Strangeness with implicit string concatenation, f-strings, and rules F541, RUF027

[dougthor42]

Summary

Seems like f-string-missing-placeholders (F541) and missing-f-string-syntax (RUF027) don't play nicely with implicit string concatenation.

$ ruff --version
ruff 0.4.4
$ python --version
Python 3.8.13

Examples:

Commands used:

$ ruff check --select F541 --preview --isolated ruff_bug.py
$ ruff check --select RUF027 --preview --isolated ruff_bug.py

Code	Expected F541	Actual F541	Expected RUF027	Actual RUF027
# ruff_bug.py a = "hello" print( f"some text here" f" bar {a}." f" {a}" )	Error on line 4	All checks passed!	All checks passed!	All checks passed!
# ruff_bug.py a = "hello" print( f"some text here" f" bar {a}." " {a}" )	Error on line 4	All checks passed!	Error on line 6	All checks passed!
# ruff_bug.py a = "hello" print( "some text here" f" bar {a}." " {a}" )	All checks passed!	All checks passed!	Error on line 6	All checks passed!
# ruff_bug.py a = "hello" print( f"some text here" " bar {a}." " {a}" )	Error on line 4	Error on line 4	Error on line 5 and 6	All checks passed!
# ruff_bug.py a = "hello" print( "some text here" " bar {a}." f" {a}" )	All checks passed!	All checks passed!	Error on line 5	All checks passed!

[dhruvmanila]

Thank you providing such a detailed set of examples along with the diagnosis!

Regarding F541, we decided to maintain this behavior where the F541 rule is only flagged when all of the f-strings in an implicit string concatenation doesn't have any placeholders. Refer to #10885 for more details.

I've pasted all of the examples in the playground (https://play.ruff.rs/0de4d4a3-bbb3-44bd-b163-2778dba84f64) and I'm seeing the expected behavior for RUF027 for examples 2, 3, 4, and 5.

And, I'm seeing the same behavior on the CLI as well:

$ ruff check --select RUF027 --preview --isolated src/RUF027.py --output-format=concise
src/RUF027.py:14:5: RUF027 Possible f-string without an `f` prefix
src/RUF027.py:22:5: RUF027 Possible f-string without an `f` prefix
src/RUF027.py:29:5: RUF027 Possible f-string without an `f` prefix
src/RUF027.py:30:5: RUF027 Possible f-string without an `f` prefix
src/RUF027.py:37:5: RUF027 Possible f-string without an `f` prefix
Found 5 errors.
No fixes available (5 hidden fixes can be enabled with the `--unsafe-fixes` option).

I'm not sure why you're seeing different behavior for RUF027. Can you re-check?

[dougthor42]

F541

Ah ok, I didn't realize F541 was expected behavior.:+1:. I plan on making a PR to update the docs https://docs.astral.sh/ruff/rules/f-string-missing-placeholders/ to include something like your comment. Perhaps:

Note regarding implicit string concatenation: In order to maintain compatibilty with the source pyflakes rule, this rule is only flagged when all of the f-strings in an implicit string concatenation don't have any placeholders. For example:

Please see Issue #10885 for more details.

RUF027

Strange! Yeah I'll double check.

...

Oh son of a b*. My CLI arg was set to RUF029 🤦

Sorry about that! It looks like everything is WAI so I'll close this. Thanks for double checking!

[dhruvmanila]

No worries, thanks for confirming!

Ah ok, I didn't realize F541 was expected behavior.:+1:. I plan on making a PR to update the docs docs.astral.sh/ruff/rules/f-string-missing-placeholders to include something like your comment. Perhaps:

Yeah, I think that would be useful