Add trailing newlines to STANDALONE_COMMENT nodes to prevent parsing errors

Author: Nikhil172913832

CHANGES.md

@@ -17,6 +17,8 @@
 
 - Fix bug where comments preceding `# fmt: off`/`# fmt: on` blocks were incorrectly
   removed, particularly affecting Jupytext's `# %% [markdown]` comments (#4845)
+- Fix crash when multiple `# fmt: skip` comments are used in a multi-part if-clause
+  (#4872)
 
 ### Preview style
 

src/black/comments.py

@@ -409,6 +409,17 @@ def _handle_regular_fmt_block(
     else:
         standalone_comment_prefix = prefix[:previous_consumed] + "\n" * comment.newlines
 
+    # Ensure STANDALONE_COMMENT nodes have trailing newlines when stringified
+    # This prevents multiple fmt: skip comments from being concatenated on one line
+    for node in ignored_nodes:
+        if isinstance(node, Leaf) and node.type == STANDALONE_COMMENT:
+            if not node.value.endswith("\n"):
+                node.value += "\n"
+        elif isinstance(node, Node):
+            for leaf in node.leaves():
+                if leaf.type == STANDALONE_COMMENT and not leaf.value.endswith("\n"):
+                    leaf.value += "\n"
+
     hidden_value = "".join(str(n) for n in ignored_nodes)
     comment_lineno = leaf.lineno - comment.newlines
 

tests/data/cases/fmtskip_multiple_in_clause.py

@@ -0,0 +1,43 @@
+# Issue 4731: Multiple fmt: skip in multi-part if-clause
+class ClassWithALongName:
+    Constant1 = 1
+    Constant2 = 2
+    Constant3 = 3
+
+
+def test():
+    if (
+        "cond1" == "cond1"
+        and "cond2" == "cond2"
+        and 1 in (
+            ClassWithALongName.Constant1,
+            ClassWithALongName.Constant2,
+            ClassWithALongName.Constant3, # fmt: skip
+        ) # fmt: skip
+    ):
+        return True
+    return False
+
+
+# output
+
+
+# Issue 4731: Multiple fmt: skip in multi-part if-clause
+class ClassWithALongName:
+    Constant1 = 1
+    Constant2 = 2
+    Constant3 = 3
+
+
+def test():
+    if (
+        "cond1" == "cond1"
+        and "cond2" == "cond2"
+        and 1 in (
+            ClassWithALongName.Constant1,
+            ClassWithALongName.Constant2,
+            ClassWithALongName.Constant3, # fmt: skip
+        ) # fmt: skip
+    ):
+        return True
+    return False