Check Watch does not show output format error message from clearing too late

[MeGaGiGaGon]

Summary

I found this while helping on #19550. When running check in watch mode with a non-full output-format, this error:

warning: `--output-format full` is always used in watch mode.

Does not get displayed due to the terminal being cleared directly after. Here's the contents of out.txt I get when running uvx ruff check --watch . --output-format json 2>&1 > out.txt:

warning: `--output-format full` is always used in watch mode.
�[H�[2J�[3J[23:04:31 PM] Starting linter in watch mode...

[23:04:31 PM] Found 7 errors. Watching for file changes.

... normal error output here

I assume �[H�[2J�[3J is the magic characters that cause a terminal clear in PowerShell, which shows that it's erasing the warning. This should be fixable by clearing the terminal prior to warnings being output, though I haven't looked at the code to see how easy it would be.

Also just a side tangent thought, but it might be better if the error said concise instead of full, since it definitely does not give the same output as non-watch check on full.

Version

ruff 0.12.5 (d13228a 2025-07-24)

[ntBre]

Possibly even more surprising is the fact that --preview enables the full format:

ruff/crates/ruff/src/printer.rs

Lines 460 to 462 in 6663571

	TextEmitter::default()
	.with_show_fix_status(show_fix_status(self.fix_mode, fixables.as_ref()))
	.with_show_source(preview)

Compare the with_show_source call to the write_once implementation:

ruff/crates/ruff/src/printer.rs

Lines 262 to 265 in 6663571

	TextEmitter::default()
	.with_show_fix_status(show_fix_status(self.fix_mode, fixables.as_ref()))
	.with_show_fix_diff(self.flags.intersects(Flags::SHOW_FIX_DIFF))
	.with_show_source(self.format == OutputFormat::Full)

But yeah, without --preview, the default is definitely concise.

[mikeleppane]

Hi! I've addressed the main issue, but I have a few comments regarding the output formatting:

• What is the reason for using only "one" formatting output logic in --watch mode compared to non-watch mode?
• As mentioned earlier, is it an oversight that the --preview option enables full mode formatting?
• Is the warning: warning: --output-format full is always used in watch mode. needed in the first place? Or is it trying to say that in watch mode, we only apply full mode?

Note: I looked at the #19550 for more details. I think these two can be combined and tackled together.

[ntBre]

Those are good questions. I'm not sure I know the exact answers, but I can at least weigh in :)

• I'm sure there was a reason at some point only to use one output format with --watch, but I think it makes sense to support --output-format now (possibly --watch was added well before we had multiple output formats)
• I think I view the --preview flag as changing the default output format, in the context of adding --output-format support. I think we can do both by continuing to default to concise output (with no --ouput-format specified) but default to full when --preview is specified. We should still stabilize that behavior in a future minor release. We should probably open a separate issue for that and add it to the 0.13 milestone. However, it might be a bit less necessary with --output-format support, so we could also consider dropping the preview feature. In either case, we should hold off there until the next minor release.
• If we add --output-format support, we should be able to drop the warning entirely, as you suggested.

[MichaReiser]

What is the reason for using only "one" formatting output logic in --watch mode compared to non-watch mode?

I don't know the reason but there's an argument that some output formats seem less useful with --watch. I'm not sure if it's worth restricting the output formats because of it but it could make sense to take a quick look at what other tools support with --watch.

[MeGaGiGaGon]

It might have been to maintain backwards compatibility? Since very old versions of ruff only supported concise-style outputs, and it looks like the decision to only support one output style with watch was made here #181 (comment)

Edit: I did some more searching, and it looks also like watch got forgotten about when #7350 happened, since prior to that change (in ruff@0.1) --watch with --show-source does work, but after (in ruff@0.2 +) it no longer works.

[MichaReiser]

We should stabilize #21097 in the next release which will fix this issue (I assigned this issue to 0.15) to make sure we stabilizie the behavior change