什么是metaclass???

假如你想创建一个类（注意是类，不是类的实例），那么你可以使用__mateclass__属性来告诉这个类怎么来构造。

生成类的方式

使用class关键字创建一个类
使用type生成类
使用type的子类来生成类

Metaclass

class Interface(object):
    __metaclass__ = InterfaceMeta
    file = 'tmp.txt'

print(Interface.class_id)
print(Interface.file)

__metaclass__

by defining the __metaclass__ attribute of the class, we've told the class that it should be constructed using InterfaceMeta rather than using type. To make this more definite, observe that the type of Interface is now InterfaceMeta:

type(Interface)

__main__.InterfaceMeta

class

object

type

classes are objects, and they are object of type type

type(name, bases, dict) -> a new type

type(name, bases, dct)

name is a string giving the name of the class to be constructed
bases is a tuple giving the parent classes of the class to be constructed
dct is a dictionary of the attributes and methods of the class to be constructed decorator

Custom Metaclasses(定制属于你的元类)

Now things get really fun. Just as we can inherit from and extend a class we've created, we can also inherit from and extend the type metaclass, and create custom behavior in our metaclass.

metaclass和继承的区别？

python cookbook 3

9.13 using a metaclass to control instance creation

Using a metaclass to implement various instance create pattern can offer be a much elegant approach than other solutions not invoving metaclasses.

singleton(单例模式) 或者不允许实例化的类

example

Django meta

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