8000 Guard clause prevents `match` from seeing that all paths return · Issue #12534 · python/mypy · GitHub
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bkovitz opened this issue Apr 6, 2022 · 2 comments · Fixed by #15882
Closed

Guard clause prevents match from seeing that all paths return #12534

bkovitz opened this issue Apr 6, 2022 · 2 comments · Fixed by #15882
Labels
bug mypy got something wrong topic-match-statement Python 3.10's match statement topic-reachability Detecting unreachable code

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@bkovitz
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bkovitz commented Apr 6, 2022

Bug Report

In a match statement in which all cases include a return statement, mypy 0.942 falsely reports "Missing return statement [return]" if at least one case includes a guard clause.

To Reproduce

def func(e) -> int:
    match e:
        case x if True:
            return x
        case _:
            return 0

Or even more simply:

def func(e) -> int:
    match e:
        case _ if True:
            return 0

Expected Behavior

This code should type-check.

Actual Behavior

$ mypy --python-version 3.10 --show-error-codes x5.py
x5.py:1: error: Missing return statement  [return]
Found 1 error in 1 file (checked 1 source file)

Notes

If we change the first case to something that does not include a guard clause, like this:

def func(e) -> int:
    match e:
        case (x, _):
            return x
        case _:
            return 0

then mypy returns no errors.

Environment

  • Mypy version used: 0.942
  • Mypy command-line flags: --python-version 3.10 --show-error-codes
  • Mypy configuration options from mypy.ini (and other config files): (none)
  • Python version used: 3.10.2
  • Operating system and version: Ubuntu 18.04
@bkovitz bkovitz added the bug mypy got something wrong label Apr 6, 2022
@JelleZijlstra JelleZijlstra added the topic-match-statement Python 3.10's match statement label Apr 6, 2022
@ilmari ilmari mentioned this issue Jul 11, 2023
Closed
@erictraut erictraut mentioned this issue Aug 13, 2023
Closed
@AlexWaygood AlexWaygood added the topic-reachability Detecting unreachable code label Aug 15, 2023
hauntsaninja added a commit to hauntsaninja/mypy that referenced this issue Aug 16, 2023
@hauntsaninja
Improve match narrowing and reachability analysis
2409044 
Fixes python#12534, python#15878
@hauntsaninja hauntsaninja mentioned this issue Aug 16, 2023
Merged
hauntsaninja added a commit to hauntsaninja/mypy that referenced this issue Aug 16, 2023
@hauntsaninja
Improve match narrowing and reachability analysis
8033909 
Fixes python#12534, python#15878
@hauntsaninja
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hauntsaninja commented Aug 16, 2023 •
edited
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Opened a PR that should fix this. Are any of the people here encountering this in open source projects? I could use some more match statement coverage in https://github.com/hauntsaninja/mypy_primer

hauntsaninja added a commit that referenced this issue Aug 19, 2023
@hauntsaninja
Improve match narrowing and reachability analysis (#15882)
5af7671 
Fixes #12534, fixes #15878
@hauntsaninja
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I've merged the fix, but if your project is open source, please let me know!

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bug mypy got something wrong topic-match-statement Python 3.10's match statement topic-reachability Detecting unreachable code
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Improve match narrowing and reachability analysis hauntsaninja/mypy
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@JelleZijlstra @bkovitz @hauntsaninja @AlexWaygood
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