Python override metaclass set attribute

Question

Suppose I have a class:

class ABC:
    pass

And I do:

ABC.x = 123

This does not give me an error, even though I haven't created an object. Which of the __func__ methods is being called, and how do I override it?

You're setting the attribute on the class object. So __setattr__ is called, but it's called on the metaclass. Look up metaclasses to get an idea of what you're in for. — BrenBarn
– BrenBarn, Commented Feb 21, 2017 at 6:14
@BrenBarn thanks for the advice, I shall do that soon. However, to solve my immediate problem, do you mind submitting an answer? :) — Abhishek Divekar
– Abhishek Divekar, Commented Feb 21, 2017 at 6:23

BrenBarn · Accepted Answer · 2017-02-21 06:49:34Z

7

You need to define a metaclass and override __setattr__ there, then make your class use that metaclass.

class Meta(type):
    def __new__(meta, name, bases, attrs):
        return super().__new__(meta, name, bases, attrs)

    def __setattr__(cls, attr, value):
        print("I am setting {}.{} = {}".format(cls.__name__, attr, value))
        return super().__setattr__(attr, value)

class Foo(metaclass=Meta):
    pass

Then:

>>> Foo.blah = 2
I am setting Foo.blah = 2
>>> Foo.blah
2

answered Feb 21, 2017 at 6:49

BrenBarn

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