Fix crash on standalone comment in lambda default arguments (#4993)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: cobalt <61329810+cobaltt7@users.noreply.github.com>

CHANGES.md

diff --git a/CHANGES.md b/CHANGES.md
index 36cbd00..ad1bd01 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -14,6 +14,7 @@
 <!-- Changes that affect Black's stable style -->
 
 - Don't double-decode input, causing non-UTF-8 files to be corrupted (#4964)
+- Fix crash on standalone comment in lambda default arguments (#4993)
 
 ### Preview style
 

src/black/nodes.py

diff --git a/src/black/nodes.py b/src/black/nodes.py
index 3bce0ef..90487fb 100644
--- a/src/black/nodes.py
+++ b/src/black/nodes.py
@@ -689,7 +689,7 @@ def is_one_sequence_between(
             break
 
     else:
-        raise LookupError("Opening paren not found in `leaves`")
+        return False
 
     commas = 0
     _opening_index += 1

tests/data/cases/comments_in_lambda_default.py

diff --git a/tests/data/cases/comments_in_lambda_default.py b/tests/data/cases/comments_in_lambda_default.py
new file mode 100644
index 0000000..9aee769
--- /dev/null
+++ b/tests/data/cases/comments_in_lambda_default.py
@@ -0,0 +1,27 @@
+help(lambda x=(
+    # comment
+    "bar",
+): False)
+
+result = (lambda x=(
+    # a standalone comment
+    1,
+    2,
+    3,
+): x)
+
+# output
+
+help(
+    lambda x=(
+    # comment
+    "bar",
+    ): False,
+)
+
+result = lambda x=(
+    # a standalone comment
+    1,
+    2,
+    3,
+): x