Special usage of metaclasses in python

Question

I was messing around with python metaclasses and I came across something very intriguing that I didn't know existed or was valid:

class Meta(type):
    def __new__(metacls, name, bases, kwargs, extra_thing=None):
        if extra_thing is not None:
            print(extra_thing)
        return super().__new__(metacls, name, bases, kwargs)
    def __init__(metacls, name, bases, kwargs, *args, **kwargs_):
        super().__init__(metacls, name, bases, kwargs)

class TestClass(metaclass=Meta, extra_thing='foo'):
    pass

When you run this, 'foo' is printed.

So apparently, classes can have extra keyword arguments if their metaclasses allow for it.

If a scenario in which this functionality may be useful is found, is it okay to take advantage of it? Or should it be left alone in favour of other methods (such as defining a class wide variable extra_thing)?

What do you mean by "is it okay"? Do you mean "is it documented behavior that I can rely on" or "is it good design"? — BrenBarn
– BrenBarn, Commented May 27, 2014 at 1:50

Blckknght · Accepted Answer · 2014-05-27 01:51:58Z

2

The behavior is documented, so it's probably safe to use, if perhaps confusing to other programmers reading your code. Here's what the docs say:

Any other keyword arguments that are specified in the class definition are passed through to all metaclass operations described below.

The "operations" it describes are __prepare__ and calling metaclass itself (which will usually call the __new__ and/or __init__ methods of the metaclass).

answered May 27, 2014 at 1:51

Blckknght

106k11 gold badges135 silver badges188 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Special usage of metaclasses in python

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related