assert_type() not matching function type

[DiegoBaldassarMilleuno]

Bug Report

'assert_type()' refuses to match a function to its own type.

To Reproduce

from typing import assert_type, Callable
def myfunc(arg: int) -> None: pass
assert_type(myfunc, Callable[[int], None])

# Gist URL: https://gist.github.com/mypy-play/50dde8e891d971d823c4c6af2e639834
# Playground URL: https://mypy-play.net/?mypy=latest&python=3.11&gist=50dde8e891d971d823c4c6af2e639834

Expected Behavior

No error.

Actual Behavior

main.py:5: error: Expression is of type "Callable[[int], None]", not "Callable[[int], None]"  [assert-type]
Found 1 error in 1 file (checked 1 source file)

The error disappears if the function takes no arguments.

Your Environment

• Mypy version used: both 1.2.0 and current master branch (commit 29cc561)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.11

[erictraut]

The two types are not equivalent. The function myfunc accepts a keyword argument for a parameter named arg, but Callable[[int], None] accepts an argument by position only. If you add a positional-only parameter marker to myfunc, they will be equivalent.

def myfunc(arg: int, /) -> None:
    pass

[ikonst]

What should we show in the error message, given there's no special form for this?

[AlexWaygood]

@erictraut is correct, but the error message mypy gives here isn't particularly helpful. Let's keep this issue open to track improving the error message.

[erictraut]

FWIW, here's the error message that pyright emits:

"assert_type" mismatch: expected "(int) -> None" but received "(arg: int) -> None"

In pyright's error messages, I output a callable syntax that is based on the proposed (but unfortunately rejected) PEP 677. Not sure if this is something mypy maintainers would consider adopting.

[DiegoBaldassarMilleuno]

I see, thank you very much for the clarification. The error message had me assuming it was a bug.

[JelleZijlstra]

It's definitely an error message bug, since we mention the same string on both sides.

[madt2709]

This has been fixed on the master branch. The error message now shows:

main.py:5: error: Expression is of type "Callable[[Arg(int, 'arg')], None]", not "Callable[[int], None]"  [assert-type]
Found 1 error in 1 file (checked 1 source file)

You can see this in the playground: https://mypy-play.net/?mypy=master&python=3.11&gist=50dde8e891d971d823c4c6af2e639834

[ikonst]

Apparently fixed by #15184.

@AlexWaygood, can we close?

[AlexWaygood]

Apparently fixed by #15184.

@AlexWaygood, can we close?

Doesn't look like a regression test was added for this specific case — fancy making a PR? :)