super().__init__ doesn't work in explicit protocol subclasses

[ilevkivskyi]

A possible solution is to append a call to existing __init__ while overwriting (after the check for _is_protocol=True, where we unconditionally raise because protocols can't be instantiated). This way explicit non-protocol subclasses can use a protocol __init__ as a "default constructor". Default implementations are already supported for other protocol members, so it is natural to expect that __init__ is not different. See also python/mypy#3823

cc: @JukkaL

[grahamc]

I have run in to this problem with @adisbladis, and he came up with this workaround:

if not TYPE_CHECKING:
    from typing import Generic
    Protocol = Generic
    def runtime_checkable(f):
        return f
else:
    from typing_extensions import Protocol, runtime_checkable

I accidentally commented this on #561 at first, my apologies.

[Yobmod]

Hi, I've just come across this in our testing when updating from 3.10.0.b4 to 3.10.0.rc1.
There's been some change made to protocols for 3.10.0 that change runtime behaviour?

In my case, the protocol classes don't have an __init__(), so I didn't expect them to cause an error, and they didn't with 3.8 or 3.9.

I can't see what the change is in the 3.10 release notes, maybe the fix for runtime_checkable protocols with only attributes? (But we have methods, just no init().)

[grahamc]

@Yobmod this bug has been open for 3 years, I wonder if you did some research that ties such a very recent change to this bug in particular?

[hauntsaninja]

https://bugs.python.org/issue44806 / python/cpython#27545

[Yobmod]

@hauntsaninja Thanks for the heads-up! (I put in a workaround for now, but I'll check if fixed next rc).

[srittau]

Closing here as this has been filed to bpo.