Improve fmt:skip handling in nested expressions with checks

CHANGES.md

@@ -74,6 +74,8 @@
 - Fix `fix_fmt_skip_in_one_liners` crashing on `with` statements (#4853)
 - Fix `fix_fmt_skip_in_one_liners` crashing on annotated parameters (#4854)
 - Fix new lines being added after imports with `# fmt: skip` on them (#4894)
+- Add defensive checks to prevent `AttributeError` crashes when processing `# fmt: skip`
+  in nested expressions (#4903)
 
 ### Packaging
 

src/black/comments.py

@@ -645,6 +645,9 @@ def _generate_ignored_nodes_from_fmt_skip(
         return
 
     if Preview.fix_fmt_skip_in_one_liners in mode and not prev_sibling and parent:
+        # Simple one-level check: if the leaf has no prev_sibling,
+        # check parent's prev_sibling. This handles some nested cases
+        # without the complexity and indentation issues of deeper tree climbing
         prev_sibling = parent.prev_sibling
 
     if prev_sibling is not None:

src/black/lines.py

@@ -864,6 +864,10 @@ def is_line_short_enough(line: Line, *, mode: Mode, line_str: str = "") -> bool:
     for i, leaf in enumerate(line.leaves):
         if max_level_to_update == math.inf:
             had_comma: int | None = None
+            # Skip multiline_string_handling logic for leaves without bracket_depth
+            # (e.g., newly created leaves not yet processed by bracket tracker)
+            if not hasattr(leaf, "bracket_depth"):
+                continue
             if leaf.bracket_depth + 1 > len(commas):
                 commas.append(0)
             elif leaf.bracket_depth + 1 < len(commas):
@@ -879,17 +883,22 @@ def is_line_short_enough(line: Line, *, mode: Mode, line_str: str = "") -> bool:
                     # MLS was in parens with at least one comma - force split
                     return False
 
-        if leaf.bracket_depth <= max_level_to_update and leaf.type == token.COMMA:
-            # Inside brackets, ignore trailing comma
-            # directly after MLS/MLS-containing expression
-            ignore_ctxs: list[LN | None] = [None]
-            ignore_ctxs += multiline_string_contexts
-            if (line.inside_brackets or leaf.bracket_depth > 0) and (
-                i != len(line.leaves) - 1 or leaf.prev_sibling not in ignore_ctxs
-            ):
-                commas[leaf.bracket_depth] += 1
-        if max_level_to_update != math.inf:
-            max_level_to_update = min(max_level_to_update, leaf.bracket_depth)
+        # Skip bracket-depth-dependent processing for leaves without the attribute
+        if not hasattr(leaf, "bracket_depth"):
+            # Still process multiline string detection below
+            pass
+        else:
+            if leaf.bracket_depth <= max_level_to_update and leaf.type == token.COMMA:
+                # Inside brackets, ignore trailing comma
+                # directly after MLS/MLS-containing expression
+                ignore_ctxs: list[LN | None] = [None]
+                ignore_ctxs += multiline_string_contexts
+                if (line.inside_brackets or leaf.bracket_depth > 0) and (
+                    i != len(line.leaves) - 1 or leaf.prev_sibling not in ignore_ctxs
+                ):
+                    commas[leaf.bracket_depth] += 1
+            if max_level_to_update != math.inf:
+                max_level_to_update = min(max_level_to_update, leaf.bracket_depth)
 
         if is_multiline_string(leaf):
             if leaf.parent and (

tests/data/cases/fmtskip_nested_expression.py

@@ -0,0 +1,41 @@
+# flags: --preview
+class ClassWithALongName:
+    Constant1 = 1
+    Constant2 = 2
+    Constant3 = 3
+
+
+def test():
+    if (
+        "cond1" == "cond1"
+        and "cond2" == "cond2"
+        and 1 in (  # fmt: skip
+            ClassWithALongName.Constant1,
+            ClassWithALongName.Constant2,
+            ClassWithALongName.Constant3,
+        )
+    ):
+        return True
+    return False
+
+# output
+
+class ClassWithALongName:
+    Constant1 = 1
+    Constant2 = 2
+    Constant3 = 3
+
+
+def test():
+    if (
+        "cond1" == "cond1"
+        and "cond2" == "cond2"
+        and 1
+        in (  # fmt: skip
+            ClassWithALongName.Constant1,
+            ClassWithALongName.Constant2,
+            ClassWithALongName.Constant3,
+        )
+    ):
+        return True
+    return False