Avoid reusing nested, interpolated quotes before Python 3.12

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/fstring.py

@@ -748,3 +748,7 @@
 
 # Regression tests for https://github.com/astral-sh/ruff/issues/15536
 print(f"{ {}, 1, }")
+
+
+# The inner quotes should not be changed to double quotes before Python 3.12
+f"{f'''{'nested'} inner'''} outer"

crates/ruff_python_formatter/src/context.rs

@@ -144,6 +144,12 @@ pub(crate) enum InterpolatedStringState {
     ///
     /// The containing `FStringContext` is the surrounding f-string context.
     InsideInterpolatedElement(InterpolatedStringContext),
+    /// The formatter is inside more than one nested f-string, such as in `nested` in:
+    ///
+    /// ```py
+    /// f"{f'''{'nested'} inner'''} outer"
+    /// ```
+    NestedInterpolatedElement(InterpolatedStringContext),
     /// The formatter is outside an f-string.
     #[default]
     Outside,
@@ -152,12 +158,18 @@ pub(crate) enum InterpolatedStringState {
 impl InterpolatedStringState {
     pub(crate) fn can_contain_line_breaks(self) -> Option<bool> {
         match self {
-            InterpolatedStringState::InsideInterpolatedElement(context) => {
+            InterpolatedStringState::InsideInterpolatedElement(context)
+            | InterpolatedStringState::NestedInterpolatedElement(context) => {
                 Some(context.is_multiline())
             }
             InterpolatedStringState::Outside => None,
         }
     }
+
+    /// Returns `true` if the interpolated string state is [`NestedInterpolatedElement`].
+    pub(crate) fn is_nested(self) -> bool {
+        matches!(self, Self::NestedInterpolatedElement(..))
+    }
 }
 
 /// The position of a top-level statement in the module.

crates/ruff_python_formatter/src/other/interpolated_string_element.rs

@@ -181,10 +181,16 @@ impl Format<PyFormatContext<'_>> for FormatInterpolatedElement<'_> {
 
             let item = format_with(|f: &mut PyFormatter| {
                 // Update the context to be inside the f-string expression element.
-                let f = &mut WithInterpolatedStringState::new(
-                    InterpolatedStringState::InsideInterpolatedElement(self.context),
-                    f,
-                );
+                let state = match f.context().interpolated_string_state() {
+                    InterpolatedStringState::InsideInterpolatedElement(_)
+                    | InterpolatedStringState::NestedInterpolatedElement(_) => {
+                        InterpolatedStringState::NestedInterpolatedElement(self.context)
+                    }
+                    InterpolatedStringState::Outside => {
+                        InterpolatedStringState::InsideInterpolatedElement(self.context)
+                    }
+                };
+                let f = &mut WithInterpolatedStringState::new(state, f);
 
                 write!(f, [bracket_spacing, expression.format()])?;
 

crates/ruff_python_formatter/src/string/normalize.rs

@@ -46,8 +46,15 @@ impl<'a, 'src> StringNormalizer<'a, 'src> {
             .unwrap_or(self.context.options().quote_style());
         let supports_pep_701 = self.context.options().target_version().supports_pep_701();
 
+        // Preserve the existing quote style for nested interpolations more than one layer deep, if
+        // PEP 701 isn't supported.
+        if !supports_pep_701 && self.context.interpolated_string_state().is_nested() {
+            return QuoteStyle::Preserve;
+        }
+
         // For f-strings and t-strings prefer alternating the quotes unless The outer string is triple quoted and the inner isn't.
-        if let InterpolatedStringState::InsideInterpolatedElement(parent_context) =
+        if let InterpolatedStringState::InsideInterpolatedElement(parent_context)
+        | InterpolatedStringState::NestedInterpolatedElement(parent_context) =
             self.context.interpolated_string_state()
         {
             let parent_flags = parent_context.flags();

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__fstring.py.snap

@@ -28,12 +28,11 @@ but none started with prefix {parentdir_prefix}"
  f'{{NOT \'a\' "formatted" "value"}}'
  f"some f-string with {a} {few():.2f} {formatted.values!r}"
 -f'some f-string with {a} {few(""):.2f} {formatted.values!r}'
--f"{f'''{'nested'} inner'''} outer"
++f"some f-string with {a} {few(''):.2f} {formatted.values!r}"
+ f"{f'''{'nested'} inner'''} outer"
 -f"\"{f'{nested} inner'}\" outer"
 -f"space between opening braces: { {a for a in (1, 2, 3)}}"
 -f'Hello \'{tricky + "example"}\''
-+f"some f-string with {a} {few(''):.2f} {formatted.values!r}"
-+f"{f'''{"nested"} inner'''} outer"
 +f'"{f"{nested} inner"}" outer'
 +f"space between opening braces: { {a for a in (1, 2, 3)} }"
 +f"Hello '{tricky + 'example'}'"
@@ -49,7 +48,7 @@ f"{{NOT a formatted value}}"
 f'{{NOT \'a\' "formatted" "value"}}'
 f"some f-string with {a} {few():.2f} {formatted.values!r}"
 f"some f-string with {a} {few(''):.2f} {formatted.values!r}"
-f"{f'''{"nested"} inner'''} outer"
+f"{f'''{'nested'} inner'''} outer"
 f'"{f"{nested} inner"}" outer'
 f"space between opening braces: { {a for a in (1, 2, 3)} }"
 f"Hello '{tricky + 'example'}'"
@@ -72,17 +71,3 @@ f'Hello \'{tricky + "example"}\''
 f"Tried directories {str(rootdirs)} \
 but none started with prefix {parentdir_prefix}"
 ```
-
-## New Unsupported Syntax Errors
-
-error[invalid-syntax]: Cannot reuse outer quote character in f-strings on Python 3.10 (syntax was added in Python 3.12)
- --> fstring.py:6:9
-  |
-4 | f"some f-string with {a} {few():.2f} {formatted.values!r}"
-5 | f"some f-string with {a} {few(''):.2f} {formatted.values!r}"
-6 | f"{f'''{"nested"} inner'''} outer"
-  |         ^
-7 | f'"{f"{nested} inner"}" outer'
-8 | f"space between opening braces: { {a for a in (1, 2, 3)} }"
-  |
-warning: Only accept new syntax errors if they are also present in the input. The formatter should not introduce syntax errors.

crates/ruff_python_formatter/tests/snapshots/format@expression__fstring.py.snap

@@ -754,6 +754,10 @@ print(f"{  # Tuple with multiple elements that doesn't fit on a single line gets
 
 # Regression tests for https://github.com/astral-sh/ruff/issues/15536
 print(f"{ {}, 1, }")
+
+
+# The inner quotes should not be changed to double quotes before Python 3.12
+f"{f'''{'nested'} inner'''} outer"
 ```
 
 ## Outputs
@@ -1532,7 +1536,7 @@ f'{f"""other " """}'
 f'{1: hy "user"}'
 f'{1:hy "user"}'
 f'{1: abcd "{1}" }'
-f'{1: abcd "{"aa"}" }'
+f'{1: abcd "{'aa'}" }'
 f'{1=: "abcd {'aa'}}'
 f"{x:a{z:hy \"user\"}} '''"
 
@@ -1581,6 +1585,10 @@ print(
 
 # Regression tests for https://github.com/astral-sh/ruff/issues/15536
 print(f"{ {}, 1 }")
+
+
+# The inner quotes should not be changed to double quotes before Python 3.12
+f"{f'''{'nested'} inner'''} outer"
 ```
 
 
@@ -2359,7 +2367,7 @@ f'{f"""other " """}'
 f'{1: hy "user"}'
 f'{1:hy "user"}'
 f'{1: abcd "{1}" }'
-f'{1: abcd "{"aa"}" }'
+f'{1: abcd "{'aa'}" }'
 f'{1=: "abcd {'aa'}}'
 f"{x:a{z:hy \"user\"}} '''"
 
@@ -2408,6 +2416,10 @@ print(
 
 # Regression tests for https://github.com/astral-sh/ruff/issues/15536
 print(f"{ {}, 1 }")
+
+
+# The inner quotes should not be changed to double quotes before Python 3.12
+f"{f'''{'nested'} inner'''} outer"
 ```