docs: Clarify COM819 and formatter interaction#25045
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MichaReiser merged 3 commits intoMay 13, 2026
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When fixing #25044 and review what other rules might not be documented, I've stumbled upon https://docs.astral.sh/ruff/rules/prohibited-trailing-comma/ which is documented, but it seems to be more than just redundant. Using this rule changes the meaning of using magic comma in some cases. E.g. `foo = (1, 2, 3,)` - if user would run `ruff check` first, then it would strip the magic comma and `ruff format` would do nothing. And if we'd do `ruff format` first, then it would expand this to multiple lines and then `ruff check` would do nothing. Given that when you're writing code you might leave this magic command, knowing that it will expand later in an expected way, - with COM819 it might if you run linter first. So I've added a warning when this rule is enabled and adjusted the documentation.
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Thank you. I don't see COM819 qualify as being incompatible with the formatter. A rule is only incompatible if the formatter can introduce new violations when running the formatter after a clean I do agree that you want to disable |
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Yeah, incompatibility is probably too strong of a term for this, it's more of a conflict. Personally, I'd like this kind of rules conflict to surface to me when using the tool, instead of encountering inconsistency and exploring the docs, but warnings can be annoying - especially since this one would become a new default ignore for everyone using select Anyways, I've dropped the warning and updated the docs, mentioning |
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May 23, 2026
When fixing astral-sh#25044 and review what other rules might not be documented, I've stumbled upon https://docs.astral.sh/ruff/rules/prohibited-trailing-comma/ which is documented, but it seems to be more than just redundant. Using this rule changes the meaning of using magic comma in some cases. E.g. `foo = (1, 2, 3,)` - if user would run `ruff check` first, then it would strip the magic comma and `ruff format` would do nothing. And if we'd do `ruff format` first, then it would expand this to multiple lines and then `ruff check` would do nothing. Given that when you're writing code you might leave this magic command, knowing that it will expand later in an expected way, - with COM819 it might if you run linter first. So I've added a warning when this rule is enabled and adjusted the documentation. I've executed the tests and adjusted the ones that were testing `ALL`. All tests seems to be fine now, besides that one (seems unrelated to PR) and bunch of other tests that seems to conflict with my global ruff settings, I'll report it as an issue separately (see astral-sh#25046). ``` Reviewing [1/6] [email protected]: Snapshot: semantic_syntax_errors Source: crates/ruff/tests/cli/lint.rs:3553 ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── program: ruff args: - check - "--output-format" - concise - "--preview" - "--quiet" ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── -old snapshot +new results ────────────┬───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── 1 1 │ success: false 2 2 │ exit_code: 1 3 3 │ ----- stdout ----- 4 4 │ main.py:1:3: error[invalid-syntax] assignment expression cannot rebind comprehension variable 5 │-main.py:1:20: error[F821] Undefined name `foo` 6 5 │ 7 6 │ ----- stderr ----- ────────────┴───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── ``` --------- Co-authored-by: Micha Reiser <[email protected]>
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When fixing #25044 and review what other rules might not be documented, I've stumbled upon https://docs.astral.sh/ruff/rules/prohibited-trailing-comma/ which is documented, but it seems to be more than just redundant.
Using this rule changes the meaning of using magic comma in some cases. E.g.
foo = (1, 2, 3,)- if user would runruff checkfirst, then it would strip the magic comma andruff formatwould do nothing. And if we'd doruff formatfirst, then it would expand this to multiple lines and thenruff checkwould do nothing. Given that when you're writing code you might leave this magic command, knowing that it will expand later in an expected way, - with COM819 it might if you run linter first.So I've added a warning when this rule is enabled and adjusted the documentation.
I've executed the tests and adjusted the ones that were testing
ALL.All tests seems to be fine now, besides that one (seems unrelated to PR) and bunch of other tests that seems to conflict with my global ruff settings, I'll report it as an issue separately (see #25046).