看Django（1.6）的Form相關原始碼時比較迷惑，於是節選了django.forms.forms.py中的幾個程式碼片段來分析Django中是怎麼使用元類的：
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載入上面的Python模組，控制檯會輸出：
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}
雖然沒有直接在程式碼中為MyForm指定metaclass，但由於MyForm繼承自Form，而Form繼承自DeclarativeFieldsMetaclass生成的類名為"NewBase"的類，所以DeclarativeFieldsMetaclass實際上就是MyForm的metaclass。