the name of the node class, or generic_visit
if that method doesn't exist.
generic_visit(node)
This visitor calls visit
on all children of the node.
Note that child nodes of nodes that have a custom visitor method won't be visited unless the visitor calls generic_visit
or visits them itself.
Don't use the NodeVisitor
if you want to apply changes to nodes during traversal. For this a special visitor exists (NodeTransformer
) that allows modifications.
3.8
Methods visit_Num
, visit_Str
, visit_Bytes
, visit_NameConstant
and visit_Ellipsis
are deprecated now and will not be called in future Python versions. Add the visit_Constant
method to handle all constant nodes.
A NodeVisitor
subclass that walks the abstract syntax tree and allows modification of nodes.
The NodeTransformer
will walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method is None
, the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place.
Here is an example transformer that rewrites all occurrences of name lookups (foo
) to data['foo']
:
class RewriteName(NodeTransformer):
def visit_Name(self, node):
return Subscript(
value=Name(id='data', ctx=Load()),
slice=Constant(value=node.id),
ctx=node.ctx
)
Keep in mind that if the node you're operating on has child nodes you must either transform the child nodes yourself or call the generic_visit
method for the node first.
For nodes that were part of a collection of statements (that applies to all statement nodes), the visitor may also return a list of nodes rather than just a single node.
If NodeTransformer
introduces new nodes (that weren't part of original tree) without giving them location information (such as lineno
), fix_missing_locations
should be called with the new sub-tree to recalculate the location information:
tree = ast.parse('foo', mode='eval')
new_tree = fix_missing_locations(RewriteName().visit(tree))
Usually you use the transformer like this:
node = YourTransformer().visit(node)
dump(node, annotate_fields=True, include_attributes=False, *, indent=None)
Return a formatted dump of the tree in node. This is mainly useful for debugging purposes. If annotate_fields is true (by default), the returned string will show the names and the values for fields. If annotate_fields is false, the result string will be more compact by omitting unambiguous field names. Attributes such as line numbers and column offsets are not dumped by default. If this is wanted, include_attributes can be set to true.
If indent is a non-negative integer or string, then the tree will be pretty-printed with that indent level. An indent level of 0, negative, or ""
will only insert newlines. None
(the default) selects the single line representation. Using a positive integer indent indents that many spaces per level. If indent is a string (such as "\t"
), that string is used to indent each level.
3.9 Added the indent option.
The following flags may be passed to compile
in order to change effects on the compilation of a program:
PyCF_ALLOW_TOP_LEVEL_AWAIT
Enables support for top-level await
, async for
, async with
and async comprehensions.
3.8
PyCF_ONLY_AST
Generates and returns an abstract syntax tree instead of returning a compiled code object.
PyCF_TYPE_COMMENTS
Enables support for 484
and 526
style type comments (# type: <type>
, # type: ignore <stuff>
).
3.8
3.9
The ast
module can be executed as a script from the command line. It is as simple as:
python -m ast [-m <mode>] [-a] [infile]
The following options are accepted:
ast
-h, --help
Show the help message and exit.
-m <mode> --mode <mode>
Specify what kind of code must be compiled, like the mode argument in parse
.
--no-type-comments
Don't parse type comments.
-a, --include-attributes
Include attributes such as line numbers and column offsets.
-i <indent> --indent <indent>
Indentation of nodes in AST (number of spaces).
If infile
is specified its contents are parsed to AST and dumped to stdout. Otherwise, the content is read from stdin.
Green Tree Snakes, an external documentation resource, has good details on working with Python ASTs.
ASTTokens annotates Python ASTs with the positions of tokens and text in the source code that generated them. This is helpful for tools that make source code transformations.
leoAst.py unifies the token-based and parse-tree-based views of python programs by inserting two-way links between tokens and ast nodes.
LibCST parses code as a Concrete Syntax Tree that looks like an ast tree and keeps all formatting details. It's useful for building automated refactoring (codemod) applications and linters.
Parso is a Python parser that supports error recovery and round-trip parsing for different Python versions (in multiple Python versions). Parso is also able to list multiple syntax errors in your python file.