实现用于更改类级别变量的python元类

时间:2018-09-05 11:53:20

标签: python python-3.x metaclass

您能解释一下如何实现AFactory类吗?看来我需要使用元类,但是如何使用?我所有的尝试都因KeyError

而失败
dct = {
    'key1': 'value1',
    'key2': 'value2'
}

class AFactory:
    pass

class A:
    NAME = ''
    VARIABLE = dct[NAME]

A1 = AFactory('key1')
a1 = A1()
assert a1.NAME == 'key1'
assert a1.VARIABLE == 'value1'

A2 = AFactory('key2')
a2 = A2()
assert a2.NAME == 'key2'
assert a2.VARIABLE == 'value2'

1 个答案:

答案 0 :(得分:2)

听起来您真的想要一个类工厂,而不是一个元类。 (是的,元类也是类工厂,但不是唯一的类工厂。)因此,最简单的解决方案是将AFactory定义为一个函数:

def AFactory(name):
    class A:
        NAME = name
        VARIABLE = dct[NAME]

    return A

如果您确实需要一个元类,那么您should implement an alternative constructor而不是尝试使该元类可调用为AFactory(name)

class AFactory(type):
    @classmethod
    def make(mcs, name):
        clsname = 'A'
        bases = ()
        attrs = {
            'NAME': name,
            'VARIABLE': dct[name]
        }

        return mcs(clsname, bases, attrs)

然后您可以使用like

A1 = AFactory.make('key1')