Python-Function-Guide

Basic python3 function guide

• [Built-in functions](#built-in functions)
  • chr

Awesome websites

• Python魔术方法介绍 http://www.diveintopython3.net/special-method-names.html
• Python3官方网站介绍内置函数 https://docs.python.org/3/library/functions.html

Built-in functions

chr(i)

Return a Unicode string of one character with ordinal i; 0 <= i <= 0x10ffff.

Note: chr() takes exactly one argument.

Examples:

>>> chr(65)
'A'

>>> chr(1)
'\x01'

>>> chr(0x30)
'0'

>>> chr(100000000)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
ValueError: chr() arg not in range(0x110000)

complex(real[,imag])

complex(real[, imag]) -> complex number

Create a complex number from a real part and an optional imaginary part. This is equivalent to (real + imag*1j) where imag defaults to 0.

Note: complex object's real and imag is a float number.

Examples:

>>> complex(1)
(1+0j)

>>> complex(1,2)
(1+2j)

>>> complex(real=1)
(1+0j)

>>> complex(imag=1)
1j

>>> type(complex())
<class 'complex'>

dir([object]) -> list of strings

If called without an argument, return the names in the current scope. Else, return an alphabetized list of names comprising (some of) the attributes of the given object, and of attributes reachable from it. If the object supplies a method named dir, it will be used; otherwise the default dir() logic is used and returns: for a module object: the module's attributes. for a class object: its attributes, and recursively the attributes of its bases. for any other object: its attributes, its class's attributes, and recursively the attributes of its class's base classes.

Note: This function returns the attributes of the object.

Examples:

>>> import sys
>>> dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__interactivehook__', '__loader__', '__name__', '__package__', '__spec__', '__stderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_current_frames', '_debugmallocstats', '_enablelegacywindowsfsencoding', '_getframe', '_git', '_home', '_xoptions', 'api_version', 'argv', 'base_exec_prefix', 'base_prefix', 'builtin_module_names', 'byteorder', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle', 'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix', 'executable', 'exit', 'flags', 'float_info', 'float_repr_style', 'get_asyncgen_hooks', 'get_coroutine_wrapper', 'getallocatedblocks', 'getcheckinterval', 'getdefaultencoding', 'getfilesystemencodeerrors', 'getfilesystemencoding', 'getprofile', 'getrecursionlimit', 'getrefcount', 'getsizeof', 'getswitchinterval', 'gettrace', 'getwindowsversion', 'hash_info', 'hexversion', 'implementation', 'int_info', 'intern', 'is_finalizing', 'maxsize', 'maxunicode', 'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'set_asyncgen_hooks', 'set_coroutine_wrapper', 'setcheckinterval', 'setprofile', 'setrecursionlimit', 'setswitchinterval', 'settrace', 'stderr', 'stdin', 'stdout', 'thread_info', 'version', 'version_info', 'warnoptions', 'winver']

>>> def function():
...     print("hello")
...     
>>> dir(function)
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

>>> class Animal:
...     def sound(self):
...         return "miaomiaomiao"
...     
>>> dir(Animal)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'sound']

divmode(x, y)

Return the tuple (x//y, x%y). Invariant: div*y + mod == x.

Examples:

>>> divmod(5, 2)
(2, 1) # This is a tuple

>>> divmod(5.0, 2)
(2.0, 1.0)

enumerate(iterable[, start]) -> iterator for index, value of iterable

Return an enumerate object. iterable must be another object that supports iteration. The enumerate object yields pairs containing a count (from start, which defaults to zero) and a value yielded by the iterable argument. enumerate is useful for obtaining an indexed list: (0, seq[0]), (1, seq[1]), (2, seq[2]), ...

Note: The optional start parameter represents the number of the first item which default to zero.

Examples:

>>> items = ['Cat', 'Dog', 'Rabbit']
... for item in enumerate(items):
...     print(item, type(item))
... 
(0, 'Cat') <class 'tuple'>
(1, 'Dog') <class 'tuple'>
(2, 'Rabbit') <class 'tuple'>
>>> for index, value in enumerate(items):
...     print(index, value)
...     
0 Cat
1 Dog
2 Rabbit
>>> for index, value in enumerate(items, 5):
...     print(index, value)
...         
5 Cat
6 Dog
7 Rabbit

eval(expression[, globals[, locals]])

Evaluate the given source in the context of globals and locals.

The source may be a string representing a Python expression or a code object as returned by compile(). The globals must be a dictionary and locals can be any mapping, defaulting to the current globals and locals. If only globals is given, locals defaults to it.

Notes: globals or locals optional parameter is a dictionary object.

Examples: This example shows three different position int variables which are added together in eval() function as a string.

>>> global_var = 1
... 
... def function():
...     local_var = 2
...     local_env = locals()
...     local_env.update({"additional_var":3})
...     result = eval("global_var + global_var + additional_var", globals(), local_env)
...     print(result)
... 
... function()
... 
5

exec(compile("run.py"), "run.py", exec)

This function is removed from python3. The execfile can be replaced with exec, compile,read.

Examples:

>>> exec(compile(open('helloworld.py').read(),'helloworld.py','exec'))
helloworld

getattr(object, name[, default])

getattr(object, name[, default]) Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

Examples:

>>> class Animal():
...     name = "Animal"
... 
...     def sound(self):
...         return "miaomiao"
>>> getattr(Animal, 'name')
'Animal'
>>> getattr(Animal, 'sound') # Get the function address in the memory
<function Animal.sound at 0x0315C078>
>>> getattr(Animal, 'sound')(None) # You can run the function attribute with (...)
'miaomiao'

abs(x)

Return the absolute value of a number. The argument may be an integer or a floating point number. If the argument is a complex number, its magnitude is returned.

Examples:

>>> abs(-10) # return type is integer if argument is integer
10
>>> abs(-10.0) # return type is float if argument is float
-10.0
>>> abs(complex(3,4)) # return type is float if argument is a complex number
5.0

all(iterable)

Return True if all elements of the iterable are true (or if the iterable is empty). Equivalent to:

def all(iterable):
    for element in iterable:
        if not element:
            return False
    return True

Note: The function return True only if all items in iterable object must be True.Otherwise returns False. False items: {}, (), [], 0, False

Examples:

>>> items = [0, 0, 0]
>>> all(items)
False
>>> items = [1, 2, 3]
>>> all(items)
True
>>> items = [[], {}, ()]
>>> all(items)
False

any(iterable)

Return True if any element of the iterable is true. If the iterable is empty, return False. Equivalent to:

def any(iterable):
    for element in iterable:
        if element:
            return True
    return False

Note: The function returns True if any one item in iterable object is True.Otherwise returns False.

Examples:

>>> items = [0, 0, 0]
>>> any(items)
False
>>> items = [0, 2, 0]
>>> any(items)
True
>>> items = [[1], {}, ()]
>>> any(items)
True

repr(object)

Return a string containing a printable representation of an object. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a repr() method.

Note: Difference between str and repr:(__repr__ is more important than __str__)

reference from https://stackoverflow.com/questions/1436703/difference-between-str-and-repr-in-python

The default implementation is useless (it’s hard to think of one which wouldn’t be, but yeah)

• __repr__ goal is to be unambiguous
• __str__ goal is to be readable
• Container’s __str__ uses contained objects’ __repr__

if __repr__ is defined, and __str__ is not, the object will behave as though __str__=__repr__.

Summary:

Implement __repr__ for any class you implement. This should be second nature. Implement __str__ if you think it would be useful to have a string version which errs on the side of more readability in favor of more ambiguity

Examples:

class Animal():
    name = "Animal"

    def sound(self):
        return "miaomiao"

    def __repr__(self):
        return 'This is my repr'

print(repr(Animal())) # 'This is my repr'
print(repr(Animal)) # '<class '__main__.Animal'>

>>> str(0.1)
'0.1'
>>> repr(0.1)
'0.1'
>>> hello = 'hello\n'
>>> str(hello)
'hello\n'
>>> repr(hello)
"'hello\\n'"

reversed(seq)

reversed(seq) Return a reverse iterator. seq must be an object which has a __reversed__() method or supports the sequence protocol (the __len__() method and the __getitem__() method with integer arguments starting at 0).

Examples:

class Animal():
    """
    Create customize reversible class with sequence protocol.
    
    implement __len__() and __getitem__()
    """
    name = "Animal"
    values = [1, 2, 3, 4, 5]

    def sound(self):
        return "miaomiao"

    def __repr__(self):
        return 'This is my repr'
        
    def __len__(self):
        return len(self.values)
        
    def __getitem__(self, item):
        return self.values[item]
        

for item in reversed(Animal()):
    print(item) # 5,4,3,2,1

hex(x)

Convert an integer number to a lowercase hexadecimal string prefixed with “0x”. If x is not a Python int object, it has to define an index() method that returns an integer.

Examples:

>>> hex(255)
'0xff'
>>> hex(-42)
'-0x2a'

# If you want to convert an integer number to an uppercase or lower hexadecimal string with prefix or not, you can use either of the following ways:
>>> '%#x' % 255, '%x' % 255, '%X' % 255
('0xff', 'ff', 'FF')
>>> format(255, '#x'), format(255, 'x'), format(255, 'X')
('0xff', 'ff', 'FF')
>>> f'{255:#x}', f'{255:x}', f'{255:X}'
('0xff', 'ff', 'FF')

id(object)

Return the “identity” of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same id() value.

CPython implementation detail: This is the address of the object in memory.

Examples:

>>> x=1
>>> id(x)
9310240
>>> id(1)
9310240
>>> id(2)
9310272
>>> id(2.0)
140662947445952

input([prompt])

If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read, EOFError is raised.

Example:

>>> s = input('--> ')
--> >? hello python
>>> s
'hello python'

int(x=0); int(x, base=10)

Return an integer object constructed from a number or string x, or return 0 if no arguments are given. If x is a number, return x.int(). For floating point numbers, this truncates towards zero.

If x is not a number or if base is given, then x must be a string, bytes, or bytearray instance representing an integer literal in radix base. Optionally, the literal can be preceded by + or - (with no space in between) and surrounded by whitespace. A base-n literal consists of the digits 0 to n-1, with a to z (or A to Z) having values 10 to 35. The default base is 10. The allowed values are 0 and 2–36. Base-2, -8, and -16 literals can be optionally prefixed with 0b/0B, 0o/0O, or 0x/0X, as with integer literals in code. Base 0 means to interpret exactly as a code literal, so that the actual base is 2, 8, 10, or 16, and so that int('010', 0) is not legal, while int('010') is, as well as int('010', 8).

Notes: If the object is a class object. The class has a function int(self), this function must return a integer number.

Examples:

>>> int()
0
>>> int(3.1)
3
>>> int(3.5)
3
>>> int(3.8)
3
>>> class Animal:
...     weight = 10
...
...     def __init__(self, weight:int):
...         self.weight = 100
...
...     def __int__(self):
...         return str(self.weight)
...
>>> int(Animal(100))
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: __int__ returned non-int (type str)

>>> class Animal:
...     weight = 10
...
...     def __init__(self, weight:int):
...         self.weight = 100
...
...     def __int__(self):
...         return self.weight
...
>>> int(Animal(100))
100

isinstance(object, classinfo)

Return true if the object argument is an instance of the classinfo argument, or of a (direct, indirect or virtual) subclass thereof. If object is not an object of the given type, the function always returns false. If classinfo is a tuple of type objects (or recursively, other such tuples), return true if object is an instance of any of the types. If classinfo is not a type or tuple of types and such tuples, a TypeError exception is raised.

Examples:

>>> class Animal:
...     weight = 10
...
...     def __init__(self, weight:int):
...         self.weight = 100
...
...     def __int__(self):
...         return self.weight
...
>>> cat = Animal(100)
>>> isinstance(cat, Animal)
True