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Django中的元类

程序员文章站 2022-04-24 08:48:09
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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: , name: NewBase, bases: (,), attrs: {}

cls: , name: Form, bases: (,), attrs: {'__module__': '__main__'}

cls: , name: MyForm, bases: (,), 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。

相关标签: Django中的元类