Django中的元类

看Django（1.6）的Form相关源代码时比较迷惑，于是节选了django.forms.forms.py中的几个代码片段来分析Django中是怎么使用元类的：

 1 def with_metaclass(meta, *bases):
 2     """Create a base class with a metaclass."""
 3     return meta("NewBase", bases, {})
 4 
 5 class DeclarativeFieldsMetaclass(type):
 6     def __new__(cls, name, bases, attrs):
 7         print('cls: %s, name: %s, bases: %s ,attrs: %s
' % (cls, name, bases, attrs))
 8 
 9         new_class = super(DeclarativeFieldsMetaclass, cls).__new__(cls, name, bases, attrs)
10 #        new_class._meta = '123'
11         return new_class
12 
13 class BaseForm(object):
14     pass
15 
16 class Form(with_metaclass(DeclarativeFieldsMetaclass, BaseForm)):
17     pass
18     
19 class MyForm(Form):
20     a = 1
21     b = 2

加载上面的Python模块，控制台会输出：

cls: <class '__main__.DeclarativeFieldsMetaclass'>, name: NewBase, bases: (<class '__main__.BaseForm'>,), attrs: {}

cls: <class '__main__.DeclarativeFieldsMetaclass'>, name: Form, bases: (<class '__main__.NewBase'>,), attrs: {'__module__': '__main__'}

cls: <class '__main__.DeclarativeFieldsMetaclass'>, name: MyForm, bases: (<class '__main__.Form'>,), attrs: {'a': 1, '__module__': '__main__', 'b': 2}

虽然没有直接在代码中为MyForm指定metaclass，但由于MyForm继承自Form，而Form继承自DeclarativeFieldsMetaclass生成的类名为"NewBase"的类，所以DeclarativeFieldsMetaclass实际上就是MyForm的metaclass。