python / python/mypy

Generic Type[T] function argument rejected as base class

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bug
Dominant language
Python
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Description

Bug Report

Since https://github.com/python/mypy/issues/5865 was fixed, the following code now type checks:

from typing import Any, Type


def f(typ: Type[Any]) -> Type[Any]:
    class C(typ):
        pass

    return C

However if we make the superclass generic, it doesn't type check anymore which I would expect it to do.

To Reproduce

from typing import Type, TypeVar

T = TypeVar("T")


def f(typ: Type[T]) -> Type[T]:
    class C(typ):
        pass

    return C

Actual Behavior

mypy_return_type.py:7: error: Variable "typ" is not valid as a type  [valid-type]
mypy_return_type.py:7: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
mypy_return_type.py:7: error: Invalid base class "typ"  [misc]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 1.0.0+dev.e9f5858c44b61c2d02819940debe3c31d099f9d5
  • Mypy command-line flags: mypy mypy_return_type.py
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.10.9

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.

Research direction

Reproduce the report with the provided generic Type[T] example using mypy mypy_return_type.py, then trace the type-checking path for variable expressions used as base classes and compare it with the non-generic Type[Any] case. Done means the generic example type-checks without the valid-type and Invalid base class errors while retaining its Type[T] return type.

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
Clearly specified
Newbie friendliness
38/100

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