implementing python metaclass for changing class level variable

Question

Can you explain, how to implement AFactory class for doing this. It seems that I need to use metaclass but how? All my tries failed with KeyError

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'

Why does AFactory have to be a metaclass? Can't it be a function instead? — Aran-Fey
– Aran-Fey, Commented Sep 5, 2018 at 12:06
ok, it can be function or simple class, but how to pass dynamically NAME in A class? — timKa
– timKa, Commented Sep 5, 2018 at 12:13

Aran-Fey · Accepted Answer · 2018-09-05 18:48:53Z

2

It sounds like you really want a class factory, not a metaclass. (Yes, metaclasses are also class factories, but they're not the only ones.) So the easiest solution is to define AFactory as a function:

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

    return A

If you really need a metaclass, you should implement an alternative constructor rather than trying to make the metaclass callable as AFactory(name):

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

        return mcs(clsname, bases, attrs)

Which you could then use like

A1 = AFactory.make('key1')

edited Sep 5, 2018 at 18:48

answered Sep 5, 2018 at 12:40

Aran-Fey

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