Python dunder

Dunder是 Double UNDERscore的缩写。指的就是以两个下划线开头和结尾的函数，同时也称作为魔法函数Magic 魔术方法。这些方法通常不直接调用，而是由解释器调用。

运算符重载魔术方法

魔术方法	方法描述
`__add__`(self, other)	add operation using `+` operator
`__sub__`(self, other)	subtraction operation using `-` operator.
`__mul__`(self, other)	multiplication operation using `*` operator.
`__floordiv__`(self, other)	floor division operation using `//` operator.
`__truediv__`(self, other)	division operation using `/` operator.
`__mod__`(self, other)	modulo operation using `%` operator.
`__pow__`(self, other[, modulo])	calculating the power using `**` operator.
`__lt__`(self, other)	comparison using `<` operator.
`__le__`(self, other)	comparison using `<=` operator.
`__eq__`(self, other)	comparison using `==` operator.
`__ne__`(self, other)	comparison using `!=` operator.
`__ge__`(self, other)	comparison using `>=` operator.

示例： `add`

Python

class Test:
    def __add__(self,dest):
        return self.x + dest.x

    def __init__(self):
        self.x = 10

    def __str__(self):
        return str(self.x)

a = Test()
b = Test()
print(a+b)

示例：`ge`

这个方法重载了操作符>=

Python

class Test:
    def __add__(self,dest):
        return self.x + dest.x

    def __init__(self):
        self.x = 10

    def __str__(self):
        return str(self.x)

    def __ge__(self,dest):
        return self.x >= dest.x

a = Test()
a.x = 15
b = Test()
b.x = 10

print(a>=b)

属性魔法方法

魔术方法	方法描述
`__getattr__`(self, name)	Is called when the accessing attribute of a class that does not exist.
`__setattr__`(self, name, value)	Is called when assigning a value to the attribute of a class.
`__delattr__`(self, name)	Is called when deleting an attribute of a class.

字符串魔术方法

魔术方法	方法描述
`__str__`(self)	To get called by built-int str() method to return a string representation of a type.
`__repr__`(self)	To get called by built-int repr() method to return a machine readable representation of a type.
`__unicode__`(self)	To get called by built-int unicode() method to return an unicode string of a type.
`__format__`(self,formatstr)	To get called by built-int string.format() method to return a new style of string.
`__hash__`(self)	To get called by built-int hash() method to return an integer.
`__nonzero__`(self)	To get called by built-int bool() method to return True or False.
`__dir__`(self)	To get called by built-int dir() method to return a list of attributes of a class.
`__sizeof__`(self)	To get called by built-int sys.getsizeof() method to return the size of an object.

类型转换魔术方法

魔术方法	方法描述
`__int__`(self)	To get called by built-int int() method to convert a type to an int.
`__float__`(self)	To get called by built-int float() method to convert a type to float.
`__complex__`(self)	To get called by built-int complex() method to convert a type to complex.
`__oct__`(self)	To get called by built-int oct() method to convert a type to octal.
`__hex__`(self)	To get called by built-int hex() method to convert a type to hexadecimal.
`__index__`(self)	To get called on type conversion to an int when the object is used in a slice expression.
`__trunc__`(self)	To get called from math.trunc() method.

增加魔术方法

魔术方法	方法描述
`__iadd__`(self, other)	To get called on addition with assignment e.g. a +=b.
`__isub__`(self, other)	To get called on subtraction with assignment e.g. a -=b.
`__imul__`(self, other)	To get called on multiplication with assignment e.g. a *=b.
`__ifloordiv__`(self, other)	To get called on integer division with assignment e.g. a //=b.
`__idiv__`(self, other)	To get called on division with assignment e.g. a /=b.
`__itruediv__`(self, other)	To get called on true division with assignment
`__imod__`(self, other)	To get called on modulo with assignment e.g. a%=b.
`__ipow__`(self, other)	To get called on exponents with assignment e.g. a `**`=b.
`__ilshift__`(self, other)	To get called on left bitwise shift with assignment e.g. a<<=b.
`__irshift__`(self, other)	To get called on right bitwise shift with assignment e.g. a >>=b.
`__iand__`(self, other)	To get called on bitwise AND with assignment e.g. a&=b.
`__ior__`(self, other)	To get called on bitwise OR with assignment e.g. a
`__ixor__`(self, other)	To get called on bitwise XOR with assignment e.g. a ^=b.

一元魔术方法

魔术方法	方法描述
`__pos__`(self)	To get called for unary positive e.g. +someobject.
`__neg__`(self)	To get called for unary negative e.g. -someobject.
`__abs__`(self)	To get called by built-in abs() function.
`__invert__`(self)	To get called for inversion using the ~ operator.
`__round__`(self,n)	To get called by built-in round() function.
`__floor__`(self)	To get called by built-in math.floor() function.
`__ceil__`(self)	To get called by built-in math.ceil() function.
`__trunc__`(self)	To get called by built-in math.trunc() function.

初始化魔术方法

魔术方法	方法描述
`__new__`(cls, other)	To get called in an object's instantiation.
`__init__`(self, other)	To get called by the new method.
`__del__`(self)	Destructor method.

属性描述方法

魔术方法	方法描述
`__get__`(self, instance, owner)
`__set__`(self, instance, value)
`__delete__`

这不是一个好理解的方法。可以借助官方手册进行理解。

示例1：

Python

class Ten:
    def __get__(self, obj, objtype=None):
        return 10
class A:
    x = 5                       # Regular class attribute
    y = Ten()                   # Descriptor instance

a = A()      
print(a.x)                         # Normal attribute lookup
# output 5

print(a.y)                         # Descriptor lookup
# output 10

上下文管理

官方手册

魔术方法	方法描述
`__enter__`(self)
`__exit__`(self, *exc)
`__aenter__`(self)
`__aexit__`(self, *exc)

Python

from contextlib import ContextDecorator

class Context(ContextDecorator):
    def __enter__(self):
        print("__enter__")
        return self

    def __exit__(self, *exc):
        print("__exit__")
        return None

with Context() as c:
    print("in context")

运算符重载魔术方法

示例： __add__

示例：__ge__

属性魔法方法

字符串魔术方法

类型转换魔术方法

增加魔术方法

一元魔术方法

初始化魔术方法

属性描述方法

上下文管理

示例： `add`

示例：`ge`