Fix for "# fmt: on" with decorators

black.py

@@ -3118,9 +3118,33 @@ def generate_ignored_nodes(leaf: Leaf) -> Iterator[LN]:
         if is_fmt_on:
             return
 
-        yield container
+        # fix for fmt: on in children
+        if contains_fmt_on_at_column(container, leaf.column):
+            for child in container.children:
+                if not contains_fmt_on_at_column(child, leaf.column):
+                    yield child
+            return
+        else:
+            yield container
+            container = container.next_sibling
+
+
+def contains_fmt_on_at_column(container, column):
+    for child in container.children:
+        if (isinstance(child, Node) and first_leaf_column(child) == column
+            or isinstance(child, Leaf) and child.column == column):
+            for comment in list_comments(child.prefix, is_endmarker=False):
+                if comment.value in FMT_ON:
+                    return True
 
-        container = container.next_sibling
+    return False
+
+
+def first_leaf_column(node):
+    for child in node.children:
+        if isinstance(child, Leaf):
+            return child.column
+    return None
 
 
 def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:

tests/data/fmtonoff4.py

@@ -0,0 +1,31 @@
+# fmt: off
+@test([
+    1, 2,
+    3, 4,
+])
+# fmt: on
+def f(): pass
+
+@test([
+    1, 2,
+    3, 4,
+])
+def f(): pass
+
+# output
+
+# fmt: off
+@test([
+    1, 2,
+    3, 4,
+])
+# fmt: on
+def f():
+    pass
+
+
+@test(
+    [1, 2, 3, 4,]
+)
+def f():
+    pass

tests/test_black.py

@@ -632,6 +632,14 @@ def test_fmtonoff3(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_fmtonoff4(self) -> None:
+        source, expected = read_data("fmtonoff4")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     @patch("black.dump_to_file", dump_to_stderr)
     def test_remove_empty_parentheses_after_class(self) -> None:
         source, expected = read_data("class_blank_parentheses")