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## Summary _This is preview only feature and is available using the `--preview` command-line flag._ With the implementation of [PEP 701] in Python 3.12, f-strings can now be broken into multiple lines, can contain comments, and can re-use the same quote character. Currently, no other Python formatter formats the f-strings so there's some discussion which needs to happen in defining the style used for f-string formatting. Relevant discussion: https://github.com/astral-sh/ruff/discussions/9785 The goal for this PR is to add minimal support for f-string formatting. This would be to format expression within the replacement field without introducing any major style changes. ### Newlines The heuristics for adding newline is similar to that of [Prettier](https://prettier.io/docs/en/next/rationale.html#template-literals) where the formatter would only split an expression in the replacement field across multiple lines if there was already a line break within the replacement field. In other words, the formatter would not add any newlines unless they were already present i.e., they were added by the user. This makes breaking any expression inside an f-string optional and in control of the user. For example, ```python # We wouldn't break this aaaaaaaaaaa = f"asaaaaaaaaaaaaaaaa { aaaaaaaaaaaa + bbbbbbbbbbbb + ccccccccccccccc } cccccccccc" # But, we would break the following as there's already a newline aaaaaaaaaaa = f"asaaaaaaaaaaaaaaaa { aaaaaaaaaaaa + bbbbbbbbbbbb + ccccccccccccccc } cccccccccc" ``` If there are comments in any of the replacement field of the f-string, then it will always be a multi-line f-string in which case the formatter would prefer to break expressions i.e., introduce newlines. For example, ```python x = f"{ # comment a }" ``` ### Quotes The logic for formatting quotes remains unchanged. The existing logic is used to determine the necessary quote char and is used accordingly. Now, if the expression inside an f-string is itself a string like, then we need to make sure to preserve the existing quote and not change it to the preferred quote unless it's 3.12. For example, ```python f"outer {'inner'} outer" # For pre 3.12, preserve the single quote f"outer {'inner'} outer" # While for 3.12 and later, the quotes can be changed f"outer {"inner"} outer" ``` But, for triple-quoted strings, we can re-use the same quote char unless the inner string is itself a triple-quoted string. ```python f"""outer {"inner"} outer""" # valid f"""outer {'''inner'''} outer""" # preserve the single quote char for the inner string ``` ### Debug expressions If debug expressions are present in the replacement field of a f-string, then the whitespace needs to be preserved as they will be rendered as it is (for example, `f"{ x = }"`. If there are any nested f-strings, then the whitespace in them needs to be preserved as well which means that we'll stop formatting the f-string as soon as we encounter a debug expression. ```python f"outer { x = !s :.3f}" # ^^ # We can remove these whitespaces ``` Now, the whitespace doesn't need to be preserved around conversion spec and format specifiers, so we'll format them as usual but we won't be formatting any nested f-string within the format specifier. ### Miscellaneous - The [`hug_parens_with_braces_and_square_brackets`](https://github.com/astral-sh/ruff/issues/8279) preview style isn't implemented w.r.t. the f-string curly braces. - The [indentation](https://github.com/astral-sh/ruff/discussions/9785#discussioncomment-8470590) is always relative to the f-string containing statement ## Test Plan * Add new test cases * Review existing snapshot changes * Review the ecosystem changes [PEP 701]: https://peps.python.org/pep-0701/ |
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