remove obviously unnecessary parentheses (#850)

Fixes #548

black.py

@@ -1602,6 +1602,26 @@ def visit_default(self, node: LN) -> Iterator[Line]:
                 self.current_line.append(node)
         yield from super().visit_default(node)
 
+    def visit_atom(self, node: Node) -> Iterator[Line]:
+        # Always make parentheses invisible around a single node, because it should
+        # not be needed (except in the case of yield, where removing the parentheses
+        # produces a SyntaxError).
+        if (
+            len(node.children) == 3
+            and isinstance(node.children[0], Leaf)
+            and node.children[0].type == token.LPAR
+            and isinstance(node.children[2], Leaf)
+            and node.children[2].type == token.RPAR
+            and isinstance(node.children[1], Leaf)
+            and not (
+                node.children[1].type == token.NAME
+                and node.children[1].value == "yield"
+            )
+        ):
+            node.children[0].value = ""
+            node.children[2].value = ""
+        yield from super().visit_default(node)
+
     def visit_INDENT(self, node: Node) -> Iterator[Line]:
         """Increase indentation level, maybe yield a line."""
         # In blib2to3 INDENT never holds comments.

tests/data/expression.diff

@@ -158,7 +158,8 @@
 +{"2.7": dead, "3.7": long_live or die_hard}
 +{"2.7", "3.6", "3.7", "3.8", "3.9", "4.0" if gilectomy else "3.10"}
  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or A, 11 or B, 12 or C]
- (SomeName)
+-(SomeName)
++SomeName
  SomeName
  (Good, Bad, Ugly)
  (i for i in (1, 2, 3))

tests/data/expression.py

@@ -410,7 +410,7 @@ async def f():
 {"2.7": dead, "3.7": long_live or die_hard}
 {"2.7", "3.6", "3.7", "3.8", "3.9", "4.0" if gilectomy else "3.10"}
 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or A, 11 or B, 12 or C]
-(SomeName)
+SomeName
 SomeName
 (Good, Bad, Ugly)
 (i for i in (1, 2, 3))

tests/data/remove_parens.py

@@ -0,0 +1,10 @@
+print((1))
+x = (1)
+x = (1.2)
+(x) = (3)
+
+# output
+print(1)
+x = 1
+x = 1.2
+x = 3

tests/test_black.py

@@ -436,6 +436,14 @@ def test_empty_lines(self) -> None:
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, black.FileMode())
 
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_remove_parens(self) -> None:
+        source, expected = read_data("remove_parens")
+        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_string_prefixes(self) -> None:
         source, expected = read_data("string_prefixes")