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Alternate nested quotes inside format spec interpolations - #28259
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MichaReiser merged 4 commits intoSep 7, 2026
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A replacement field inside a format spec belongs to the same string as the field enclosing it, but it was being treated as a further level of nesting. Before Python 3.12 that made the formatter preserve the inner quotes and reuse the outer quote character, which is a syntax error the parser rejects. Only entering another string literal is a further level of nesting, so keep the state unchanged for format spec fields.
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cargo hawk flags them as unnecessarily visible: both are only used inside the module that defines them.
Format-spec interpolations belong to the containing f-string or t-string. Advance the nesting depth when entering a string, and retain each interpolation’s own layout context without introducing format-spec markers. Regression: astral-sh#28218
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Thank you. I made a few smaller changes, mainly to avoid introducing more state. The implementation now has an explicit |
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Fixes #28218.
When Ruff formats an f-string, it normally alternates the inner quotes so they don't conflict with the outer ones. For example, a normal interpolation like
f'x {d["k"]} y'is formatted asf"x {d['k']} y".However, this didn't work correctly for interpolations inside a format spec. For example,
f'x {v:,.{d["n"]}f} y'was left with{d["n"]}using the same quote character as the outer f-string.Reusing the outer quote character inside an f-string is only valid starting with Python 3.12, thanks to PEP 701. With
target-version = "py311", this meant Ruff's formatter could produce code that its own checker would reject with:Cannot reuse outer quote character in f-strings on Python 3.11 (syntax was added in Python 3.12)The issue was caused by how Ruff tracks nesting inside interpolated strings. A field inside a format spec was incorrectly treated as an additional nesting level, even though it's still part of the same f-string and doesn't introduce another string literal.
That incorrect nesting level caused the formatter to preserve the existing quotes for Python versions before 3.12. In this case, preserving them resulted in invalid Python 3.11 syntax.
The fix is to only increase the nesting level when entering another string literal. Entering a format spec does not count as deeper string nesting, so fields inside format specs now stay at the current nesting level.