(🐞) Strings as `TypeAlias`s are incorrectly assumed to be `type`s
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bug
topic-runtime-semantics
topic-type-alias
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
from typing import TypeAlias
A: TypeAlias = "str"
isinstance(1, A) # no error
reveal_type(A) # type[str]
Parent issue of #11582
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 Python reproducer in the issue and read parent issue #11582 for the intended TypeAlias behavior. Trace how mypy handles a string-valued TypeAlias in isinstance and reveal_type; done means it no longer treats the string alias as a runtime type incorrectly, with the reported example covered by verification.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100