Confusing message if callable is incompatible due to argument name

Open
#4,530 6 comments 1 reaction 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Start by running the supplied Python example through mypy and compare the diagnostic with the callable annotations, especially the missing argument name in y. Trace the callable compatibility diagnostic and update it so the incompatibility identifies the argument-name mismatch; done means the error gives that hint for this reproduction.

Written by the indexing model from the issue text.

Description

bug priority-1-normal topic-calls topic-error-reporting topic-usability

Mypy generates a confusing error for this program:

from typing import Callable, Union
from mypy_extensions import Arg

def f(x: Callable[[Arg(int, 'x')], None]) -> None: pass

y: Callable[[Union[int, str]], None]
f(y)  # error

Here is the output:

t.py:7: error: Argument 1 to "f" has incompatible type "Callable[[Union[int, str]], None]"; expected "Callable[[int], None]"

The actual error is the missing argument name in the type of y, but the error message gives no hint about this.

Dominant language
Python
Stars
20.6k
Forks
3.3k
Avg merge
1d 18h
Merged PRs (30d)
54

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from python/mypy

All issues in python/mypy

Similar issues

More Python issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.