Unsound typing using isinstance
Open
Nobody has claimed this yet.
bug
priority-0-high
topic-type-narrowing
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
isinstance make mypy assume a too strong hypothesis on the type of the value, for example:
def foo(t : list[int | str]) -> int :
assert(isinstance(t[0], int))
t[0] = "coucou"
return t[0]
x : list[int | str] = [1, 2]
foo(x)
typechecks correctly while the type annotation is not correct.
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 reproducing the minimal Python example in the issue and inspect how mypy handles isinstance-based narrowing for mutable list elements. The issue names no source files or tests, so locate the relevant type-checking entry point and existing narrowing tests first; done means the unsound example is no longer accepted while valid isinstance narrowing remains supported.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100