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For loop assumed to execute at least once based on inferred type #18629

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JukkaL opened this issue Feb 7, 2025 · 1 comment · Fixed by #18631
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For loop assumed to execute at least once based on inferred type #18629

JukkaL opened this issue Feb 7, 2025 · 1 comment · Fixed by #18631
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@JukkaL
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bug mypy got something wrong topic-type-narrowing Conditional type narrowing / binder

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@JukkaL
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JukkaL commented Feb 7, 2025 •
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Bug Report

Mypy seems to assume that for loop body executes at least once when inferring index variable types.

To Reproduce

x: int | str
x = "abc"
for x in list[int]():
    pass
reveal_type(x) # int

Expected Behavior

Revealed type is int | str.

Actual Behavior

Revealed type is int.

Your Environment

Latest master.
@JukkaL JukkaL added bug mypy got something wrong topic-type-narrowing Conditional type narrowing / binder labels Feb 7, 2025
@JukkaL JukkaL self-assigned this Feb 7, 2025
@JukkaL JukkaL mentioned this issue Feb 7, 2025
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@Hnasar
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Hnasar commented Feb 7, 2025

Nice! thanks for the fix — this will also close #16321

JukkaL added a commit that referenced this issue Feb 10, 2025
@JukkaL
Don't assume that for loop body index variable is always set (#18631)
f946af4 
Fixes #18629. Fixes #16321. Fixes #8637.
x612skm pushed a commit to x612skm/mypy-dev that referenced this issue Feb 24, 2025
@JukkaL @x612skm
Don't assume that for loop body index variable is always set (python#…
7d0f4d6 
…18631)

Fixes python#18629. Fixes python#16321. Fixes python#8637.
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bug mypy got something wrong topic-type-narrowing Conditional type narrowing / binder
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Don't assume that for loop body index variable is always set python/mypy
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@Hnasar @JukkaL