mypy throws `assignment` error for a list created from an empty list
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feature
topic-type-variables
topic-union-types
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- Python
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Description
Bug Report
mypy fails to proceed sorting of the empty list, considering it's a list[object], see the reproduce script and the output
To Reproduce
a: list[str] | list[int] = []
a = sorted(a)
Expected Behavior
There shouldn't be errors
Actual Behavior
> mypy .
/tmp/test.py:2: error: Incompatible types in assignment (expression has type "list[SupportsDunderLT[Any] | SupportsDunderGT[Any]]", variable has type "list[str] | list[int]") [assignment]
Your Environment
- Mypy version used:
mypy 1.14.0 (compiled: no) - Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.13.2
In the scope of https://github.com/ClickHouse/ClickHouse/pull/76679
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
Use /tmp/test.py as the reproducer and run mypy . with mypy 1.14.0 and Python 3.13.2. Investigate the assignment diagnostic for sorted(a) when a is list[str] | list[int]. Done when this example type-checks without an error and a regression check covers it.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers, devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100