utils: add KeyedDefaultdict

This is a subclass of defaultdict allowing the key to be passed to the default factory.

utils.py

@@ -10,11 +10,25 @@ import re
 import inspect
 import importlib
 import os
+import collections
 
 from log import log
 import world
 import conf
 
+class KeyedDefaultdict(collections.defaultdict):
+    """
+    Subclass of defaultdict allowing the key to be passed to the default factory.
+    """
+    def __missing__(self, key):
+        if self.default_factory is None:
+            # If there is no default factory, just let defaultdict handle it
+            super().__missing__(self, key)
+        else:
+            value = self[key] = self.default_factory(key)
+            return value
+
+
 class NotAuthenticatedError(Exception):
     """
     Exception raised by checkAuthenticated() when a user fails authentication