python中的下划线及双下划线
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 |
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