diff --git a/hgext/remotenames.py b/hgext/remotenames.py
--- a/hgext/remotenames.py
+++ b/hgext/remotenames.py
@@ -22,7 +22,12 @@
 
 from __future__ import absolute_import
 
-import UserDict
+try:
+    import UserDict
+    dictmixin = UserDict.DictMixin
+except ImportError:
+    import collections
+    dictmixin = collections.MutableMapping
 
 from mercurial.i18n import _
 
@@ -57,7 +62,7 @@
     default=True,
 )
 
-class lazyremotenamedict(UserDict.DictMixin):
+class lazyremotenamedict(dictmixin):
     """
     Read-only dict-like Class to lazily resolve remotename entries
 
@@ -110,6 +115,18 @@
         else:
             raise KeyError()
 
+    def __iter__(self):
+        return iter(self.potentialentries)
+
+    def __len__(self):
+        return len(self.potentialentries)
+
+    def __setitem__(self):
+        raise NotImplementedError
+
+    def __delitem__(self):
+        raise NotImplementedError
+
     def _fetchandcache(self, key):
         if key in self.cache:
             return self.cache[key]