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Class-scope comprehension with lambda raises SystemError #150700
Description
Bug report
Bug description:
class D:
vals = [(lambda: __class__)() for _ in range(1)]gives
SystemError: compiler_lookup_arg(name='__class__') with reftype=5 failed in D; freevars of code <lambda>: ('__class__',)
tested on main (f4bda4d) and on 3.12.8 (debug build). Issue #149806 looks related, but I tried that example on #149806 and it's not fixed by it.
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
Linked PRs
- gh-150700: Fix class-scope inline comprehensions when nested scopes reference
__class__and friends #150735 - [3.15] gh-150700: Fix class-scope inline comprehensions when nested scopes reference
__class__and friends (GH-150735) #151211 - [3.14] gh-150700: Fix class-scope inline comprehensions when nested scopes reference
__class__and friends (GH-150735) #151212
Activity
[-]Class-scope inlined comprehensions can crash the compiler when a nested function captures class[/-][+]Class-scope comprehension with lambda raises SystemError[/+]terryjreedy commentedon Jun 1, 2026
Same on Windows back to 3.12. In 3.11, "NameError: cannot access free variable 'class' where it is not associated with a value in enclosing scope".
StanFromIreland commentedon Jun 1, 2026
Out of curiosity, how was this found? I'm asking because it seems quite like our other OSS-Fuzz things (like the one you linked), and I'm wondering if we missed a finding?
steve-s commentedon Jun 1, 2026
As part of implementation of PEP 709 in GraalPy done by my colleague @msimacek, during review I asked Codex to look for corner cases and gaps around the code handling the name resolution and it came up with this (manifested by different internal error). Then we realized it is also not working on CPython.
johnslavik commentedon Jun 1, 2026
MREs:
class C: [lambda: __class__ for () in ()],class C: [lambda: super() for () in ()]StanFromIreland commentedon Jun 1, 2026
Also don't forget
__classdict__and__conditional_annotations__(which I suppose I technically broke some time ago), they can also be dropped.johnslavik commentedon Jun 1, 2026
I was just about to mention them, yes.
class C: [lambda: __conditional_annotations__ for () in ()],class C: [lambda: __classdict__ for () in ()]reproduce too.johnslavik commentedon Jun 1, 2026
#149806 is unrelated because it fixes annotation visiting code and the issue here lies in
inline_comprehension.This is an edge case of #120295 (which is the likely cause of regression).
In
inline_comprehension(), when__class__/__classdict__/__conditional_annotations__appears asFREEin a comprehension's symbol table because a nested scope captured it (e.g. these lambdas), this name is still discarded fromcomp_freeunconditionally. This preventsdrop_class_free()from seeing it, so the appropriateste_needs_(...)flag is never set on the enclosing class. That leads tocodegen_make_closure()throwingSystemErrorwhen it couldn't find__class__/__classdict__/__conditional_annotations__in the class's cellvars.We could just only discard from
comp_freewhen no child scope (e.g. a lambda) still needs the name asFREE. When a child scope does need it, keep it incomp_freesodrop_class_free()can set the appropriate flag and the class creates the implicit cell.Merge branch 'main' into pythongh-150700-class-scope-comprehension-wi…
gh-150700: Fix class-scope inline comprehensions when nested scopes r…
[3.14] gh-150700: Fix class-scope inline comprehensions when nested s…
[3.15] gh-150700: Fix class-scope inline comprehensions when nested s…
Fix SystemError during REPL input compilation corrupting display
Fix SystemError during REPL input compilation corrupting display
Fix SystemError during REPL input compilation corrupting display
Fix SystemError during REPL input compilation corrupting display
Fix SystemError during REPL input compilation corrupting display
Fix SystemError during REPL input compilation corrupting display
Fix SystemError during REPL input compilation corrupting display
pythongh-150700: Fix class-scope inline comprehensions when nested sc…