Fix crash when f-string follows # fmt: off inside brackets#5097
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Fix crash when f-string follows # fmt: off inside brackets#5097
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When an f-string appeared after a `# fmt: off` comment inside brackets, Black would crash with an AssertionError about invalid string prefix characters. The root cause: `normalize_fmt_off` (which runs before line generation) converts content between `# fmt: off` and `# fmt: on` into STANDALONE_COMMENT nodes. When this content is inside an f-string node, the f-string's children get replaced with a STANDALONE_COMMENT leaf. However, `visit_fstring` still tried to call `fstring_tstring_to_string` on the modified node, producing a "string" value like `# fmt: off\nf"""\n"""` which fails the string prefix assertion. The fix adds a guard in `visit_fstring` (and `visit_tstring`) to detect when the node has been modified by `normalize_fmt_off` and fall back to `visit_default` instead of attempting the f-string-to-string conversion. Fixes psf#4511.
cobaltt7
reviewed
Apr 12, 2026
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
cobaltt7
reviewed
Apr 13, 2026
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| # Same as above but unindented inside brackets. | ||
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| # fmt: off |
Collaborator
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Ideally the comments wouldn't move. Would that be a lot of extra work? If so it's fine like this.
Please also add a changelog entry
Contributor
Author
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The comment movement (re-indenting # fmt: off/# fmt: on to match bracket depth) is existing Black behavior for any comment inside brackets — it's not introduced by this PR. Changing that would require modifying the general comment normalization/line generation logic, which feels out of scope for this crash fix. Happy to look into it as a follow-up if you think it's worth pursuing.
Added a changelog entry under Stable style in 67adc97.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
JelleZijlstra
approved these changes
Apr 13, 2026
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Summary
Fixes #4511.
Black crashes with an
AssertionErrorwhen formatting code that contains an f-string after a# fmt: offcomment inside brackets:Error:
Root Cause
normalize_fmt_offruns before line generation and converts content between# fmt: off/# fmt: onintoSTANDALONE_COMMENTnodes. When this content is inside an f-string node, the f-string's children (FSTRING_START, FSTRING_MIDDLE, FSTRING_END) get replaced with a single STANDALONE_COMMENT leaf.When
visit_fstringis later called for this modified node, it callsfstring_tstring_to_string()which doesstr(node)[len(node.prefix):]. This produces a value like# fmt: off\nf"""\n"""— the comment text becomes part of the "string value", which fails the string prefix assertion.Fix
Added a guard in
visit_fstring(andvisit_tstring) to detect when the node contains a STANDALONE_COMMENT child (indicating it was modified bynormalize_fmt_off). In that case, fall back tovisit_defaultinstead of attempting the f-string-to-string conversion.Test plan
tests/data/cases/fmtonoff6.pyfor both triple-quoted and single-line f-strings with# fmt: offinside brackets