Class "keyword arguments" in Python 3

Question

I noticed this from the docstring of __build_class__:

__build_class__(func, name, *bases, metaclass=None, **kwds) -> class

Internal helper function used by the class statement.

The part that intrigued me was the **kwds part. Can class definitions take keyword arguments? I tried it, but I got a very strange error:

>>> class Test(a=1):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type() takes 1 or 3 arguments

What's the deal here? Can classes in Python 3 somehow accept keyword arguments? Maybe a special metaclass is required?

user2357112 · Accepted Answer · 2014-01-26 05:55:31Z

3

Can classes in Python 3 somehow accept keyword arguments?

Yes. Any keyword arguments in the class statement besides metaclass are passed to the metaclass. If the metaclass argument is specified, it's used as the metaclass; otherwise, the metaclass is type. See PEP 3115 for more details.

answered Jan 26, 2014 at 5:55

user2357112

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1 Comment

asmeurer Over a year ago

I see. Assumedly this was just implemented for the sake of the new metaclass= syntax, since I think the same thing should be possible by just using a metaclass factory, like class MyClass(metaclass=get_metaclass(**kwargs)).

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