Implement RUF027: Missing F-String Syntax lint

[snowsignal]

Summary

Fixes #8151

This PR implements a new rule, RUF027.

What it does

Checks for strings that contain f-string syntax but are not f-strings.

Why is this bad?

An f-string missing an f at the beginning won't format anything, and instead treat the interpolation syntax as literal.

Example

name = "Sarah"
dayofweek = "Tuesday"
msg = "Hello {name}! It is {dayofweek} today!"

It should instead be:

name = "Sarah"
dayofweek = "Tuesday"
msg = f"Hello {name}! It is {dayofweek} today!"

Heuristics

Since there are many possible string literals which contain syntax similar to f-strings yet are not intended to be,
this lint will disqualify any literal that satisfies any of the following conditions:

1. The string literal is a standalone expression. For example, a docstring.
2. The literal is part of a function call with keyword arguments that match at least one variable (for example: format("Message: {value}", value = "Hello World"))
3. The literal (or a parent expression of the literal) has a direct method call on it (for example: "{value}".format(...))
4. The string has no {...} expression sections, or uses invalid f-string syntax.
5. The string references variables that are not in scope, or it doesn't capture variables at all.
6. Any format specifiers in the potential f-string are invalid.

Test Plan

I created a new test file, RUF027.py, which is both an example of what the lint should catch and a way to test edge cases that may trigger false positives.

[charliermarsh]

Excellent.

[MichaReiser]

This is looking good. I mainly left a few nit comments and some suggestions for more tests (I fear, some of them will be painful)

[github-actions]

ruff-ecosystem results

Linter (stable)

ℹ️ ecosystem check encountered linter errors. (no lint changes; 1 project error)

sphinx-doc/sphinx (error)

ruff failed
  Cause: Selection of unstable rules without the `--preview` flag is not allowed. Enable preview or remove selection of:
	- FURB113
	- FURB131
	- FURB132

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+91 -0 violations, +0 -0 fixes in 7 projects; 36 projects unchanged)

apache/airflow (+4 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select ALL

+ airflow/models/dag.py:2155:30: RUF027 Possible f-string without an `f` prefix
+ airflow/providers/amazon/aws/hooks/s3.py:746:21: RUF027 Possible f-string without an `f` prefix
+ tests/providers/docker/operators/test_docker.py:566:28: RUF027 Possible f-string without an `f` prefix
+ tests/test_utils/asserts.py:94:16: RUF027 Possible f-string without an `f` prefix

bokeh/bokeh (+9 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select ALL

+ examples/plotting/customjs_expr.py:26:49: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:47:13: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:48:13: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:49:13: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:50:13: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:51:13: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:52:13: RUF027 Possible f-string without an `f` prefix
+ examples/styling/mathtext/latex_normal_distribution.py:53:13: RUF027 Possible f-string without an `f` prefix
+ src/bokeh/models/annotations/legends.py:581:28: RUF027 Possible f-string without an `f` prefix

ibis-project/ibis (+2 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview

+ ibis/backends/exasol/__init__.py:70:13: RUF027 Possible f-string without an `f` prefix
+ ibis/backends/snowflake/__init__.py:338:9: RUF027 Possible f-string without an `f` prefix

pandas-dev/pandas (+5 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview

+ pandas/io/formats/format.py:1266:27: RUF027 Possible f-string without an `f` prefix
+ pandas/io/formats/format.py:1268:27: RUF027 Possible f-string without an `f` prefix
+ pandas/tests/dtypes/test_dtypes.py:1063:13: RUF027 Possible f-string without an `f` prefix
+ pandas/tests/indexes/base_class/test_formats.py:138:35: RUF027 Possible f-string without an `f` prefix
+ pandas/tests/indexes/base_class/test_formats.py:141:16: RUF027 Possible f-string without an `f` prefix

rotki/rotki (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview

+ rotkehlchen/tests/utils/blockchain.py:410:38: RUF027 Possible f-string without an `f` prefix

scikit-build/scikit-build-core (+1 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview

+ src/scikit_build_core/builder/wheel_tag.py:84:27: RUF027 Possible f-string without an `f` prefix

zulip/zulip (+69 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select ALL

+ tools/droplets/create.py:321:21: RUF027 Possible f-string without an `f` prefix
+ zerver/actions/streams.py:1575:13: RUF027 Possible f-string without an `f` prefix
+ zerver/actions/streams.py:1576:13: RUF027 Possible f-string without an `f` prefix
+ zerver/actions/streams.py:1577:13: RUF027 Possible f-string without an `f` prefix
+ zerver/lib/redis_utils.py:44:40: RUF027 Possible f-string without an `f` prefix
+ zerver/lib/typed_endpoint.py:366:32: RUF027 Possible f-string without an `f` prefix
+ zerver/management/commands/edit_linkifiers.py:14:12: RUF027 Possible f-string without an `f` prefix
+ zerver/migrations/0423_fix_email_gateway_attachment_owner.py:93:21: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/curl_param_value_generators.py:281:9: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1031:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1046:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1055:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1096:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1133:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1157:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1307:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1398:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1409:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:1487:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:303:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:315:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:321:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:333:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:343:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:355:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:362:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:379:17: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:454:25: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:469:25: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:477:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:572:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:588:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:627:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:796:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:809:17: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:934:45: RUF027 Possible f-string without an `f` prefix
+ zerver/openapi/python_examples.py:959:45: RUF027 Possible f-string without an `f` prefix
... 32 additional changes omitted for project

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
RUF027	91	91	0	0	0

[codspeed-hq]

CodSpeed Performance Report

Merging #9728 will not alter performance

_{Comparing jane/linter/rule/fstring (3e5953e) with main (25d9305)}

Summary

✅ 30 untouched benchmarks

[zanieb]

Really weird false positive

https://github.com/pandas-dev/pandas/blob/c3f7fee653fef5598055227821b6974f1eb87968/pandas/core/window/doc.py#L96-L115

They're basically doing "{foo}".replace("{foo}", foo) which such an anti-pattern that I'm not sure I mind?

[zanieb]

Here's another false positive

https://github.com/DisnakeDev/disnake/blob/a24abf43297215b6fe70e0bc61b2ec1dada84b9f/disnake/client.py#L2446

They're doing data = {"foo": 1, "bar": 2}; "{foo} {bar}".format_map(data) which seems like a true false positive. Perhaps we should ignore strings with a .<call> attached

[zanieb]

This false positive is no good

https://github.com/apache/airflow/blob/3ec781946a7fcb5fa5bc99449d59d5981e6257ab/airflow/www/utils.py#L437-L446

e.g. "{foo}".format(foo=1) should definitely not be flagged

The vast majority of the false positives in the ecosystem are this.

I think we should also not flag " = "{foo}"; x.format(foo=1) if feasible.

[zanieb]

This false positive is due to some inline JavaScript written as a string

https://github.com/bokeh/bokeh/blob/829b2a75c402d0d0abd7e37ff201fbdfd949d857/examples/plotting/customjs_expr.py#L26-L38

I think that's okay...

[snowsignal]

e.g. "{foo}".format(foo=1) should definitely not be flagged

I think this ecosystem check might be outdated, this should be fixed.

[zanieb]

e.g. "{foo}".format(foo=1) should definitely not be flagged

I think this ecosystem check might be outdated, this should be fixed.

Sweet. It'll update when the build succeeds.

[snowsignal]

Perhaps we should ignore strings with a .<call> attached

This is probably the best approach, honestly.