Redundant cast warning in nested function in a narrowed block
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bug
false-positive
priority-0-high
topic-casts
- Dominant language
- Python
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Description
mypy version: 0.641
Given the following (in a file named test.py):
from typing import Optional, List, cast
things: Optional[List[int]] = None
others: List[int] = []
if things is not None:
filtered = filter(lambda x: x in cast(List[int], things), others)
Running mypy --warn-redundant-casts test.py gives the following warning:
test.py:7: note: Redundant cast to "List[int]"
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 example in test.py with mypy 0.641 and --warn-redundant-casts. Trace how narrowing is handled inside the lambda, then add a regression test for this case; done means the nested-function example no longer produces an incorrect redundant-cast warning.
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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