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inlined comprehension and closed over variables have buggy interactions #156664

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inlined comprehension and closed over variables have buggy interactions#156664
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interpreter-core(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or error

Description

@cfbolz
cfbolz
opened on Aug 30, 2026

Bug report

Bug description:

We are a bit late to report this, but the following of PyPy's unit tests got broken by inlined comprehensions in CPython 3.12:

def test_inlined_comprehension_private_name_reuse():
    x = 3

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

    assert f() == [3]

test_inlined_comprehension_private_name_reuse()

this passes fine in CPython 3.11 but breaks like this in 3.12 onwards, including on main:

$ ./python /tmp/bug.py
Traceback (most recent call last):
  File "/tmp/bug.py", line 10, in <module>
    test_inlined_comprehension_private_name_reuse()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/tmp/bug.py", line 8, in test_inlined_comprehension_private_name_reuse
    assert f() == [3]
           ~^^
  File "/tmp/bug.py", line 6, in f
    return [x for _ in [1]]
            ^
UnboundLocalError: cannot access local variable 'x' where it is not associated with a value

I wonder whether that is the kind of bug that would be easier to fix when doing #124697

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

Activity

  1. cfbolz
    added
    type-bugAn unexpected behavior, bug, or error
    on Aug 30, 2026

  2. iritkatriel commented on Aug 30, 2026

    @iritkatriel
    iritkatriel
    on Aug 30, 2026
    Member

    CC @carljm

  3. JelleZijlstra commented on Aug 30, 2026

    @JelleZijlstra
    JelleZijlstra
    on Aug 30, 2026
    Member

    Surprised this has never been caught before! I even ran a fuzzer when we were implementing PEP 709 to find things like this. I'm attempting a fix now.

  4. JelleZijlstra commented on Aug 30, 2026

    @JelleZijlstra
    JelleZijlstra
    on Aug 30, 2026
    Member

    An analogous case that is also broken in 3.12+:

    def test_inlined_comprehension_then_nested_function():
    x = 3

    def f():
        [x for x in [1]]

        def g():
            return x

        return g()

    assert f() == 3


test_inlined_comprehension_then_nested_function()

    And a more complicated version:

    def test_captured_target_then_sibling_comprehension():
    x = 3

    def f():
        funcs = [lambda: x for x in [1]]
        return funcs[0](), [x for _ in [1]]

    assert f() == (1, [3])


test_captured_target_then_sibling_comprehension()
  5. picnixz
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    on Aug 30, 2026
  6. JelleZijlstra
    added 2 commits that reference this issue on Sep 1, 2026

    pythongh-156664: Preserve enclosing cells in nested comprehensions

    c93c1d8

    pythongh-156664: Deduplicate comprehension frame locals

    f2c0db4
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