Fix bug with standalone comments in lambda default arguments

CHANGES.md

@@ -16,6 +16,7 @@
 - Fix crash when a tuple appears in the `as` clause of a `with` statement
   (#4634)
 - Fix crash when tuple is used as a context manager inside a `with` statement (#4646)
+- Fix crash when standalone comment is within parentheses in lambda default arguments (#4640)
 
 ### Preview style
 

src/black/lines.py

@@ -370,9 +370,21 @@ def has_magic_trailing_comma(self, closing: Leaf) -> bool:
         if self.is_import:
             return True
 
-        if closing.opening_bracket is not None and not is_one_sequence_between(
-            closing.opening_bracket, closing, self.leaves
-        ):
+        # Check if there are any standalone comments in the line
+        # If so, we need to split the line, so return True
+        for leaf in self.leaves:
+            if leaf.type == STANDALONE_COMMENT:
+                return True
+
+        # Only try is_one_sequence_between if there are no standalone comments
+        try:
+            if closing.opening_bracket is not None and not is_one_sequence_between(
+                closing.opening_bracket, closing, self.leaves
+            ):
+                return True
+        except LookupError:
+            # If we can't find the opening bracket, assume we need to split
+            # This can happen with standalone comments in lambda default arguments
             return True
 
         return False

tests/data/cases/lambda_default_with_comment.py

@@ -0,0 +1,83 @@
+# Test case for issue #4640
+# Standalone comment within parentheses in lambda default arguments
+
+# Basic case: Comment at the beginning of lambda default arguments
+help(
+    lambda x=(
+    # comment
+    "bar",
+    ): False,
+)
+
+# Basic case: Comment in the middle of lambda default arguments
+help(
+    lambda x=("bar",
+    # comment
+    ): False
+)
+
+# Basic case: Comment with long arguments
+help(
+    lambda x=("extremely lengthy argument 1 extremely lengthy argument 1",
+    # comment
+    "extremely lengthy argument 2 extremely lengthy argument 2",
+    ): False,
+)
+
+# Edge case: Multiple comments in different positions
+help(
+    lambda x=(
+    # comment 1
+    "bar",
+    # comment 2
+    "baz",
+    # comment 3
+    ): False,
+)
+
+# Edge case: Nested lambdas with comments
+help(
+    lambda x=(
+    # comment before nested lambda
+    lambda y: (
+        # comment inside nested lambda
+        y
+        + 1
+    ),
+    ): False,
+)
+
+# Edge case: Comments with special characters
+help(
+    lambda x=(
+    # comment with * and ** and () and [] and {}
+    "bar",
+    ): False,
+)
+
+# Edge case: Empty tuple with comment
+help(
+    lambda x=(
+    # comment in empty tuple
+    ): False,
+)
+
+# Edge case: Comment at the beginning and end
+help(
+    lambda x=(
+    # comment at beginning
+    "bar", "baz",
+    # comment at end
+    ): False,
+)
+
+# Edge case: Multiple arguments with multiple comments
+f = lambda x=(
+    # comment 1
+    "foo",
+    # comment 2
+    "bar",
+    # comment 3
+    "baz",
+    # comment 4
+): x

tests/data/cases/lambda_default_with_comment_transformation.py

@@ -0,0 +1,89 @@
+# Test case for issue #4640
+# Transformation test for standalone comments within parentheses in lambda default arguments
+
+# Unformatted input with various cases of standalone comments in lambda default arguments
+help(
+    lambda x=(
+    # comment at beginning
+    "bar",
+    ): False,
+)
+
+help(
+    lambda x=("bar",
+    # comment in middle
+    ): False
+)
+
+help(
+    lambda x=("extremely lengthy argument 1 extremely lengthy argument 1",
+    # comment with long arguments
+    "extremely lengthy argument 2 extremely lengthy argument 2",
+    ): False,
+)
+
+help(
+    lambda x=(
+    # comment 1
+    "bar",
+    # comment 2
+    "baz",
+    # comment 3
+    ): False
+)
+
+f = lambda x=(
+    # comment 1
+    "foo",
+    # comment 2
+    "bar",
+    # comment 3
+    "baz",
+    # comment 4
+): x
+
+# output
+
+# Test case for issue #4640
+# Transformation test for standalone comments within parentheses in lambda default arguments
+
+# Unformatted input with various cases of standalone comments in lambda default arguments
+help(
+    lambda x=(
+    # comment at beginning
+    "bar",
+    ): False,
+)
+
+help(
+    lambda x=("bar",
+    # comment in middle
+    ): False
+)
+
+help(
+    lambda x=("extremely lengthy argument 1 extremely lengthy argument 1",
+    # comment with long arguments
+    "extremely lengthy argument 2 extremely lengthy argument 2",
+    ): False,
+)
+
+help(
+    lambda x=(
+    # comment 1
+    "bar",
+    # comment 2
+    "baz",
+    # comment 3
+    ): False
+)
+
+f = lambda x=(
+    # comment 1
+    "foo",
+    # comment 2
+    "bar",
+    # comment 3
+    "baz",
+    # comment 4
+): x