mypy cannot decide if a cast is necessary or redundant
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bug
topic-inference
topic-literal-types
- Dominant language
- Python
- Stars
- 20.6k
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Description
Bug Report
Here a case where mypy says a cast is redundant, but if you remove the cast, mypy cannot infer the right type.
To Reproduce
Expected Behavior
Either no error with the cast or no error without the cast.
Actual Behavior
Using a type cast, mypy says it's not needed.
Without the type cast, mypy cannot infer the type.
Your Environment
- Mypy version used: 2.10
- Mypy configuration options from
mypy.ini(and other config files): warn_redundant_casts = true - Python version used: 3.14
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 with the linked Gist and mypy-playground reproduction using warn_redundant_casts enabled, then trace how mypy checks the cast and infers the expression without it. Confirm the behavior on the reported Python and mypy versions. Done means the example produces no error with the cast or without it.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 58/100