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：

def with_metaclass(meta, *bases):
"""Create a base class with a metaclass."""
return meta("NewBase", bases, {})

class DeclarativeFieldsMetaclass(type):
def __new__(cls, name, bases, attrs):
print('cls: %s, name: %s, bases: %s ,attrs: %s\n' % (cls, name, bases, attrs))

new_class = super(DeclarativeFieldsMetaclass, cls).__new__(cls, name, bases, attrs)
#        new_class._meta = '123'
return new_class

class BaseForm(object):
pass

class Form(with_metaclass(DeclarativeFieldsMetaclass, BaseForm)):
pass

class MyForm(Form):
a = 1
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。