[`pylint`] Update `missing-maxsplit-arg` docs and error to suggest proper usage (`PLC0207`) #18949

junhsonjb · 2025-06-26T00:41:59Z

Summary

Fix #18383 by updating the documentation and error message to explain that users should use rsplit in order to access the last element of the result with maxsplit=1

Test Plan

Only documentation and an error message was changed. As such, snapshots were updated to reflect the new error message. With this change, all existing tests pass.

…maxsplit=1

github-actions · 2025-06-26T00:50:21Z

`ruff-ecosystem` results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+46 -46 violations, +0 -0 fixes in 13 projects; 42 projects unchanged)

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

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

- noxfile.py:453:16: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ noxfile.py:453:16: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`

apache/airflow (+9 -9 violations, +0 -0 fixes)

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

- dev/backport/update_backport_status.py:78:21: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ dev/backport/update_backport_status.py:78:21: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- dev/breeze/src/airflow_breeze/commands/common_options.py:482:26: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ dev/breeze/src/airflow_breeze/commands/common_options.py:482:26: PLC0207 Pass `maxsplit=1` into `str.split()`
- dev/breeze/src/airflow_breeze/commands/release_management_commands.py:2631:50: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ dev/breeze/src/airflow_breeze/commands/release_management_commands.py:2631:50: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- dev/breeze/src/airflow_breeze/prepare_providers/provider_documentation.py:701:34: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ dev/breeze/src/airflow_breeze/prepare_providers/provider_documentation.py:701:34: PLC0207 Pass `maxsplit=1` into `str.split()`
- dev/breeze/src/airflow_breeze/utils/run_utils.py:120:25: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ dev/breeze/src/airflow_breeze/utils/run_utils.py:120:25: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
... 8 additional changes omitted for project

apache/superset (+2 -2 violations, +0 -0 fixes)

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

- tests/common/query_context_generator.py:214:29: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ tests/common/query_context_generator.py:214:29: PLC0207 Pass `maxsplit=1` into `str.split()`
- tests/common/query_context_generator.py:255:22: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ tests/common/query_context_generator.py:255:22: PLC0207 Pass `maxsplit=1` into `str.split()`

bokeh/bokeh (+3 -3 violations, +0 -0 fixes)

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

- src/bokeh/io/notebook.py:663:26: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ src/bokeh/io/notebook.py:663:26: PLC0207 Pass `maxsplit=1` into `str.split()`
- src/bokeh/util/token.py:142:41: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ src/bokeh/util/token.py:142:41: PLC0207 Pass `maxsplit=1` into `str.split()`
- src/bokeh/util/token.py:155:41: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ src/bokeh/util/token.py:155:41: PLC0207 Pass `maxsplit=1` into `str.split()`

freedomofpress/securedrop (+1 -1 violations, +0 -0 fixes)

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

- securedrop/store.py:345:49: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ securedrop/store.py:345:49: PLC0207 Pass `maxsplit=1` into `str.split()`

ibis-project/ibis (+1 -1 violations, +0 -0 fixes)

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

- ibis/examples/gen_registry.py:125:52: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ ibis/examples/gen_registry.py:125:52: PLC0207 Pass `maxsplit=1` into `str.split()`

langchain-ai/langchain (+9 -9 violations, +0 -0 fixes)

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

- libs/core/langchain_core/_api/deprecation.py:360:17: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ libs/core/langchain_core/_api/deprecation.py:360:17: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- libs/core/langchain_core/_api/deprecation.py:360:56: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ libs/core/langchain_core/_api/deprecation.py:360:56: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- libs/core/langchain_core/_api/deprecation.py:368:17: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ libs/core/langchain_core/_api/deprecation.py:368:17: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- libs/core/langchain_core/_api/deprecation.py:369:16: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ libs/core/langchain_core/_api/deprecation.py:369:16: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- libs/core/langchain_core/_api/deprecation.py:473:24: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ libs/core/langchain_core/_api/deprecation.py:473:24: PLC0207 Pass `maxsplit=1` into `str.split()`
... 8 additional changes omitted for project

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

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

- pandas/compat/_optional.py:166:14: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ pandas/compat/_optional.py:166:14: PLC0207 Pass `maxsplit=1` into `str.split()`

pypa/cibuildwheel (+3 -3 violations, +0 -0 fixes)

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

- bin/update_pythons.py:150:22: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ bin/update_pythons.py:150:22: PLC0207 Pass `maxsplit=1` into `str.split()`
- cibuildwheel/platforms/macos.py:153:31: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ cibuildwheel/platforms/macos.py:153:31: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- cibuildwheel/selector.py:79:26: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ cibuildwheel/selector.py:79:26: PLC0207 Pass `maxsplit=1` into `str.split()`

zulip/zulip (+11 -11 violations, +0 -0 fixes)

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

- zerver/lib/send_email.py:287:16: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ zerver/lib/send_email.py:287:16: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- zerver/lib/send_email.py:429:21: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ zerver/lib/send_email.py:429:21: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- zerver/lib/upload/local.py:67:17: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ zerver/lib/upload/local.py:67:17: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- zerver/lib/upload/s3.py:176:25: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ zerver/lib/upload/s3.py:176:25: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`
- zerver/lib/webhooks/common.py:242:26: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ zerver/lib/webhooks/common.py:242:26: PLC0207 Pass `maxsplit=1` into `str.split()`
- zerver/lib/webhooks/common.py:269:16: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
... 11 additional changes omitted for project

wntrblm/nox (+1 -1 violations, +0 -0 fixes)

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

- tests/test_sessions.py:1199:44: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ tests/test_sessions.py:1199:44: PLC0207 Instead of `str.split()`, call `str.rsplit()` and pass `maxsplit=1`

pytest-dev/pytest (+3 -3 violations, +0 -0 fixes)

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

- src/_pytest/config/findpaths.py:160:16: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ src/_pytest/config/findpaths.py:160:16: PLC0207 Pass `maxsplit=1` into `str.split()`
- src/_pytest/terminal.py:1012:40: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ src/_pytest/terminal.py:1012:40: PLC0207 Pass `maxsplit=1` into `str.split()`
- src/_pytest/terminal.py:462:41: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ src/_pytest/terminal.py:462:41: PLC0207 Pass `maxsplit=1` into `str.split()`

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

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

- astropy/io/ascii/latex.py:438:16: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
+ astropy/io/ascii/latex.py:438:16: PLC0207 Pass `maxsplit=1` into `str.split()`

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
PLC0207	92	46	46	0	0

ntBre

I don't think this really addresses the problem in the issue. The confusion isn't from the rule applying to either split or rsplit calls, it's the fact that the fix might change which call you need.

This is the confusing case from the issue:

url = "www.example.com"
url.split(".")[-1]  # result is "com"

And the diagnostic from Ruff:

-:2:1: PLC0207 Accessing only the first or last element of `str.split()` without setting `maxsplit=1`
  |
1 | url = "www.example.com"
2 | url.split(".")[-1]  # result is "com"
  | ^^^^^^^^^^^^^^^^^^ PLC0207
  |

The error message implies that the fix is simply to add maxsplit=1, but that gives a different result:

```python
url = "www.example.com"
url.split(".", maxsplit=1)[-1]  # result is now "example.com" instead of "com"

The correct fix is to add maxplit=1 and change split to rsplit, restoring the original behavior.

That's the kind of information we need to include in the rule documentation and in the error message. In the latter case, we'll need to inspect the slice ([0] or [-1]) and call type (split or rsplit) and customize the error message accordingly so that we actually suggest the right thing for the specific situation.

junhsonjb · 2025-06-30T23:27:23Z

Thanks for the comments, that makes sense! I just patched some updates to address them!

MeGaGiGaGon · 2025-06-30T23:33:53Z

crates/ruff_linter/src/rules/pylint/rules/missing_maxsplit_arg.rs

+        let correct_function = match index {
+            0 => "split",
+            -1 => "rsplit",
+            _ => "", // We should never hit this case


This should probably be an unreachable! instead, since if that case is hit it should cause a failure instead of a silent issue.

I think we should probably define a simple enum instead of using an i64. That way we can check for known values up front and bail out of the whole diagnostic. Then we can match exhaustively here.

...les/pylint/snapshots/ruff_linter__rules__pylint__tests__PLC0207_missing_maxsplit_arg.py.snap

junhsonjb · 2025-07-08T14:39:06Z

Hey @ntBre — just wanted to check in on this. I’ve pushed some updates addressing your feedback above. When you get the chance, let me know if there’s anything else you’d like me to adjust. Thanks again! 🙂

ntBre

This looks great, thank you! And thanks for the ping.

I'm pretty sure these changes give us all the information we need to generate an autofix too. Feel free to work on that in a follow-up PR if you're interested :)

…re_help * 'main' of https://github.com/astral-sh/ruff: (34 commits) [docs] add capital one to who's using ruff (astral-sh#19248) [`pyupgrade`] Keyword arguments in `super` should suppress the `UP008` fix (astral-sh#19131) [`flake8-use-pathlib`] Add autofixes for `PTH100`, `PTH106`, `PTH107`, `PTH108`, `PTH110`, `PTH111`, `PTH112`, `PTH113`, `PTH114`, `PTH115`, `PTH117`, `PTH119`, `PTH120` (astral-sh#19213) [ty] Do not run `mypy_primer.yaml` when all changed files are Markdown files (astral-sh#19244) [`flake8-bandit`] Make example error out-of-the-box (`S412`) (astral-sh#19241) [`pydoclint`] Make example error out-of-the-box (`DOC501`) (astral-sh#19218) [ty] Add "kind" to completion suggestions [ty] Add type information to `all_members` API [ty] Expand API of `all_members` to return a struct [ty] Ecosystem analyzer PR comment workflow (astral-sh#19237) [ty] Merge `ty_macros` into `ruff_macros` (astral-sh#19229) [ty] Fix `ClassLiteral.into_callable` for dataclasses (astral-sh#19192) [ty] `dataclasses.field` support (astral-sh#19140) [ty] Fix panic for attribute expressions with empty value (astral-sh#19069) [ty] Return `CallableType` from `BoundMethodType.into_callable_type` (astral-sh#19193) [`flake8-bugbear`] Support non-context-manager calls in `B017` (astral-sh#19063) [ty] Improved diagnostic for reassignments of `Final` symbols (astral-sh#19214) [ty] Use full range for assignment definitions (astral-sh#19211) [`pylint`] Update `missing-maxsplit-arg` docs and error to suggest proper usage (`PLC0207`) (astral-sh#18949) [ty] Add `set -eu` to mypy-primer script (astral-sh#19212) ... # Conflicts: # crates/ty_python_semantic/src/types/class.rs

junhsonjb · 2025-07-16T18:21:11Z

This looks great, thank you! And thanks for the ping.

I'm pretty sure these changes give us all the information we need to generate an autofix too. Feel free to work on that in a follow-up PR if you're interested :)

@ntBre Great idea! PR is here

…19387)  ## Summary As a follow-up to #18949 (suggested [here](#18949 (review))), this PR implements auto-fix logic for `PLC0207`. ## Test Plan  Existing tests pass, with updates to the snapshot so that it expects the new output that comes along with the auto-fix.

update docs and error message to clarify accessing last element with …

3680ae7

…maxsplit=1

ntBre self-requested a review June 26, 2025 01:56

ntBre added the documentation Improvements or additions to documentation label Jun 26, 2025

ntBre requested changes Jun 26, 2025

View reviewed changes

junhsonjb added 4 commits June 30, 2025 18:54

build error msg based on function call and index

970a74e

update docs

9964954

slight update to error message

82c3b77

update snapshots

e9aedf5

junhsonjb requested a review from ntBre June 30, 2025 23:27

format, lint, and pre-commit

a14a518

MeGaGiGaGon reviewed Jun 30, 2025

View reviewed changes

junhsonjb requested a review from MeGaGiGaGon July 1, 2025 14:07

junhsonjb added 2 commits July 1, 2025 19:41

better variable names

2755044

use SliceBoundary enum instead of i64 to represent slice index

00e1a4e

junhsonjb changed the title ~~Update missing-maxsplit-arg docs and error to explain rsplit usage~~ Update missing-maxsplit-arg docs and error to suggest proper split/rsplit usage along with maxsplit=1 Jul 2, 2025

junhsonjb changed the title ~~Update missing-maxsplit-arg docs and error to suggest proper split/rsplit usage along with maxsplit=1~~ Update missing-maxsplit-arg docs and error to suggest proper usage Jul 2, 2025

ntBre approved these changes Jul 8, 2025

View reviewed changes

ntBre changed the title ~~Update missing-maxsplit-arg docs and error to suggest proper usage~~ [pylint] Update missing-maxsplit-arg docs and error to suggest proper usage (PLC0207) Jul 8, 2025

ntBre merged commit fda1889 into astral-sh:main Jul 8, 2025
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BrewTestBot mentioned this pull request Jul 11, 2025

ruff 0.12.3 Homebrew/homebrew-core#229792

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junhsonjb mentioned this pull request Jul 16, 2025

[pylint] Implement auto-fix for missing-maxsplit-arg (PLC0207) #19387

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[`pylint`] Update `missing-maxsplit-arg` docs and error to suggest proper usage (`PLC0207`) #18949

[`pylint`] Update `missing-maxsplit-arg` docs and error to suggest proper usage (`PLC0207`) #18949

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[pylint] Update missing-maxsplit-arg docs and error to suggest proper usage (PLC0207) #18949

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[`pylint`] Update `missing-maxsplit-arg` docs and error to suggest proper usage (`PLC0207`) #18949

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