assert_type: Use fully qualified types when names are ambiguous

[JelleZijlstra]

On current master, this code:

from typing import TypeVar
import typing
import typing_extensions

U = TypeVar("U", int, typing_extensions.SupportsIndex)

def f(si: U) -> U:
    return si

def caller(si: typing.SupportsIndex):
    typing_extensions.assert_type(f(si), typing.SupportsIndex)

produces

test_cases/stdlib/itertools/mypybug.py:11: error: Expression is of type "SupportsIndex", not "SupportsIndex"

This is arguably correct, since the expression is of type typing_extensions.SupportsIndex and we wanted typing.SupportsIndex, but obviously the UX is terrible. We should output the fully qualified names.

More broadly, perhaps assert_type() should consider two protocols that define identical methods to be the same (so the assertion should pass). Curious what other people think here.

[jwodder]

I don't know if this is the exact same bug or not, but I just discovered that this code:

from typing_extensions import assert_type

assert_type(42, int)

fails to typecheck with mypy (both 0.960 and commit 1636a05) with the following message:

assert-type01.py:3: error: Expression is of type "int", not "int"

[JelleZijlstra]

@jwodder wow that's unfortunate. I think it's a different problem though, could you open a new issue?

[jwodder]

@JelleZijlstra Issue created: #12923

[JukkaL]

Implementation hints: to fix this, it may be sufficient to use format_type_distinctly to convert the types to strings when generating the error message.

[teresa0605]

Hi! Is this issue still open? I would like to work on it.

[JelleZijlstra]

Yes! Jukka's message above should provide a way to fix the issue. Feel free to ask here if you have any questions.

[teresa0605]

from typing import TypeVar
import typing
import typing_extensions

U = TypeVar("U", int, typing_extensions.SupportsIndex)

def f(si: U) -> U:
    return si

def caller(si: typing.SupportsIndex):
    typing_extensions.assert_type(f(si), typing.SupportsIndex)

This code doesn't produce any errors in current master branch. Is this fixed already or should the behavior be outputing the fully qualified names?

[JelleZijlstra]

The original repro case is a bit convoluted, not sure why I didn't do something simpler. However, consider this case:

import typing_extensions

class SupportsIndex:
    pass

def f(si: SupportsIndex):
    typing_extensions.assert_type(si, typing_extensions.SupportsIndex)

This still produces main.py:8: error: Expression is of type "SupportsIndex", not "SupportsIndex" [assert-type].

I would want it to say something like main.py:8: error: Expression is of type "my_module.SupportsIndex", not "typing_extensions.SupportsIndex" [assert-type]