python / python/mypy

TypeVar declared at class scope with generic bound specialized on an in-scope generic parameter TypeVar doesn't give an error

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bug
Dominant language
Python
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Description

Bug Report

I don't think that TypeVars at class scope have any particular meaning, but if they reject bound=T they ought to also reject bound=G[T] in the same way.

To Reproduce

from typing import TypeVar, Generic

T = TypeVar("T")

class A:
    pass

class B(A):
    pass

class G(Generic[T]):
    pass

anA = TypeVar("anA", bound=A)

class X(Generic[anA]):
    # error, as expected
    # AnInnerA = TypeVar("AnInnerA", bound=anA)
    AnInnerG = TypeVar("AnInnerG", bound=G[anA])

    def method(self, val: AnInnerG) -> None:
        pass

x: X[A] = X()
g: G[A] = G()

x.method(g)

playground

Expected Behavior

I would have expected to see an error on the line where AnInnerG was defined.

Actual Behavior

No error on AnInnerG's definition, but I cannot find any type I can pass to X.method without provoking an error.

main.py:27: error: Value of type variable "AnInnerG" of "method" of "X" cannot be "G[A]"  [type-var]
Found 1 error in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 1.4.1
  • Mypy command-line flags: --strict
  • Mypy configuration options from mypy.ini (and other config files): n/a
  • Python version used: 3.11

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 supplied Python reproduction with mypy 1.4.1 and --strict, then compare it with the linked playground. Investigate how class-scope TypeVars and generic bounds are validated; done means the invalid AnInnerG declaration produces the expected diagnostic rather than only failing at the method call.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
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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