Unreachability should detect redundant `isinstance`s even for `object` arguments
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bug
topic-reachability
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
Bug Report
See #20665
To Reproduce
def f(x: object) -> None:
if isinstance(x, int):
print("aaaa")
elif isinstance(x, int):
print("aaaa")
Expected Behavior
Error
Actual Behavior
No error
Your Environment
- Mypy version used: master
- Mypy command-line flags:
--warn-unreachable --strict - Mypy configuration options from
mypy.ini(and other config files): N/A - Python version used: 3.13
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
Reproduce the example with Python 3.13 using mypy master and the --warn-unreachable --strict flags. Trace the unreachability analysis for repeated isinstance checks when the argument is typed as object, and confirm that the redundant branch produces an error without changing unrelated behavior.
Written by the indexing model from the issue text.
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