[`ruff`] New rule `RUF073`: warn when using `%` operator on an f-string by RenzoMXD · Pull Request #24162 · astral-sh/ruff

RenzoMXD · 2026-03-24T21:15:09Z

Summary

Adds a new rule RUF072 (fstring-percent-format) that flags any use of the % operator on an f-string.

Mixing f-string interpolation with %-formatting is almost certainly a mistake since both serve the same purpose. There's no valid use case for using % on an f-string.

# Flagged
f"{name}" % name
f"hello %s %s" % (1, 2)
f"value: {x}" % {"key": "value"}

# OK — plain string literals are handled by existing F50x rules
"hello %s" % name
"%s %s" % (1, 2)

Test Plan

Added test cases for f-strings with % on various RHS types (variables, tuples, dicts, literals)
Added test cases for plain string literals (not flagged — handled by F50x)
Verified existing ruff and pyflakes tests still pass

cargo test -p ruff_linter -- fstring_percent_format

Test file: crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF072_RUF072.py.snap

Test cases cover:

F-string with % and variable RHS: f"{banana}" % banana
F-string with % and tuple RHS: f"hello %s %s" % (1, 2)
F-string with % and dict RHS: f"value: {x}" % {"key": "value"}
F-string with % and literal RHS: f"{x}" % 42
Plain string literals not flagged (handled by existing F50x rules)

astral-sh-bot · 2026-03-24T21:26:58Z

`ruff-ecosystem` results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+1 -0 violations, +0 -0 fixes in 1 projects; 55 projects unchanged)

pytest-dev/pytest (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --no-fix --output-format concise --preview

+ src/_pytest/main.py:622:16: RUF073 `%` operator used on an f-string

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
RUF073	1	1	0	0	0

MichaReiser · 2026-03-25T07:35:17Z

I'll close this for now until we have agreement on #24159. We can always reopen it

MichaReiser

Thanks. This overall looks good to me.

MichaReiser · 2026-03-25T09:17:23Z

crates/ruff_linter/src/rules/ruff/rules/fstring_percent_format.rs

+/// "hello %s %s" % (1, 2)
+/// ```
+#[derive(ViolationMetadata)]
+#[violation_metadata(preview_since = "0.15.7")]


Suggested change

#[violation_metadata(preview_since = "0.15.7")]

#[violation_metadata(preview_since = "RUFF_NEXT_VERSION")]

I'm sorry, it's NEXT_RUFF_VERSION

MichaReiser · 2026-03-25T09:19:48Z

crates/ruff_linter/src/rules/ruff/rules/fstring_percent_format.rs

+/// ## Example
+/// ```python
+/// f"{name}" % name
+/// f"hello %s %s" % (1, 2)


Let's use variables instead of 1 and 2 and I think we should recommend using f-string interpolation over %.

MichaReiser · 2026-03-25T12:20:27Z

This now requires a rebase.

MichaReiser · 2026-03-25T13:09:27Z

Thank you

* main: [ty] make `test-case` a dev-dependency (#24187) [ty] implement cycle normalization for more types to prevent too-many-cycle panics (#24061) [ty] Silence all diagnostics in unreachable code (#24179) [ty] Intern `InferableTypeVars` (#24161) Implement unnecessary-if (RUF050) (#24114) Recognize `Self` annotation and `self` assignment in SLF001 (#24144) Bump the npm version before publish (#24178) [ty] Disallow Self in metaclass and static methods (#23231) Use trusted publishing for NPM packages (#24171) [ty] Respect non-explicitly defined dataclass params (#24170) Add RUF072: warn when using operator on an f-string (#24162) [ty] Check return type of generator functions (#24026) Implement useless-finally (RUF-072) (#24165) [ty] Add test for a dataclass with a default field converter (#24169) [ty] Dataclass field converters (#23088) [flake8-bandit] Treat sys.executable as trusted input in S603 (#24106) [ty] Add support for `typing.Concatenate` (#23689) `ASYNC115`: autofix to use full qualified `anyio.lowlevel` import (#24166) [ty] Disallow read-only fields in TypedDict updates (#24128) Speed up diagnostic rendering (#24146)

@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

astral-sh-bot bot assigned amyreese Mar 24, 2026

astral-sh-bot bot requested a review from amyreese March 24, 2026 21:15

MichaReiser closed this Mar 25, 2026

MichaReiser reopened this Mar 25, 2026

MichaReiser added rule Implementing or modifying a lint rule preview Related to preview mode features labels Mar 25, 2026

MichaReiser approved these changes Mar 25, 2026

View reviewed changes

RenzoMXD added 2 commits March 25, 2026 13:31

Add RUF072: warn when using operator on an f-string

2416e7f

fix lint

ff21afc

RenzoMXD force-pushed the feat/ruf072-fstring-percent-format branch from bc9651f to ff21afc Compare March 25, 2026 12:40

fix conflicts

8692aad

RenzoMXD force-pushed the feat/ruf072-fstring-percent-format branch from 0ad7bb2 to 8692aad Compare March 25, 2026 12:57

MichaReiser merged commit 32f644c into astral-sh:main Mar 25, 2026
42 checks passed

amyreese changed the title ~~Add RUF072: warn when using operator on an f-string~~ Add RUF072: warn when using % operator on an f-string Mar 26, 2026

amyreese changed the title ~~Add RUF072: warn when using % operator on an f-string~~ Add RUF073: warn when using % operator on an f-string Mar 26, 2026

amyreese changed the title ~~Add RUF073: warn when using % operator on an f-string~~ [ruff] New rule RUF073: warn when using % operator on an f-string Mar 26, 2026

BrewTestBot mentioned this pull request Mar 26, 2026

ruff 0.15.8 Homebrew/homebrew-core#274426

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[`ruff`] New rule `RUF073`: warn when using `%` operator on an f-string#24162

[`ruff`] New rule `RUF073`: warn when using `%` operator on an f-string#24162
MichaReiser merged 3 commits intoastral-sh:mainfrom
RenzoMXD:feat/ruf072-fstring-percent-format

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	#[violation_metadata(preview_since = "0.15.7")]
	#[violation_metadata(preview_since = "RUFF_NEXT_VERSION")]

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RenzoMXD commented Mar 24, 2026

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