Black crashes when multiple `# fmt: skip` are added in a multi-part if-clause.

[ranjodhsingh1729]

Describe the bug

Black crashes on the following valid python code:

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

To Reproduce

Just run black on the given code.

The resulting error is:

error: cannot format temp.py: Cannot parse for target version Python 3.9: 15:5:     ): # fmt: skip

Oh no! 💥 💔 💥
1 file failed to reformat.

Environment

• Black's version: main
• OS and Python version: Ubuntu 24.04 LTS, Python3.9 (tested on others on playground)

Additional context

• Crash is also observed when the second skip is on the previous line (L15)
• Found it while looking into # fmt: skip ignored inside multi-part if-clause #4730. Don't have a solution yet.

[ranjodhsingh1729]

I've identified the root cause: when the second # fmt: skip is processed, its prev_sibling (the node that should be ignored) includes the STANDALONE_COMMENT generated by the first # fmt: skip. Since STANDALONE_COMMENT nodes do not include trailing newlines (these are added at a later stage), the formatter produces the following output:

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

A simple fix would be to traverse the ignored nodes and insert trailing newlines for any STANDALONE_COMMENT nodes encountered:

diff --git a/src/black/comments.py b/src/black/comments.py
index 8c88668..893a10b 100644
--- a/src/black/comments.py
+++ b/src/black/comments.py
@@ -174,6 +174,15 @@ def normalize_fmt_off(
         try_again = convert_one_fmt_off_pair(node, mode, lines)
 
 
+def normalize_skipped_comment_newlines(node: Node):
+    if node.type == STANDALONE_COMMENT:
+        node.value += "\n"
+
+    if not isinstance(node, Leaf):
+        for child in node.children:
+            normalize_skipped_comment_newlines(child)
+
+
 def convert_one_fmt_off_pair(
     node: Node, mode: Mode, lines: Collection[tuple[int, int]]
 ) -> bool:
@@ -209,6 +218,8 @@ def convert_one_fmt_off_pair(
                 continue
 
             first = ignored_nodes[0]  # Can be a container node with the `leaf`.
+            normalize_skipped_comment_newlines(first)
+
             parent = first.parent
             prefix = first.prefix
             if comment.value in FMT_OFF:

I'll look for a better approach. If anyone has suggestions or ideas, I'd be glad to hear them.