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1.16 changed behavior of redefined variable to None #19280

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eli-schwartz opened this issue Jun 12, 2025 · 3 comments
Open

1.16 changed behavior of redefined variable to None #19280

eli-schwartz opened this issue Jun 12, 2025 · 3 comments
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bug mypy got something wrong topic-strict-optional topic-type-narrowing Conditional type narrowing / binder

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@eli-schwartz
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https://github.com/mesonbuild/meson/blob/master/mesonbuild/cmake/fileapi.py

This file started triggering:

mesonbuild/cmake/fileapi.py:81:20: error: "None" has no attribute "__iter__" (not iterable)  [attr-defined]
mesonbuild/cmake/fileapi.py:82:20: error: Value of type "None" is not indexable  [index]
mesonbuild/cmake/fileapi.py:84:36: error: Value of type "None" is not indexable  [index]

due to reuse of for i in ......:

Reduction:

import typing as T
from pathlib import Path

for i in Path().iterdir():
    T.reveal_type(i)
    break

data = T.cast('T.Dict[str, T.Any]', {'foo': [{'kind': 1, 'b': 'two'}]})
for i in data['foo']:
    T.reveal_type(i)
    assert isinstance(i, dict)
    T.reveal_type(i)
    assert 'kind' in i

With mypy 1.16.0

$ mypy --no-strict-optional --warn-unreachable fileapi.py 
fileapi.py:5: note: Revealed type is "pathlib.Path"
fileapi.py:10: note: Revealed type is "Any"
fileapi.py:12: note: Revealed type is "None"
fileapi.py:13: error: "None" has no attribute "__iter__" (not iterable)  [attr-defined]

It starts off as Path, gets redefined as Any (which works?), but then asserting the Any is a dict results in it migrating to None. With mypy 1.15, it never redefined to Any at all, so it became unreachable:

$ pip install mypy==1.15.* 
$ mypy --no-strict-optional --warn-unreachable fileapi.py 
fileapi.py:5: note: Revealed type is "pathlib.Path"
fileapi.py:10: note: Revealed type is "pathlib.Path"
fileapi.py:11: error: Subclass of "Path" and "dict[Any, Any]" cannot exist: would have incompatible method signatures  [unreachable]
fileapi.py:12: error: Statement is unreachable  [unreachable]

I can't really say either behavior is useful. I want variables to be block-scoped and reset their expected value. But at least the 1.15 behavior was comprehensible to me. The new behavior only works if I delete the assert, which seems terribly counterproductive.
@eli-schwartz eli-schwartz added the bug mypy got something wrong label Jun 12, 2025
@brianschubert brianschubert added the topic-type-narrowing Conditional type narrowing / binder label Jun 12, 2025
@brianschubert
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Bisects to b50f3a1 (#18538), cc @ilevkivskyi

Without --no-strict-optional, i is narrowed to Never:

$ mypy --version
mypy 1.16.0+dev.b50f3a1a44038b5f6304f77263f6e08c157f9aa8 (compiled: no)

$ mypy --no-strict-optional fileapi.py
fileapi.py:5: note: Revealed type is "pathlib.Path"
fileapi.py:10: note: Revealed type is "Any"
fileapi.py:12: note: Revealed type is "None"
fileapi.py:13: error: "None" has no attribute "__iter__" (not iterable)  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

$ mypy --no-strict-optional fileapi.py
fileapi.py.:5: note: Revealed type is "pathlib.Path"
fileapi.py:10: note: Revealed type is "Any"
fileapi.py:12: note: Revealed type is "Never"
fileapi.py:13: error: "Never" has no attribute "__iter__" (not iterable)  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

After #18972, the behavior improves without --no-strict-optional

$ mypy --version
mypy 1.16.0+dev.c724a6a806655f94d0c705a7121e3d671eced96d (compiled: no)

$ mypy fileapi.py                        
fileapi.py:5: note: Revealed type is "pathlib.Path"
fileapi.py:10: note: Revealed type is "Any"
fileapi.py:12: note: Revealed type is "builtins.dict[Any, Any]"

$ mypy --no-strict-optional fileapi.py
fileapi.py:5: note: Revealed type is "pathlib.Path"
fileapi.py:10: note: Revealed type is "Any"
fileapi.py:12: note: Revealed type is "None"
fileapi.py:13: error: "None" has no attribute "__iter__" (not iterable)  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

@hauntsaninja
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I guess this branch https://github.com/python/mypy/pull/18972/files#diff-42eda644ce5003e33ea0766d7601fcfb4934ad106429abfe115954a1fccddd07R6301 could also check for no strict optional and NoneType

@eli-schwartz your best unblock might be trying out --allow-redefinition-new in 1.16

@eli-schwartz
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I'm not in an urgent hurry, we are still tangled up in python 3.7 compat :) but will probably jump straight to >=3.10 once November rolls around. Just locally exploring in the interest of being better prepared for November, actually.

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