python / python/mypy

Type of generic property subclass reveals <nothing>

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Description

Bug Report

When subclassing property and returning a generic type, mypy reveals <nothing> as the type of the property.
Pyright reveals the expected type.

To Reproduce

from typing import Any, Generic, Optional, Type, TypeVar

T = TypeVar("T")

class customProperty(property, Generic[T]):
    def __get__(self, obj: Any, owner: Optional[Type] = None) -> T:
        return super().__get__(obj, owner)

class C:
    @customProperty
    def prop(self) -> int:
        return 1

reveal_type(C().prop)

Expected Behavior

Revealed type is "builtins.int"

Actual Behavior

Revealed type is "<nothing>"

Your Environment

  • Mypy version used: 0.950
  • Mypy command-line flags: none
  • Mypy configuration options from mypy.ini (and other config files): none
  • Python version used: 3.10
  • Operating system and version: Ubuntu 22.04

Pyright 1.1.244 however seems to get it right and returns the expected type
Type of "C().prop" is "int"

This could be related to #11115

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

Start by running the provided Python reproduction with mypy 0.950 or a current checkout and compare the reveal_type result with the expected int. Trace generic property subclass handling and the related issue #11115; done means C().prop is revealed as builtins.int rather than , with a regression test for the case.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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