Fix joining of f-strings with different quotes when using quote style Preserve (#15524)

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

@@ -11,3 +11,12 @@ a = "different '" 'quote "are fine"'  # join
 
 # Already invalid Pre Python 312
 f"{'Hy "User"'}" f'{"Hy 'User'"}'
+
+
+# Regression tests for https://github.com/astral-sh/ruff/issues/15514
+params = {}
+string = "this is my string with " f'"{params.get("mine")}"'
+string = f'"{params.get("mine")} ' f"with {'nested single quoted string'}"
+string = f"{'''inner ' '''}" f'{"""inner " """}'
+string = f"{10 + len('bar')=}" f"{10 + len('bar')=}"
+string = f"{10 + len('bar')=}" f'{10 + len("bar")=}'

crates/ruff_python_formatter/src/string/normalize.rs

@@ -44,15 +44,33 @@ impl<'a, 'src> StringNormalizer<'a, 'src> {
         let preferred_quote_style = self
             .preferred_quote_style
             .unwrap_or(self.context.options().quote_style());
+        let supports_pep_701 = self.context.options().target_version().supports_pep_701();
 
+        // For f-strings prefer alternating the quotes unless The outer string is triple quoted and the inner isn't.
+        if let FStringState::InsideExpressionElement(parent_context) = self.context.f_string_state()
+        {
+            let parent_flags = parent_context.f_string().flags();
+
+            if !parent_flags.is_triple_quoted() || string.flags().is_triple_quoted() {
+                // This logic is even necessary when using preserve and the target python version doesn't support PEP701 because
+                // we might end up joining two f-strings that have different quote styles, in which case we need to alternate the quotes
+                // for inner strings to avoid a syntax error: `string = "this is my string with " f'"{params.get("mine")}"'`
+                if !preferred_quote_style.is_preserve() || !supports_pep_701 {
+                    return QuoteStyle::from(parent_flags.quote_style().opposite());
+                }
+            }
+        }
+
+        // Leave the quotes unchanged for all other strings.
         if preferred_quote_style.is_preserve() {
             return QuoteStyle::Preserve;
         }
 
+        // There are cases where it is necessary to preserve the quotes to prevent an invalid f-string.
         if let StringLikePart::FString(fstring) = string {
             // There are two cases where it's necessary to preserve the quotes if the
             // target version is pre 3.12 and the part is an f-string.
-            if !self.context.options().target_version().supports_pep_701() {
+            if !supports_pep_701 {
                 // An f-string expression contains a debug text with a quote character
                 // because the formatter will emit the debug expression **exactly** the
                 // same as in the source text.
@@ -77,16 +95,6 @@ impl<'a, 'src> StringNormalizer<'a, 'src> {
             }
         }
 
-        // For f-strings prefer alternating the quotes unless The outer string is triple quoted and the inner isn't.
-        if let FStringState::InsideExpressionElement(parent_context) = self.context.f_string_state()
-        {
-            let parent_flags = parent_context.f_string().flags();
-
-            if !parent_flags.is_triple_quoted() || string.flags().is_triple_quoted() {
-                return QuoteStyle::from(parent_flags.quote_style().opposite());
-            }
-        }
-
         // Per PEP 8, always prefer double quotes for triple-quoted strings.
         if string.flags().is_triple_quoted() {
             // ... unless we're formatting a code snippet inside a docstring,

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

@@ -17,6 +17,15 @@ a = "different '" 'quote "are fine"'  # join
 
 # Already invalid Pre Python 312
 f"{'Hy "User"'}" f'{"Hy 'User'"}'
+
+
+# Regression tests for https://github.com/astral-sh/ruff/issues/15514
+params = {}
+string = "this is my string with " f'"{params.get("mine")}"'
+string = f'"{params.get("mine")} ' f"with {'nested single quoted string'}"
+string = f"{'''inner ' '''}" f'{"""inner " """}'
+string = f"{10 + len('bar')=}" f"{10 + len('bar')=}"
+string = f"{10 + len('bar')=}" f'{10 + len("bar")=}'
 ```
 
 ## Outputs
@@ -49,6 +58,15 @@ a = "different 'quote \"are fine\""  # join
 
 # Already invalid Pre Python 312
 f"{'Hy "User"'}{"Hy 'User'"}"
+
+
+# Regression tests for https://github.com/astral-sh/ruff/issues/15514
+params = {}
+string = f"this is my string with \"{params.get('mine')}\""
+string = f'"{params.get("mine")} with {"nested single quoted string"}'
+string = f"{'''inner ' '''}" f'{"""inner " """}'
+string = f"{10 + len('bar')=}{10 + len('bar')=}"
+string = f"{10 + len('bar')=}" f'{10 + len("bar")=}'
 ```
 
 
@@ -81,4 +99,13 @@ a = "different 'quote \"are fine\""  # join
 
 # Already invalid Pre Python 312
 f"{'Hy "User"'}{"Hy 'User'"}"
+
+
+# Regression tests for https://github.com/astral-sh/ruff/issues/15514
+params = {}
+string = f"this is my string with \"{params.get("mine")}\""
+string = f'"{params.get("mine")} with {'nested single quoted string'}'
+string = f"{'''inner ' '''}{"""inner " """}"
+string = f"{10 + len('bar')=}{10 + len('bar')=}"
+string = f"{10 + len('bar')=}{10 + len("bar")=}"
 ```