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：
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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
' % (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。
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