Passing a list as an argument with * leads to confusing error

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#3,224 3 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
tooling

Research direction

The issue names no source file or test. Start by reproducing the minimal Python example and trace mypy's handling of starred list arguments and its diagnostic location. Done means the diagnostic is corrected to identify the second argument, or the valid call is accepted if that behavior is implemented.

Written by the indexing model from the issue text.

Description

topic-calls topic-error-reporting topic-usability

(Originally reported at https://github.com/python/typeshed/issues/814, which gives an example from real code.)

def foo(a: int, b: str = '') -> None: ...

foo(1)  # OK
foo(*[1])  # E: Argument 1 to "foo" has incompatible type *List[int]; expected "str"

Ideally this shouldn't be an error at all (this works fine at runtime), but I imagine it's hard for mypy to accept this code since it doesn't know how long the list is. However, we could at least fix the error message, which talks about "argument 1" needing to be a str when really it's argument 2 that is the problem.

Dominant language
Python
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