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UnboundLocalError when module is shadowed by a comprehension and then accessed by another comprehension #118513

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jmymay opened this issue May 2, 2024 · 6 comments
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UnboundLocalError when module is shadowed by a comprehension and then accessed by another comprehension #118513

jmymay opened this issue May 2, 2024 · 6 comments
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3.12 only security fixes 3.13 bugs and security fixes type-bug An unexpected behavior, bug, or error

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@jmymay
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jmymay commented May 2, 2024 •
edited by bedevere-app bot
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Bug report

Bug description:

Seems to be a regression in 3.12 when compared to 3.11, likely caused by PEP 709: Comprehension inlining, but I haven't verified this. Example below. (Noticed while running this code.)

import sys
def f():
    _ = [sys for sys in []]  # can be any module other than sys
    [sys.stdout.write('success\n') for _ in [1]]
f()
$ python3.11 main.py  # 3.11.9
success

$ python3.12 main.py  # 3.12.3
Traceback (most recent call last):
  File "/home/jkozera/imp/main.py", line 5, in <module>
    f()
  File "/home/jkozera/imp/main.py", line 4, in f
    [sys.stdout.write('success\n') for _ in [1]]
     ^^^
UnboundLocalError: cannot access local variable 'sys' where it is not associated with a value

$ python3.13 main.py  # 3.13.0a6+ (heads/main:2770d5c, May  2 2024, 18:19:03)
Traceback (most recent call last):
  File "/home/jkozera/imp/main.py", line 5, in <module>
    f()
    ~^^
  File "/home/jkozera/imp/main.py", line 4, in f
    [sys.stdout.write('success\n') for _ in [1]]
     ^^^
UnboundLocalError: cannot access local variable 'sys' where it is not associated with a value

CPython versions tested on:

3.11, 3.12, CPython main branch

Operating systems tested on:

Linux

Linked PRs
@jmymay jmymay added the type-bug An unexpected behavior, bug, or error label May 2, 2024
@Eclips4
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Eclips4 commented May 2, 2024

cc @carljm

@JelleZijlstra
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Confirmed without a module too:

x = 1
def f():
    [x for x in [1]]
    return [x for _ in [1]]
print(f())

Traceback (most recent call last):
  File "/Users/jelle/py/cpython/globs.py", line 5, in <module>
    print(f())
          ~^^
  File "/Users/jelle/py/cpython/globs.py", line 4, in f
    return [x for _ in [1]]
            ^
UnboundLocalError: cannot access local variable 'x' where it is not associated with a value

Disassembly shows we're using LOAD_FAST_CHECK for the second comprehension when we should be using LOAD_GLOBAL.

@Eclips4 Eclips4 added 3.12 only security fixes 3.13 bugs and security fixes labels May 2, 2024
@carljm
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carljm commented May 2, 2024

Thanks, taking a look.

@Eclips4
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Eclips4 commented May 2, 2024 •
edited
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Confirmed without a module too:

x = 1
def f():
    [x for x in [1]]
    return [x for _ in [1]]
print(f())

Traceback (most recent call last):
  File "/Users/jelle/py/cpython/globs.py", line 5, in <module>
    print(f())
          ~^^
  File "/Users/jelle/py/cpython/globs.py", line 4, in f
    return [x for _ in [1]]
            ^
UnboundLocalError: cannot access local variable 'x' where it is not associated with a value

Disassembly shows we're using LOAD_FAST_CHECK for the second comprehension when we should be using LOAD_GLOBAL.

FYI, the assignment x = 1 here's unnecessary (result will be the same).

@carljm
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carljm commented May 2, 2024

Yes, either way it should compile to LOAD_GLOBAL; without the global assignment it should result in NameError, not UnboundLocalError.

carljm added a commit to carljm/cpython that referenced this issue May 2, 2024
@carljm
pythongh-118513: Fix sibling comprehensions with a name bound in one …
1258fad 
…and global in the other
@bedevere-app bedevere-app bot mentioned this issue May 2, 2024
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@carljm
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carljm commented May 2, 2024

Fix in #118526

carljm added a commit that referenced this issue May 3, 2024
@carljm @JelleZijlstra @Eclips4
gh-118513: Fix sibling comprehensions with a name bound in one and gl…
c8deb1e 
…obal in the other (#118526)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>
miss-islington pushed a commit to miss-islington/cpython that referenced this issue May 3, 2024
@carljm @JelleZijlstra
@Eclips4 @miss-islington
pythongh-118513: Fix sibling comprehensions with a name bound in one …
93d9508 
…and global in the other (pythonGH-118526)

(cherry picked from commit c8deb1e)

Co-authored-by: Carl Meyer <carl@oddbird.net>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>
@bedevere-app bedevere-app bot mentioned this issue May 3, 2024
Merged
carljm added a commit that referenced this issue May 3, 2024
@miss-islington @carljm
@JelleZijlstra @Eclips4
[3.12] gh-118513: Fix sibling comprehensions with a name bound in one…
3706eef 
… and global in the other (GH-118526) (#118548)

gh-118513: Fix sibling comprehensions with a name bound in one and global in the other (GH-118526)
(cherry picked from commit c8deb1e)

Co-authored-by: Carl Meyer <carl@oddbird.net>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>
@carljm carljm closed this as completed May 3, 2024
SonicField pushed a commit to SonicField/cpython that referenced this issue May 8, 2024
@carljm @JelleZijlstra
@Eclips4 @SonicField
pythongh-118513: Fix sibling comprehensions with a name bound in one …
48b5ec7 
…and global in the other (python#118526)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>
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