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_classclass BaseForm(object):    passclass 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。