python / python/mypy

`tuple` of different `type`s narrows to `object`

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bug topic-join-v-union
Dominant language
Python
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Description

Bug Report

mypy fails to infer the type variable in a arbitrary length tuple.

To Reproduce

from typing import TypeVar, reveal_type

T = TypeVar('T')

def get_type_hint(val_type: tuple[type[T]] | tuple[type[T], ...]) -> T:
    return None  # type: ignore

a: tuple[type[int | str], ...] = (str, int)

reveal_type(a)
reveal_type(get_type_hint((str,)))
reveal_type(get_type_hint((str, int)))
reveal_type(get_type_hint(a))

Expected Behavior

The reveal_type for calling (str, int) and a should be int | str.

Actual Behavior

Type is inferred to narrow type as object.

tuple_infer.py:10: note: Revealed type is "builtins.tuple[Union[Type[builtins.int], Type[builtins.str]], ...]"
tuple_infer.py:11: note: Revealed type is "builtins.str"   
tuple_infer.py:12: note: Revealed type is "builtins.object"
tuple_infer.py:13: note: Revealed type is "builtins.object"

Your Environment

  • Mypy version used: 1.1.1
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.11.1

Contributor guide

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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

Reproduce the example in tuple_infer.py with mypy 1.1.1 and inspect the reveal_type output for the heterogeneous tuple calls. Trace the type inference behavior behind get_type_hint; done when the calls with (str, int) and a reveal int | str rather than object.

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
35/100

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