python / python/mypy

Generic type not inferred when valid constraint available

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#18,837 0 comments 0 reactions 0 assignees View on GitHub

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bug topic-type-variables
Dominant language
Python
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Description

from typing import reveal_type


def foo[T: (int, str, int | str)](*args: T) -> tuple[T, ...]:
    return args


a = foo(0)
b = foo('')
c = foo(0, '')
d = foo('', 0)

reveal_type(a)
reveal_type(b)
reveal_type(c)
reveal_type(d)
$ mypy example.py
example.py:10: error: Value of type variable "T" of "foo" cannot be "object"  [type-var]
example.py:11: error: Value of type variable "T" of "foo" cannot be "object"  [type-var]
example.py:13: note: Revealed type is "builtins.tuple[builtins.int, ...]"
example.py:14: note: Revealed type is "builtins.tuple[builtins.str, ...]"
example.py:15: note: Revealed type is "builtins.tuple[builtins.object, ...]"
example.py:16: note: Revealed type is "builtins.tuple[builtins.object, ...]"
Found 2 errors in 1 file (checked 1 source file)

I'd expect c and d to be of type tuple[int | str, ...] as allowed by the constraints on T

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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.

Research direction

Start by running the example.py reproduction with mypy and compare the revealed types and errors for all four calls. Trace type-variable constraint inference for the generic foo definition; done when the mixed int and str calls are accepted and reveal tuple[int | str, ...].

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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