New rule to prevent implicit string concatenation in collections #21972
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| code | total | + violation | - violation | + fix | - fix |
|---|---|---|---|---|---|
| ISC004 | 342 | 342 | 0 | 0 | 0 |
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FWIW This was one of the more impactful lint rules in Fixit that prevented several serious outages at Instagram/Meta |
MichaReiser
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Dec 15, 2025
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MichaReiser
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Thank you. This rule makes sense to me
crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/collection_literal.rs
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ntBre
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Dec 16, 2025
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ntBre
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Thank you, this looks great!
I only had one comment with a couple of small ideas, but I'm also happy with the current state of the diagnostic.
I do agree with Micha that we should make this an unsafe fix, if we keep the fix, though.
I think we should also touch base upstream. They expressed interest in adding this rule as well (flake8-implicit-str-concat/flake8-implicit-str-concat#55), so we should double check the error code to stay in sync, if we make it an ISC rule, which I think makes sense otherwise.
Have you had a chance to go through the ecosystem results? Cases like this feel a bit unfortunate to me:
assert not np.isclose(val, expected, rtol=1e-16, atol=0.0), (
f"Permittivity value test gives {val} and should not be "
f"equal to {expected}.",
)Maybe we should exclude cases with a single element?
crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/collection_literal.rs
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crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/collection_literal.rs
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I think I'm fine with assert not np.isclose(val, expected, rtol=1e-16, atol=0.0), (
f"Permittivity value test gives {val} and should not be "
f"equal to {expected}.",
)because the trailing |
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Thank you both!
Like Micha says, this should be a string not a tuple. There were several accidental tuples in my work codebase, so this would be good to keep. (Most of these were asserts, we could consider a separate rule that complains about single element str tuples as assert message)
Moved it to a note, thanks!
Made it an unsafe fix, thanks!
I prefer "unparenthesized" because it makes it clearer what the lint rule wants you to do
Commented upstream
Yeah, I looked at them, felt mostly positive / expected hits to me. In a large internal codebase this has caught dozens and dozens and dozens of bugs, and is a readability improvement in most of the other cases. |
ntBre
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Dec 16, 2025
crates/ruff_linter/src/rules/flake8_implicit_str_concat/rules/collection_literal.rs
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Fixed, and all is green! |
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Thanks again! I might give upstream at least until tomorrow to make sure there are no objections on the rule code (I take the 🚀 react on your comment as a positive sign), but this is otherwise good to go and should land before our release on Thursday. |
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This is a common footgun, see the example in #13014 (comment)
Fixes #13014 , fixes #13031