Add parentheses around tuple unpack assignment (#832)

Fixes #656

black.py

@@ -2726,6 +2726,14 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
 
     check_lpar = False
     for index, child in enumerate(list(node.children)):
+        # Add parentheses around long tuple unpacking in assignments.
+        if (
+            index == 0
+            and isinstance(child, Node)
+            and child.type == syms.testlist_star_expr
+        ):
+            check_lpar = True
+
         if check_lpar:
             if child.type == syms.atom:
                 if maybe_make_parens_invisible_in_atom(child, parent=node):
@@ -2757,7 +2765,11 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
                 lpar = Leaf(token.LPAR, "")
                 rpar = Leaf(token.RPAR, "")
                 index = child.remove() or 0
-                node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+                prefix = child.prefix
+                child.prefix = ""
+                new_child = Node(syms.atom, [lpar, child, rpar])
+                new_child.prefix = prefix
+                node.insert_child(index, new_child)
 
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 

tests/data/tupleassign.py

@@ -0,0 +1,10 @@
+sdfjklsdfsjldkflkjsf, sdfjsdfjlksdljkfsdlkf, sdfsdjfklsdfjlksdljkf, sdsfsdfjskdflsfsdf = 1, 2, 3
+
+
+# output
+(
+    sdfjklsdfsjldkflkjsf,
+    sdfjsdfjlksdljkfsdlkf,
+    sdfsdjfklsdfjlksdljkf,
+    sdsfsdfjskdflsfsdf,
+) = (1, 2, 3)

tests/test_black.py

@@ -542,6 +542,14 @@ def test_bracket_match(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_tuple_assign(self) -> None:
+        source, expected = read_data("tupleassign")
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, black.FileMode())
+
     def test_tab_comment_indentation(self) -> None:
         contents_tab = "if 1:\n\tif 2:\n\t\tpass\n\t# comment\n\tpass\n"
         contents_spc = "if 1:\n    if 2:\n        pass\n    # comment\n    pass\n"