Use parenthesis with equality check in walrus/assigment statements

Closes psf#449

src/black/linegen.py

@@ -99,6 +99,27 @@ def visit_default(self, node: LN) -> Iterator[Line]:
                 self.current_line.append(node)
         yield from super().visit_default(node)
 
+    def visit_comparison(self, node: Node) -> Iterator[Line]:
+        parent: Optional[Node] = node.parent
+        grandparent: Optional[Node] = parent.parent if parent else None
+        if (
+            parent is not None
+            and Leaf(token.EQEQUAL, "==") in node.children
+            and (
+                parent.type == syms.namedexpr_test
+                or (
+                    grandparent is not None
+                    and grandparent.type in (syms.expr_stmt, syms.annassign)
+                )
+            )
+        ):
+            lpar = Leaf(token.LPAR, "(")
+            rpar = Leaf(token.RPAR, ")")
+            node.insert_child(0, lpar)
+            node.insert_child(len(node.children), rpar)
+
+        yield from self.visit_default(node)
+
     def visit_INDENT(self, node: Leaf) -> Iterator[Line]:
         """Increase indentation level, maybe yield a line."""
         # In blib2to3 INDENT never holds comments.

tests/data/paren_eq_check_in_assigments.py

@@ -0,0 +1,83 @@
+match_count += new_value == old_value
+
+if on_windows := os.name == "nt":
+    ...
+
+on_windows: bool = (os.name == "nt")
+
+implementation_version = (
+    platform.python_version() if platform.python_implementation() == "CPython" else "Unknown"
+)
+
+is_mac = platform.system() == 'Darwin'
+
+s = y == 2 + y == 4
+
+name1 = name2 = name3
+
+name1 == name2 == name3
+
+check_sockets(on_windows=os.name == "nt")
+
+if a := b == c:
+    pass
+
+a = [y := f(x) == True, y ** 2, y ** 3]
+
+a = lambda line: (m := re.match(pattern, line) == True)
+
+a = b in c and b == d
+
+a = b == c == d
+
+a = b == c in d
+
+a = b >= c == True
+
+a = b in c
+
+a = b > c
+
+# output
+
+match_count += (new_value == old_value)
+
+if on_windows := (os.name == "nt"):
+    ...
+
+on_windows: bool = (os.name == "nt")
+
+implementation_version = (
+    platform.python_version()
+    if platform.python_implementation() == "CPython"
+    else "Unknown"
+)
+
+is_mac = (platform.system() == "Darwin")
+
+s = (y == 2 + y == 4)
+
+name1 = name2 = name3
+
+name1 == name2 == name3
+
+check_sockets(on_windows=os.name == "nt")
+
+if a := (b == c):
+    pass
+
+a = [y := (f(x) == True), y ** 2, y ** 3]
+
+a = lambda line: (m := (re.match(pattern, line) == True))
+
+a = b in c and b == d
+
+a = (b == c == d)
+
+a = (b == c in d)
+
+a = (b >= c == True)
+
+a = b in c
+
+a = b > c