[flake8-comprehensions] Document RecursionError edge case in __len__ (C416)#25286
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… `__len__` Reaching `[item for item in self]` (and the dict/set equivalents) from inside `__len__` is safe today, but the C416 auto-fix replaces it with `list(self)`, which CPython calls back into `__len__` via the length hint optimization, recursing infinitely. The rule's heuristic intentionally doesn't try to detect this control flow. Document the failure mode in the rule's "Known problems" so users can suppress with `# noqa: C416` rather than discovering it at runtime. Closes astral-sh#13752.
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flake8-comprehensions] Document C416 RecursionError edge case in __len__flake8-comprehensions] DocumentRecursionError edge case in __len__ (C416)
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flake8-comprehensions] DocumentRecursionError edge case in __len__ (C416)flake8-comprehensions] Document RecursionError edge case in __len__ (C416)
thejchap
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May 23, 2026
…en__` (`C416`) (astral-sh#25286) C416's autofix rewrites a list comprehension like `[item for item in self]` into `list(self)`, but when the original is reached from inside `__len__`, the rewrite recurses: CPython's `list()` constructor probes `__length_hint__` (which dispatches to `__len__`) as a sizing optimization while iterating, so `list(self)` from inside `__len__` calls `__len__` again. The same applies to `dict()` and `set()` calls on `self`. In astral-sh#13752, MichaReiser noted that extending the docs would be reasonable and that detecting this control flow isn't the goal. I added a paragraph to the C416 docstring with a runnable example of the failure mode, plus the `# noqa: C416` workaround for when the comprehension is reached (directly or transitively) from `__len__`. The change is docstring-only. `cargo dev generate-all` and `uvx prek run --from-ref main` both pass. fixes astral-sh#13752 --------- Co-authored-by: Brent Westbrook <[email protected]>
anishgirianish
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May 28, 2026
…en__` (`C416`) (astral-sh#25286) C416's autofix rewrites a list comprehension like `[item for item in self]` into `list(self)`, but when the original is reached from inside `__len__`, the rewrite recurses: CPython's `list()` constructor probes `__length_hint__` (which dispatches to `__len__`) as a sizing optimization while iterating, so `list(self)` from inside `__len__` calls `__len__` again. The same applies to `dict()` and `set()` calls on `self`. In astral-sh#13752, MichaReiser noted that extending the docs would be reasonable and that detecting this control flow isn't the goal. I added a paragraph to the C416 docstring with a runnable example of the failure mode, plus the `# noqa: C416` workaround for when the comprehension is reached (directly or transitively) from `__len__`. The change is docstring-only. `cargo dev generate-all` and `uvx prek run --from-ref main` both pass. fixes astral-sh#13752 --------- Co-authored-by: Brent Westbrook <[email protected]>
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C416's autofix rewrites a list comprehension like
[item for item in self]intolist(self), but when the original is reached from inside__len__, the rewrite recurses: CPython'slist()constructor probes__length_hint__(which dispatches to__len__) as a sizing optimization while iterating, solist(self)from inside__len__calls__len__again. The same applies todict()andset()calls onself.In #13752, MichaReiser noted that extending the docs would be reasonable and that detecting this control flow isn't the goal. I added a paragraph to the C416 docstring with a runnable example of the failure mode, plus the
# noqa: C416workaround for when the comprehension is reached (directly or transitively) from__len__.The change is docstring-only.
cargo dev generate-allanduvx prek run --from-ref mainboth pass.fixes #13752