Overzealous Optimization For `isinstance`

[George-Ogden]

Bug report

Bug description:

The custom __instancecheck__ dunder method allows full customization (described in the Data model).

However, it produces a spurious result:

class CustomIsInstanceMeta(type):
    def __instancecheck__(cls, instance: object) -> bool:
        return type(instance) is int

class CustomIsInstance(metaclass=CustomIsInstanceMeta):
    ...

class CustomIsInstanceSubclass(CustomIsInstance):
    ...


print(isinstance(5, CustomIsInstance)) # True
print(isinstance(CustomIsInstance(), CustomIsInstance)) # True (should be False)
print(isinstance(CustomIsInstanceSubclass(), CustomIsInstance)) # False

This is caused by:

cpython/Objects/abstract.c

Lines 2641 to 2644 in 23c488d

	/* Quick test for an exact match */
	if (Py_IS_TYPE(inst, (PyTypeObject *)cls)) {
	return 1;
	}

which is invalid in this case.

This is a reframing of #79264 (request to fix documentation)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-144873: Remove Overzealous isinstance Optimization #144874

[ZeroIntensity]

Removing this will likely harm performance, and I'm sure people are relying on this in some way already.

I think it would be better to update the documentation. Do you have a concrete example of where it would be useful for a type to declare an instance of itself as not an instance, other than party tricks?