Add f-string formatting to the docs (#15367)

Revive https://github.com/astral-sh/ruff/pull/15341 as it got removed
from the latest rebase in https://github.com/astral-sh/ruff/pull/15238.

crates/ruff_python_formatter/README.md

@@ -19,7 +19,7 @@ filed in the issue tracker. If you've identified a new deviation, please [file a
 
 When run over _non_-Black-formatted code, the formatter makes some different decisions than Black,
 and so more deviations should be expected, especially around the treatment of end-of-line comments.
-For details, see [Black compatibility](https://docs.astral.sh/ruff/formatter/#black-compatibility).
+For details, see [Style Guide](https://docs.astral.sh/ruff/formatter/#style-guide).
 
 ## Getting started
 

docs/faq.md

@@ -28,7 +28,7 @@ to see a few differences on the margins, but the vast majority of your code shou
 When run over _non_-Black-formatted code, the formatter makes some different decisions than Black,
 and so more deviations should be expected, especially around the treatment of end-of-line comments.
 
-See [_Black compatibility_](formatter.md#black-compatibility) for more.
+See [_Style Guide_](formatter.md#style-guide) for more.
 
 ## How does Ruff's linter compare to Flake8?
 

docs/formatter.md

@@ -33,7 +33,7 @@ adoption is minimally disruptive for the vast majority of projects.
 
 Specifically, the formatter is intended to emit near-identical output when run over existing
 Black-formatted code. When run over extensive Black-formatted projects like Django and Zulip, > 99.9%
-of lines are formatted identically. (See: [_Black compatibility_](#black-compatibility).)
+of lines are formatted identically. (See: [_Style Guide](#style-guide).)
 
 Given this focus on Black compatibility, the formatter thus adheres to [Black's (stable) code style](https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html),
 which aims for "consistency, generality, readability and reducing git diffs". To give you a sense
@@ -373,21 +373,10 @@ Meanwhile, `ruff format --check` exits with the following status codes:
 - `2` if Ruff terminates abnormally due to invalid configuration, invalid CLI options, or an
     internal error.
 
-## Black compatibility
+## Style Guide
 
 The formatter is designed to be a drop-in replacement for [Black](https://github.com/psf/black).
-
+This section documents the areas where the Ruff formatter goes beyond Black in terms of code style.
-Specifically, the formatter is intended to emit near-identical output when run over Black-formatted
-code. When run over extensive Black-formatted projects like Django and Zulip, > 99.9% of lines
-are formatted identically. When migrating an existing project from Black to Ruff, you should expect
-to see a few differences on the margins, but the vast majority of your code should be unchanged.
-
-When run over _non_-Black-formatted code, the formatter makes some different decisions than Black,
-and so more deviations should be expected, especially around the treatment of end-of-line comments.
-
-If you identify deviations in your project, spot-check them against the [known deviations](formatter/black.md),
-as well as the [unintentional deviations](https://github.com/astral-sh/ruff/issues?q=is%3Aopen+is%3Aissue+label%3Aformatter)
-filed in the issue tracker. If you've identified a new deviation, please [file an issue](https://github.com/astral-sh/ruff/issues/new).
 
 ### Intentional deviations
 
@@ -398,11 +387,120 @@ Black's code style, while others fall out of differences in the underlying imple
 For a complete enumeration of these intentional deviations, see [_Known deviations_](formatter/black.md).
 
 Unintentional deviations from Black are tracked in the [issue tracker](https://github.com/astral-sh/ruff/issues?q=is%3Aopen+is%3Aissue+label%3Aformatter).
+If you've identified a new deviation, please [file an issue](https://github.com/astral-sh/ruff/issues/new).
 
 ### Preview style
+
 Similar to [Black](https://black.readthedocs.io/en/stable/the_black_code_style/future_style.html#preview-style), Ruff implements formatting changes
 under the [`preview`](https://docs.astral.sh/ruff/settings/#format_preview) flag, promoting them to stable through minor releases, in accordance with our [versioning policy](https://github.com/astral-sh/ruff/discussions/6998#discussioncomment-7016766).
 
+### F-string formatting
+
+_Stabilized in Ruff 0.9.0_
+
+Unlike Black, Ruff formats the expression parts of f-strings which are the parts inside the curly
+braces `{...}`. This is a [known deviation](formatter/black.md#f-strings) from Black.
+
+Ruff employs several heuristics to determine how an f-string should be formatted which are detailed
+below.
+
+#### Quotes
+
+Ruff will use the [configured quote style] for the f-string expression unless doing so would result in
+invalid syntax for the target Python version or requires more backslash escapes than the original
+expression. Specifically, Ruff will preserve the original quote style for the following cases:
+
+When the target Python version is < 3.12 and a [self-documenting f-string] contains a string
+literal with the [configured quote style]:
+
+```python
+# format.quote-style = "double"
+
+f'{10 + len("hello")=}'
+# This f-string cannot be formatted as follows when targeting Python < 3.12
+f"{10 + len("hello")=}"
+```
+
+When the target Python version is < 3.12 and an f-string contains any triple-quoted string, byte
+or f-string literal that contains the [configured quote style]:
+
+```python
+# format.quote-style = "double"
+
+f'{"""nested " """}'`
+# This f-string cannot be formatted as follows when targeting Python < 3.12
+f"{'''nested " '''}``
+```
+
+For all target Python versions, when a [self-documenting f-string] contains an expression between
+the curly braces (`{...}`) with a format specifier containing the [configured quote style]:
+
+```python
+# format.quote-style = "double"
+
+f'{1=:"foo}'
+# This f-string cannot be formatted as follows for all target Python versions
+f"{1=:"foo}"
+```
+
+For nested f-strings, Ruff alternates quote styles, starting with the [configured quote style] for the
+outermost f-string. For example, consider the following f-string:
+
+```python
+# format.quote-style = "double"
+
+f"outer f-string {f"nested f-string {f"another nested f-string"} end"} end"
+```
+
+Ruff formats it as:
+
+```python
+f"outer f-string {f'nested f-string {f"another nested f-string"} end'} end"
+```
+
+#### Line breaks
+
+Starting with Python 3.12 ([PEP 701](https://peps.python.org/pep-0701/)), the expression parts of an f-string can
+span multiple lines. Ruff needs to decide when to introduce a line break in an f-string expression.
+This depends on the semantic content of the expression parts of an f-string - for example,
+introducing a line break in the middle of a natural-language sentence is undesirable. Since Ruff
+doesn't have enough information to make that decision, it adopts a heuristic similar to [Prettier](https://prettier.io/docs/en/next/rationale.html#template-literals):
+it will only split the expression parts of an f-string across multiple lines if there was already a line break
+within any of the expression parts.
+
+For example, the following code:
+
+```python
+f"this f-string has a multiline expression {
+  ['red', 'green', 'blue', 'yellow',]} and does not fit within the line length"
+```
+
+... is formatted as:
+
+```python
+# The list expression is split across multiple lines because of the trailing comma
+f"this f-string has a multiline expression {
+    [
+        'red',
+        'green',
+        'blue',
+        'yellow',
+    ]
+} and does not fit within the line length"
+```
+
+But, the following will not be split across multiple lines even though it exceeds the line length:
+
+```python
+f"this f-string has a multiline expression {['red', 'green', 'blue', 'yellow']} and does not fit within the line length"
+```
+
+If you want Ruff to split an f-string across multiple lines, ensure there's a linebreak somewhere within the
+`{...}` parts of an f-string.
+
+[self-documenting f-string]: https://realpython.com/python-f-strings/#self-documenting-expressions-for-debugging
+[configured quote style]: settings.md/#format_quote-style
+
 ## Sorting imports
 
 Currently, the Ruff formatter does not sort imports. In order to both sort imports and format,

docs/formatter/black.md

@@ -253,6 +253,9 @@ f'test{inner   + "nested_string"} including math {5 ** 3 + 10}'
 f"test{inner + 'nested_string'} including math {5**3 + 10}"
 ```
 
+For more details on the formatting style, refer to the [f-string
+formatting](../formatter.md#f-string-formatting) section.
+
 ### Implicit concatenated strings
 
 Ruff merges implicitly concatenated strings if the entire string fits on a single line: