Update line break heuristic when f-string has format specifier #10040
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dhruvmanila
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bug
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Thanks for opening this as an issue. Is this something you plan to work on? |
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Yeah, although not right away. It should be a simple fix. |
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I think with #7787, this issue isn't relevant anymore because both unformatted and formatted code produces the same AST as the lexer would stop at the newline token. |
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Right, but only for single-quoted string. It's still relevant for triple-quoted string. |
dhruvmanila
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Apr 26, 2024
## Summary
This PR fixes the bug where the formatter would format an f-string and
could potentially change the AST.
For a triple-quoted f-string, the element can't be formatted into
multiline if it has a format specifier because otherwise the newline
would be treated as part of the format specifier.
Given the following f-string:
```python
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
variable:.3f} ddddddddddddddd eeeeeeee"""
```
The formatter sees that the f-string is already multiline so it assumes
that it can contain line breaks i.e., broken into multiple lines. But,
in this specific case we can't format it as:
```python
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
variable:.3f
} ddddddddddddddd eeeeeeee"""
```
Because the format specifier string would become ".3f\n", which is not
the original string (`.3f`).
If the original source code already contained a newline, they'll be
preserved. For example:
```python
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {
variable:.3f
} ddddddddddddddd eeeeeeee"""
```
The above will be formatted as:
```py
f"""aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbb ccccccccccc {variable:.3f
} ddddddddddddddd eeeeeeee"""
```
Note that the newline after `.3f` is part of the format specifier which
needs to be preserved.
The Python version is irrelevant in this case.
fixes: #10040
## Test Plan
Add some test cases to verify this behavior.
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Given:
Ruff (with
--preview) would format is as:Here, we're changing the format specifier from
.3fto.3f\n.Now, if the user had the following code:
Then, with the new heuristic to not have line breaks would mean that we'd format the above code as:
The newline is already present in the format specifier
.3f\nin the original source code.Local or Global?
Another question is whether this heuristic should be applied globally or local to an individual expression element. For example, in the following code snippet,
We've an expression element which has a line break and doesn't contain a format specifier (third field). This means that the f-string layout is multiline. So, the first element cannot have line breaks while the second and third can have. If so, then we'd potentially format it as:
If we apply the heuristic globally, then the formatter would collapse all of the expression elements.
Playground: https://play.ruff.rs/c2d0cc39-30ab-4c17-ac8c-0f4d8a9afbb6
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