Add support for PEP 572 assignment expressions (#457)

Author: asottile

pyflakes/checker.py

@@ -1329,7 +1329,7 @@ def ignore(self, node):
     # "expr" type nodes
     BOOLOP = UNARYOP = IFEXP = SET = \
         REPR = ATTRIBUTE = SUBSCRIPT = \
-        STARRED = NAMECONSTANT = handleChildren
+        STARRED = NAMECONSTANT = NAMEDEXPR = handleChildren
 
     def _handle_string_dot_format(self, node):
         try:

pyflakes/test/test_other.py

@@ -1754,6 +1754,15 @@ def test_f_string(self):
         print(f'\x7b4*baz\N{RIGHT CURLY BRACKET}')
         ''')
 
+    @skipIf(version_info < (3, 8), 'new in Python 3.8')
+    def test_assign_expr(self):
+        """Test PEP 572 assignment expressions are treated as usage / write."""
+        self.flakes('''
+        from foo import y
+        print(x := y)
+        print(x)
+        ''')
+
 
 class TestStringFormatting(TestCase):