implement visitor method for NamedExpr

fixes #139

src/pep8ext_naming.py

@@ -412,18 +412,30 @@ def _find_errors(self, assignment_target, parents, ignore):
             if error_code:
                 yield self.err(assignment_target, error_code, name=name)
 
+    @staticmethod
+    def is_namedtupe(node_value):
+        if isinstance(node_value, ast.Call):
+            if isinstance(node_value.func, ast.Attribute):
+                if node_value.func.attr == 'namedtuple':
+                    return True
+            elif isinstance(node_value.func, ast.Name):
+                if node_value.func.id == 'namedtuple':
+                    return True
+        return False
+
     def visit_assign(self, node, parents, ignore=None):
-        if isinstance(node.value, ast.Call):
-            if isinstance(node.value.func, ast.Attribute):
-                if node.value.func.attr == 'namedtuple':
-                    return
-            elif isinstance(node.value.func, ast.Name):
-                if node.value.func.id == 'namedtuple':
-                    return
+        if self.is_namedtupe(node.value):
+            return
         for target in node.targets:
             for error in self._find_errors(target, parents, ignore):
                 yield error
 
+    def visit_namedexpr(self, node, parents, ignore):
+        if self.is_namedtupe(node.value):
+            return
+        for error in self._find_errors(node.target, parents, ignore):
+            yield error
+
     def visit_with(self, node, parents, ignore):
         if PY2:
             for error in self._find_errors(

testsuite/N806_py38.py

@@ -0,0 +1,19 @@
+# python_version >= '3.8'
+#: Okay
+def f1(values):
+    total = 0
+    partial_sums = [total := total + v for v in values]
+    return partial_sums, total
+#: Okay
+GLOBAL_VAR = 0
+def f2(values):
+    global GLOBAL_VAR
+    partial_sums = [GLOBAL_VAR := GLOBAL_VAR + v for v in values]
+    return partial_sums, GLOBAL_VAR
+#: N806:2:16
+def f():
+    return 1, (BaD_WalRuS := 1), BaD_WalRuS + 1
+#: Okay
+def f():
+    (NamedTuple := namedtuple('NamedTuple', 'f1 f2'))
+    return NamedTuple

testsuite/N816_py38.py

@@ -0,0 +1,5 @@
+# python_version >= '3.8'
+#: Okay
+lambda f: (TheName := namedtuple('TheName', 'a b c')), TheName
+#: N816:1:15:
+lambda line: (BaD_WaLRuS := re.match(pattern, line)) and BaD_WaLRuS.group(1)