Cannot collect test classes that are inherited from Protocol

[ThirVondukr]

System Info

• Pytest Version: 8.3.3
• OS: Windows 10

The problem

Currently pytest doesn't collect classes that contain __init__ which seems to conflict with typing.Protocol:

class _BaseTest(Protocol):
    sample_01: bytes
    cls: type[Union[HtpasswdFile, HtdigestFile]]


@typing.final
class TestHtpasswdFile(_BaseTest):
    sample_01 = SAMPLE_01
    cls = HtpasswdFile

typing.Protocol could be very useful if you want to have fields without having to create a method for them with @property.
Protocol seems to be patching __init__ for it's own one that raises a TypeError if it's a protocol, so perhaps an exception could be made for protocol classes here?

[dongfangtianyu]

Hi, @ThirVondukr

It doesn't make sense to treat only protocol classes as exceptions.

Maybe you can try this approach:

class _BaseTest(Protocol):
    sample_01: bytes
    cls: type[Union[HtpasswdFile, HtdigestFile]]


delattr(_BaseTest, '__init__') # Remove unnecessary __init__ from parent class

[ThirVondukr]

Simply deleting init from a class seems like a hack 🤔, it's not a huge issue but I think it's something that pytest should probably support

[RonnyPfannschmidt]

It's tricky to pick reasonable exceptions

What's the intent for using a protocol there

[ThirVondukr]

@RonnyPfannschmidt It's a way for defining an interface for child test classes (I want to parametrize a bunch of tests that way) and type check them that way, it's just more brief compared to using abc.ABC:

class ABCTest(ABC):
    @property
    @abstractmethod
    def a() -> A:
        raise NotImplementedError

    @property
    @abstractmethod
    def b() -> B:
        raise NotImplementedError

class ProtocolTest(Protocol):
    a: A
    b: B

[ThirVondukr]

Honestly I could look into parametrized fixtures instead but classes seemed more explicit to me atm 🤔

[RonnyPfannschmidt]

I believe it's a good idea for a new feature, lets start by just supporting protocol as that didn't exist when init was blocked

[ThirVondukr]

Implementation wise I think you could either import typing._no_init_or_replace_init and check if SomeClass.__init__ is that, or just subclass protocol and get it's init:

from typing import Protocol


class A(Protocol):
    pass


class B(Protocol):
    pass


assert A.__init__ is B.__init__