diff -r ab3cf67740d6 -r 6e3e8575276d mercurial/context.py
--- a/mercurial/context.py	Wed Jul 24 19:13:39 2013 -0400
+++ b/mercurial/context.py	Sat Jul 13 19:59:21 2013 -0500
@@ -16,9 +16,20 @@
 
 propertycache = util.propertycache
 
-class changectx(object):
+class basectx(object):
+    """A basectx object represents the common logic for its children:
+    changectx: read-only context that is already present in the repo,
+    workingctx: a context that represents the working directory and can
+                be committed,
+    memctx: a context that represents changes in-memory and can also
+            be committed."""
+    def __new__(cls, repo, changeid='', *args, **kwargs):
+        return super(basectx, cls).__new__(cls)
+
+class changectx(basectx):
     """A changecontext object makes access to data related to a particular
-    changeset convenient."""
+    changeset convenient. It represents a read-only context already presnt in
+    the repo."""
     def __init__(self, repo, changeid=''):
         """changeid is a revision number, node, or tag"""
         if changeid == '':