10000 Typechecking differs between one-line def of "return expr" vs. "r = expr; return r" · Issue #11985 · python/mypy · GitHub
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Typechecking differs between one-line def of "return expr" vs. "r = expr; return r" #11985

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drvink opened this issue Jan 13, 2022 · 2 comments · Fixed by #18976
Closed

Typechecking differs between one-line def of "return expr" vs. "r = expr; return r" #11985

drvink opened this issue Jan 13, 2022 · 2 comments · Fixed by #18976
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bug mypy got something wrong topic-type-context Type context / bidirectional inference

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@drvink
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drvink commented Jan 13, 2022

mypy 0.931

#!/usr/bin/env python3

from collections.abc import Iterable
from typing import Optional
import os

def min_direntry_good(
        fs: 'Iterable[os.DirEntry[str]]'
) -> 'Optional[os.DirEntry[str]]':
    r = min(fs, default=None, key=lambda x: x.stat().st_mtime)
    return r

def min_direntry_bad(
        fs: 'Iterable[os.DirEntry[str]]'
) -> 'Optional[os.DirEntry[str]]':
    return min(fs, default=None, key=lambda x: x.stat().st_mtime)
hober.py:16: error: Item "None" of "Optional[DirEntry[str]]" has no attribute "stat"
Found 1 error in 1 file (checked 1 source file)
@drvink
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drvink commented Mar 12, 2022

This is still broken in mypy 0.940:

hober.py:16: error: Item "None" of "Optional[DirEntry[str]]" has no attribute "stat"  [union-attr]
hober.py:16: error: Invalid self argument "stat_result" to attribute function "st_mtime" with type "Callable[[stat_result], float]"  [misc]
Found 2 errors in 1 file (checked 1 source file)

@JelleZijlstra
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This is likely an issue with mypy's "type context", which infers different types based on the "expected" type. Here it obviously goes wrong, but chances are it's hard to fix it without breaking some other use case.

@AlexWaygood AlexWaygood added bug mypy got something wrong topic-type-context Type context / bidirectional inference labels Mar 28, 2022
@sterliakov sterliakov mentioned this issue Apr 26, 2025
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