(🎁) Infer local partial type when using `extend` with generator
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feature
- Dominant language
- Python
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- 20.6k
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Description
a = [] # main.py:1: error: Need type annotation for "a" (hint: "a: List[<type>] = ...")
a.extend(a for a in [1])
reveal_type(a) # main.py:3: note: Revealed type is "builtins.list[Any]"
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 the linked mypy playground with the shown Python snippet. Trace the type inference for the generator passed to list.extend and verify that the completed behavior infers the local list element type instead of revealing list[Any], while preserving the expected annotation diagnostic.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100