一、Python 用下划线作为变量前缀和后缀指定特殊变量

1. 单下划线开头：

_xxx：弱“内部使用”标识，如：”from Module import *”，将不导入所有以下划线开头的对象，包括包、模块、成员

2. 双下划线开头：

__xxx：模块内的私有成员，外部无法直接调用。

即：私有类型的变量。只能是允许这个类本身进行访问了。连子类也不可以

3. 双下划线开头和结尾:

__xxx__ :系统定义名字, 用户无法控制的命名空间中的“魔术”对象或属性, 如：

__name__、__doc__、__init__、__import__、__file__等；

二、python常用的魔法方法及变量

转自：http://www.cnblogs.com/DxSoft/p/3506627.html

__new__： 是一个类的初始化过程中第一个被执行的方法。它创建了类，然后把一些参数传递给__init__

_init__(self, [...): 类的构造器，当初始构造方法被执行

__del__(self)：如果 __new__ 和 __init__ 形成一个类的构造函数，__del__ 是就是析构函数

__str__(self)：定义当 str() 被你的一个类的实例调用时所要产生的行为。

Magic

Method

何时被调用(例子)

Explanation

__new__(cls [,...])

instance = MyClass(arg1, arg2)

__new__ is called on instance creation

__init__(self [,...])

instance = MyClass(arg1, arg2)

__init__ is called on instance creation

__cmp__(self, other)

self == other, self > other, etc.

Called for any comparison

__pos__(self)

+self

Unary plus sign

__neg__(self)

-self

Unary minus sign

__invert__(self)

~self

Bitwise inversion

__index__(self)

x[self]

Conversion when object is used as index

__nonzero__(self)

bool(self)

Boolean value of the object

__getattr__(self, name)

self.name # name doesn‘t exist

Accessing nonexistent attribute

__setattr__(self, name, val)

self.name = val

Assigning to an attribute

__delattr__(self, name)

del self.name

Deleting an attribute

__getattribute__(self, name)

self.name

Accessing any attribute

__getitem__(self, key)

self[key]

Accessing an item using an index

__setitem__(self, key, val)

self[key] = val

Assigning to an item using an index

__delitem__(self, key)

del self[key]

Deleting an item using an index

__iter__(self)

for x in self

Iteration

__contains__(self, value)

value in self,value not in self

Membership tests using in

__call__(self [,...])

self(args)

"Calling" an instance

__enter__(self)

with self as x:

with statement context managers

__exit__(self, exc, val, trace)

with self as x:

with statement context managers

__getstate__(self)

pickle.dump(pkl_file, self)

Pickling

__setstate__(self)

data = pickle.load(pkl_file)

Pickling

三、 双下划线开头和结尾的函数及变量枚举：

x.__add__(y)          等价于  x+y

x.__contains__(y)   等价于 y in x, 在list,str, dict,set等容器中有这个函数

__base__, __bases__, __mro__,    关于类继承和函数查找路径的

class.__subclasses__(),                 返回子类列表

x.__call__(...) 等价于   x(...)

x.__cmp__(y) 等价于   cmp(x,y)

x.__getattribute__(‘name‘) 等价于  x.name   等价于  getattr(x, ‘name‘),  比__getattr__更早调用

x.__hash__()  等价于   hash(x)

x.__sizeof__(), x在内存中的字节数, x为class得话， 就应该是x.__basicsize__

x.__delattr__(‘name‘) 等价于 del x.name

__dictoffset__ attribute tells you the offset to where you find the pointer to the __dict__ object in any instance object that has one. It is in bytes.

__flags__, 返回一串数字，用来判断该类型能否被序列化(if it‘s a heap type), __flags__ & 512

S.__format__, 有些类有用

x.__getitem__(y) == x[y], 相应还有__setitem__, 某些不可修改类型如set，str没有__setitem__

x.__getslice__(i, j) == x[i:j], 有个疑问，x=‘123456789‘, x[::2],是咋实现得

__subclasscheck__(), check if a class is subclass

__instancecheck__(), check if an object is an instance

__itemsize__, These fields allow calculating the size in bytes of instances of the type. 0是可变长度， 非0则是固定长度

x.__mod__(y) == x%y, x.__rmod__(y) == y%x

x.__module__ , x所属模块

x.__mul__(y) == x*y,  x.__rmul__(y) == y*x

__reduce__, __reduce_ex__ , for pickle

__slots__ 使用之后类变成静态一样，没有了__dict__, 实例也不可新添加属性

__getattr__ 在一般的查找属性查找不到之后会调用此函数

__setattr__ 取代一般的赋值操作，如果有此函数会调用此函数， 如想调用正常赋值途径用 object.__setattr__(self, name, value)

__delattr__ 同__setattr__, 在del obj.name有意义时会调用

Magic

Method

何时被调用(例子)

Explanation

__new__(cls [,...])

instance = MyClass(arg1, arg2)

__new__ is called on instance creation

__init__(self [,...])

instance = MyClass(arg1, arg2)

__init__ is called on instance creation

__cmp__(self, other)

self == other, self > other, etc.

Called for any comparison

__pos__(self)

+self

Unary plus sign

__neg__(self)

-self

Unary minus sign

__invert__(self)

~self

Bitwise inversion

__index__(self)

x[self]

Conversion when object is used as index

__nonzero__(self)

bool(self)

Boolean value of the object

__getattr__(self, name)

self.name # name doesn‘t exist

Accessing nonexistent attribute

__setattr__(self, name, val)

self.name = val

Assigning to an attribute

__delattr__(self, name)

del self.name

Deleting an attribute

__getattribute__(self, name)

self.name

Accessing any attribute

__getitem__(self, key)

self[key]

Accessing an item using an index

__setitem__(self, key, val)

self[key] = val

Assigning to an item using an index

__delitem__(self, key)

del self[key]

Deleting an item using an index

__iter__(self)

for x in self

Iteration

__contains__(self, value)

value in self,value not in self

Membership tests using in

__call__(self [,...])

self(args)

"Calling" an instance

__enter__(self)

with self as x:

with statement context managers

__exit__(self, exc, val, trace)

with self as x:

with statement context managers

__getstate__(self)

pickle.dump(pkl_file, self)

Pickling

__setstate__(self)

data = pickle.load(pkl_file)

Pickling

原文：http://www.cnblogs.com/tanql/p/3914583.html