Should isdataclass return True for both classes and instances?

[ericvsmith]

Raised by Carl Meyer in https://mail.python.org/pipermail/python-dev/2017-November/150850.html.

I think he's probably correct. The current behavior, where it only works on instances, is surprising. The equivalent attrs API, attr.has(), returns True for both classes and instances.

[gvanrossum]

I'm not keen on perpetuation confusion between classes and instances -- it makes code harder to read. E.g. when I see

def foo(a):
    assert isdataclass(a)

I can't tell whether a is supposed to be a class or an instance.

[ilevkivskyi]

Also note that if we are going to change this, then asdict and astuple should be updated, since they use isdataclass to work only on instances.

[ericvsmith]

I've already verified that there's an existing test that fails if isdataclass is changed.

[ericvsmith]

I think I'm just going to delete isdataclass, since I can't come up with a good use case for it. It's easy enough to add later.

[ericvsmith]

I've added on how to implement isdataclass() to the PEP.