`F507`: Fix false negative for non-tuple RHS in `%`-formatting by RenzoMXD · Pull Request #24142 · astral-sh/ruff

RenzoMXD · 2026-03-23T17:40:16Z

Summary

F507 (percent-format-positional-count-mismatch) previously only checked tuple right-hand sides in %-format expressions. This meant literal non-tuple values like '%s %s' % 42 were silently ignored.

This PR extends F507 to also flag non-tuple RHS values when the format string expects a different number of positional arguments, using ResolvedPythonType::from() to infer the type of the RHS expression.

What gets flagged now

Literal values: 42, 3.14, "hello", b"hello", True, None, ..., f"hello {name}"
Compound expressions with known types: -1, 1 + 2, not x, "a" + "b", 1 if True else 2

What is intentionally NOT flagged

Variables, attribute accesses, subscripts, and calls are skipped because they could be tuples at runtime:

x = (1, 2)
'%s %s' % x  # valid Python — % unpacks tuples

Why `ResolvedPythonType`

Per reviewer suggestion, using ResolvedPythonType::from() instead of manually matching literal expression types catches more cases (unary ops, binary ops, ternary, etc.) and is more maintainable.

Context

The original implementation only checked Expr::Tuple because List and Set were intentionally removed in #5986 — '%s' % [1, 2, 3] is valid Python (lists aren't unpacked by %). This PR takes the same conservative approach by only flagging cases where we can statically determine the type is not a tuple.

Test plan

Added test cases for all literal types (int, float, string, bytes, bool, None, ellipsis, f-string) with multiple placeholders
Added test cases for compound expressions resolved via ResolvedPythonType (unary, binary, boolean, ternary)
Added test cases for single-placeholder with literal RHS (not flagged)
Added test cases for variables, attribute access, subscripts, calls, ternary with unknowns, binary ops with unknowns (not flagged)
All 469 existing pyflakes tests pass

kaddkaka · 2026-03-23T18:03:58Z

Does this also solve the case where the LHS is an f-string?

f"{banana}" % banana

RenzoMXD · 2026-03-23T18:06:49Z

No, it doesn't. That's a separate issue.

The LHS check happens in expression.rs:1442 — it only matches Expr::StringLiteral:

if let Expr::StringLiteral(format_string @ ast::ExprStringLiteral { value, .. }) = left.as_ref()

So when the LHS is an f-string (Expr::FString), none of the F50x rules fire at all. My fix only touches the RHS handling within F507.

I will make a separate PR for that.

RenzoMXD · 2026-03-23T18:09:15Z

@ntBre Could you please review my PR?
I will make another PR that solves the case where the LHS is an f-string.

kaddkaka · 2026-03-23T18:41:45Z

No, it doesn't. That's a separate issue.

The LHS check happens in expression.rs:1442 — it only matches Expr::StringLiteral:
if let Expr::StringLiteral(format_string @ ast::ExprStringLiteral { value, .. }) = left.as_ref()
So when the LHS is an f-string (Expr::FString), none of the F50x rules fire at all. My fix only touches the RHS handling within F507.

I will make a separate PR for that.

OK, but perhaps a case with 0 placeholders and 1 item in right hand side should be considered.

I'm not sure if this case can be covered without risk of false positive:

"no placeholder" % banana

What happens for this case if banana is an empty tuple? I suppose for this particular case, with a literal string LHS, any use of % (no matters what's on RHS) could give a warning.

RenzoMXD · 2026-03-23T18:56:40Z

Good point! The current implementation already covers that case — "no placeholder" % 42 would be flagged since num_positional is 0 and a literal counts as 1 substitution (0 != 1).

For "no placeholder" % banana, we intentionally don't flag it because banana could be an empty tuple () at runtime, which would be valid. This is consistent with the conservative approach of only flagging literals where we can be certain about the value.

crates/ruff_linter/src/rules/pyflakes/rules/strings.rs

* main: [ty] Avoid eager TypedDict diagnostics in `TypedDict | dict` unions (#24151) `F507`: Fix false negative for non-tuple RHS in `%`-formatting (#24142) [ty] Update `SpecializationBuilder` hook to get both lower/upper bounds (#23848) Fix `%foo?` parsing in IPython assignment expressions (#24152) `E501`/`W505`/formatter: Exclude nested pragma comments from line width calculation (#24071) [ty] Fix Salsa panic propagation (#24141) [ty] Support `type:ignore[ty:code]` suppressions (#24096) [ty] Support narrowing for extended walrus targets (#24129)

@bitloi

##### [\`v0.15.8\`](https://github.com/astral-sh/ruff/blob/HEAD/CHANGELOG.md#0158) Released on 2026-03-26. ##### Preview features - \[`ruff`] New rule `unnecessary-if` (`RUF050`) ([#24114](astral-sh/ruff#24114)) - \[`ruff`] New rule `useless-finally` (`RUF072`) ([#24165](astral-sh/ruff#24165)) - \[`ruff`] New rule `f-string-percent-format` (`RUF073`): warn when using `%` operator on an f-string ([#24162](astral-sh/ruff#24162)) - \[`pyflakes`] Recognize `frozendict` as a builtin for Python 3.15+ ([#24100](astral-sh/ruff#24100)) ##### Bug fixes - \[`flake8-async`] Use fully-qualified `anyio.lowlevel` import in autofix (`ASYNC115`) ([#24166](astral-sh/ruff#24166)) - \[`flake8-bandit`] Check tuple arguments for partial paths in `S607` ([#24080](astral-sh/ruff#24080)) - \[`pyflakes`] Skip `undefined-name` (`F821`) for conditionally deleted variables ([#24088](astral-sh/ruff#24088)) - `E501`/`W505`/formatter: Exclude nested pragma comments from line width calculation ([#24071](astral-sh/ruff#24071)) - Fix `%foo?` parsing in IPython assignment expressions ([#24152](astral-sh/ruff#24152)) - `analyze graph`: resolve string imports that reference attributes, not just modules ([#24058](astral-sh/ruff#24058)) ##### Rule changes - \[`eradicate`] ignore `ty: ignore` comments in `ERA001` ([#24192](astral-sh/ruff#24192)) - \[`flake8-bandit`] Treat `sys.executable` as trusted input in `S603` ([#24106](astral-sh/ruff#24106)) - \[`flake8-self`] Recognize `Self` annotation and `self` assignment in `SLF001` ([#24144](astral-sh/ruff#24144)) - \[`pyflakes`] `F507`: Fix false negative for non-tuple RHS in `%`-formatting ([#24142](astral-sh/ruff#24142)) - \[`refurb`] Parenthesize generator arguments in `FURB142` fixer ([#24200](astral-sh/ruff#24200)) ##### Performance - Speed up diagnostic rendering ([#24146](astral-sh/ruff#24146)) ##### Server - Warn when Markdown files are skipped due to preview being disabled ([#24150](astral-sh/ruff#24150)) ##### Documentation - Clarify `extend-ignore` and `extend-select` settings documentation ([#24064](astral-sh/ruff#24064)) - Mention AI policy in PR template ([#24198](astral-sh/ruff#24198)) ##### Other changes - Use trusted publishing for NPM packages ([#24171](astral-sh/ruff#24171)) ##### Contributors - [@bitloi](https://github.com/bitloi) - [@Sim-hu](https://github.com/Sim-hu) - [@mvanhorn](https://github.com/mvanhorn) - [@chinar-amrutkar](https://github.com/chinar-amrutkar) - [@markjm](https://github.com/markjm) - [@RenzoMXD](https://github.com/RenzoMXD) - [@vivekkhimani](https://github.com/vivekkhimani) - [@seroperson](https://github.com/seroperson) - [@moktamd](https://github.com/moktamd) - [@charliermarsh](https://github.com/charliermarsh) - [@ntBre](https://github.com/ntBre) - [@zanieb](https://github.com/zanieb) - [@dylwil3](https://github.com/dylwil3) - [@MichaReiser](https://github.com/MichaReiser) Renovate-Branch: renovate/2024.6-ruff-0.15.x Change-Id: Ifd4216a963962ffb24a4df69802bc60fcc29628d Priv-Id: 46d2f61be3a5e65a9fdd2fef998ba41ea3388f12

Fix F507 false negative for literal non-tuple RHS in %-formatting

8190ab2

astral-sh-bot bot assigned ntBre Mar 23, 2026

astral-sh-bot bot requested a review from ntBre March 23, 2026 17:40

MichaReiser reviewed Mar 24, 2026

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crates/ruff_linter/src/rules/pyflakes/rules/strings.rs Outdated Show resolved Hide resolved

Use ResolvedPythonType for F507 non-tuple RHS detection

e802e57

MichaReiser approved these changes Mar 24, 2026

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MichaReiser added rule Implementing or modifying a lint rule accepted Ready for implementation labels Mar 24, 2026

MichaReiser changed the title ~~Fix F507 false negative for literal non-tuple RHS in %-formatting~~ F507: Fix false negative for non-tuple RHS in %-formatting Mar 24, 2026

MichaReiser merged commit 6cff034 into astral-sh:main Mar 24, 2026
41 checks passed

RenzoMXD deleted the fix/f507-non-tuple-rhs-count-mismatch branch March 24, 2026 14:20

RenzoMXD mentioned this pull request Mar 24, 2026

Warn when using % operator on an f-string #24159

Closed

kaddkaka mentioned this pull request Mar 24, 2026

Mismatching number of %-placeholders in string #24164

Closed

BrewTestBot mentioned this pull request Mar 26, 2026

ruff 0.15.8 Homebrew/homebrew-core#274426

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`F507`: Fix false negative for non-tuple RHS in `%`-formatting#24142

`F507`: Fix false negative for non-tuple RHS in `%`-formatting#24142
MichaReiser merged 2 commits intoastral-sh:mainfrom
RenzoMXD:fix/f507-non-tuple-rhs-count-mismatch

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RenzoMXD commented Mar 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

What gets flagged now

What is intentionally NOT flagged

Why ResolvedPythonType

Context

Test plan

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RenzoMXD commented Mar 23, 2026 •

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Why `ResolvedPythonType`

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