Avoid treating pass and ... as no-op for reachability
#20488
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Fixes python#16232 The inclusion of these statements dates back to the original `--warn-unreachable` PR (and doesn't provide justification) I've been advocating for more consistent heuristics for allowable unreachable code across type checkers and I can't come up with a good reason that this should be allowed
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The aiohttp one seems kind of useful to know about, the other two are maybe a little annoying to see change on, but defensible |
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Would half-fix #20462, & docunenting the consensus reached at microsoft/pyright#11041 would fix the other half. |
A5rocks
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I think there are (weak) justifications for these, but I agree with removing it.
Maybe we can remove the logic that checks multiple statements now? Maybe that's a followup.
JukkaL
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Jan 8, 2026
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Add test?
This looks good to me.
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Diff from mypy_primer, showing the effect of this PR on open source code: aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/client.py:870:17: error: Statement is unreachable [unreachable]
+ aiohttp/client.py:870:17: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-unreachable for more info
Tanjun (https://github.com/FasterSpeeding/Tanjun)
+ tanjun/abc.py:2774: error: Statement is unreachable [unreachable]
pytest (https://github.com/pytest-dev/pytest)
+ testing/code/test_code.py:151: error: Statement is unreachable [unreachable]
+ testing/code/test_code.py:167: error: Statement is unreachable [unreachable]
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This was referenced Jan 11, 2026
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There is a special case about unreachability, `return NotImplemented`, which we did not until now mention, which we hereby now mention in the documentation of `--warn-unreachable`. I also add a test to test this claimed behavior thoroughly. The current test tests its desired behavior, in a class, but the actual behavior of mypy is much broader than that, and applies all the time, it seems. Fixes #20462. That issue is not correct anymore, as it was obsoleted by #20488. However, during its discussion @sterliakov brought up this `NotImplemented` edge case as something that should be documented. I think the documentation after this PR will be satisfactory by everyone's standards. Similarly, this PR closes #20558. Theoretically, there could be further special cases of unreachability of which I am not aware, but these can be noted by later PRs if so.
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Fixes #16232
The inclusion of these statements dates back to the original
--warn-unreachablePR (and doesn't provide justification)I've been advocating for more consistent heuristics for allowable unreachable code across type checkers and I can't come up with a good reason that this should be allowed