(🐞) False error with assignment of generic union of types to the same union of types
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bug
- Dominant language
- Python
- Stars
- 20.6k
- Forks
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Description
from typing import TypeVar
class A: pass
class B: pass
T_AB = TypeVar("T_AB", bound=A | B)
def foo(t: type[T_AB]) -> None:
x: type[A | B] = t # error
It works when the type is in the bound:
from typing import TypeVar
class A: pass
class B: pass
T_AB = TypeVar("T_AB", bound=type[A | B])
def foo(t: T_AB) -> None:
x: type[A | B] = t # no error
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running the two provided snippets through mypy and comparing the diagnostic on the assignment to type[A | B]. Trace the generic type-variable and union compatibility checks involved. Done means the first snippet is accepted without an error while the second example remains accepted.
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