python中那些双下划线开头得函数和变量

python中以双下划线的是一些系统定义得名称，让python以更优雅得语法实行一些操作，本质上还是一些函数和变量，与其他函数和变量无二。

比如x.__add__(y) 等价于 x+y

有一些很常见，有一些可能比较偏，在这里罗列一下，做个笔记，备忘。

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__(...) == x(...)

x.__cmp__(y) == cmp(x,y)

x.__getattribute__('name') == x.name == getattr(x, 'name'), 比__getattr__更早调用

x.__hash__() == 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') == 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

This is an explaination about "__mro__" which quoted from stackoverflow.com:

ref : http://stackoverflow.com/questions/16663514/why-does-mro-not-show-up-in-dirmyclass
From the Python documentation:

Because dir() is supplied primarily as a convenience for use at an interactive prompt, it tries to supply an interesting set of names more than it tries to supply a rigorously or consistently defined set of names, and its detailed behavior may change across
releases. For example, metaclass attributes are not in the result list when the argument is a class.

__mro__

is a read-only attribute that's used to determine method resolution in case your class inherits from multiple base classes. If you wish to customize this behavior, you should use a metaclass (a special kind of object whose purpose is
to create class instances) which overrides the

mro()

method.

__mro__

is left unchanged in any case.