Avoid stack exhaustion in superclass_names()

Because this function is recursive, it can exhaust the stack if the
class hierarchy contains duplicate names. Avoid that using simple
memoization.

Also, these two supporting functions can be classmethods because they
don't use any instance-level state.

src/pep8ext_naming.py

@@ -296,23 +296,24 @@ class ClassNameCheck(BaseASTCheck):
     N801 = "class name '{name}' should use CapWords convention"
     N818 = "exception name '{name}' should be named with an Error suffix"
 
-    def get_class_def(self, name, parents):
+    @classmethod
+    def get_classdef(cls, name, parents):
         for parent in parents:
-            for definition in parent.body:
-                is_class_definition = isinstance(definition, ast.ClassDef)
-                if is_class_definition and definition.name == name:
-                    return definition
-
-    def superclass_names(self, name, parents):
-        class_ids = set()
-        class_def = self.get_class_def(name, parents)
-        if not class_def:
-            return class_ids
-        for base in class_def.bases:
-            if hasattr(base, "id"):
-                class_ids.add(base.id)
-                class_ids.update(self.superclass_names(base.id, parents))
-        return class_ids
+            for node in parent.body:
+                if isinstance(node, ast.ClassDef) and node.name == name:
+                    return node
+
+    @classmethod
+    def superclass_names(cls, name, parents, _names=None):
+        names = _names or set()
+        classdef = cls.get_classdef(name, parents)
+        if not classdef:
+            return names
+        for base in classdef.bases:
+            if isinstance(base, ast.Name) and base.id not in names:
+                names.add(base.id)
+                names.update(cls.superclass_names(base.id, parents, names))
+        return names
 
     def visit_classdef(self, node, parents, ignore=None):
         name = node.name

testsuite/N818.py

@@ -28,3 +28,7 @@ class Mixin:
     pass
 class MixinActionClass(Mixin, MixinError):
     pass
+#: Okay
+from decimal import Decimal
+class Decimal(Decimal):
+    pass