Avoid expanding single-element tuple patterns

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/match.py

@@ -501,3 +501,24 @@ def foo():
     # own line
     ):
         ...
+
+
+# Single-element tuples.
+match pattern:
+    case (a,):
+        pass
+
+    case (a, b):
+        pass
+
+    case (a, b,):
+        pass
+
+    case a,:
+        pass
+
+    case a, b:
+        pass
+
+    case a, b,:
+        pass

crates/ruff_python_formatter/src/pattern/pattern_match_sequence.rs

@@ -1,4 +1,4 @@
-use ruff_formatter::{Format, FormatResult};
+use ruff_formatter::{format_args, Format, FormatResult};
 use ruff_python_ast::node::AnyNodeRef;
 use ruff_python_ast::PatternMatchSequence;
 use ruff_python_trivia::{SimpleTokenKind, SimpleTokenizer};
@@ -20,14 +20,25 @@ impl FormatNodeRule<PatternMatchSequence> for FormatPatternMatchSequence {
         let dangling = comments.dangling(item);
 
         let sequence_type = SequenceType::from_pattern(item, f.context().source());
-        if patterns.is_empty() {
-            return match sequence_type {
-                SequenceType::List => empty_parenthesized("[", dangling, "]").fmt(f),
-                SequenceType::Tuple | SequenceType::TupleNoParens => {
-                    empty_parenthesized("(", dangling, ")").fmt(f)
-                }
-            };
+
+        match (patterns.as_slice(), sequence_type) {
+            // If the sequence is empty, the parentheses with any dangling comments.
+            ([], SequenceType::Tuple | SequenceType::TupleNoParens) => {
+                return empty_parenthesized("(", dangling, ")").fmt(f)
+            }
+            ([], SequenceType::List) => return empty_parenthesized("[", dangling, "]").fmt(f),
+
+            // A single-element tuple should always be parenthesized, and the trailing comma
+            // should never cause it to expand.
+            ([elt], SequenceType::Tuple | SequenceType::TupleNoParens) => {
+                return parenthesized("(", &format_args![elt.format(), token(",")], ")")
+                    .with_dangling_comments(dangling)
+                    .fmt(f)
+            }
+
+            _ => {}
         }
+
         let items = format_with(|f| {
             f.join_comma_separated(range.end())
                 .nodes(patterns.iter())

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

@@ -507,6 +507,27 @@ match pattern_match_or:
     # own line
     ):
         ...
+
+
+# Single-element tuples.
+match pattern:
+    case (a,):
+        pass
+
+    case (a, b):
+        pass
+
+    case (a, b,):
+        pass
+
+    case a,:
+        pass
+
+    case a, b:
+        pass
+
+    case a, b,:
+        pass
 ```
 
 ## Output
@@ -1038,6 +1059,33 @@ match pattern_match_or:
         # own line
     ):
         ...
+
+
+# Single-element tuples.
+match pattern:
+    case (a,):
+        pass
+
+    case (a, b):
+        pass
+
+    case (
+        a,
+        b,
+    ):
+        pass
+
+    case (a,):
+        pass
+
+    case a, b:
+        pass
+
+    case (
+        a,
+        b,
+    ):
+        pass
 ```