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AckslD opened this issue Apr 14, 2022 · 4 comments
Closed

False positive unreachable for generator that might not yield #12587

AckslD opened this issue Apr 14, 2022 · 4 comments
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bug mypy got something wrong topic-reachability Detecting unreachable code

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@AckslD
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AckslD commented Apr 14, 2022 •
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Bug Report
(Maybe related to #8766)

Consider the following code:

from typing import Iterator


def gen() -> Iterator[int]:
    if 1 == 2:
        yield 0


def test() -> None:
    x = None
    for x in gen():
        pass
    if x is None:
        print('reachable')

To Reproduce

mypy above_file.py --warn-unreachable

Expected Behavior
No error.

Actual Behavior

tmp.py:14: error: Statement is unreachable
            print('reachable')
            ^
Found 1 error in 1 file (checked 1 source file)

Your Environment

  • Mypy version used: 0.931
  • Mypy command-line flags: --warn-unreachable
  • Python version used: Python 3.9.10
  • Operating system and version: arch 5.17.1-arch1-1
@AckslD AckslD added the bug mypy got something wrong label Apr 14, 2022
@AlexWaygood AlexWaygood added the topic-reachability Detecting unreachable code label Apr 14, 2022
@JelleZijlstra
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I think there's an existing issue for this: the problem is essentially that mypy thinks for loops always execute at least once.

@AlexWaygood
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I think there's an existing issue for this

#8637?

@JelleZijlstra
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Yes, thanks!

@AckslD
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AckslD commented Apr 14, 2022

Thanks both!

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@JelleZijlstra @AckslD @AlexWaygood