--- a/src/topic/__init__.py	Wed Jun 10 15:03:39 2015 -0400
+++ b/src/topic/__init__.py	Wed Jun 10 16:13:20 2015 -0400
@@ -19,6 +19,7 @@
 from mercurial import phases
 from mercurial import util
 
+from . import constants
 from . import revset as topicrevset
 
 cmdtable = {}
@@ -30,7 +31,7 @@
 
 def _nodemap(repo, node):
     ctx = repo[node]
-    t = ctx.extra().get('topic', '')
+    t = ctx.extra().get(constants.extrakey, '')
     if t and ctx.phase() > phases.public:
         return [t]
     return []
@@ -41,14 +42,14 @@
         def commitctx(self, ctx, error=None):
             current = self.currenttopic
             if current:
-                ctx.extra()['topic'] = current
+                ctx.extra()[constants.extrakey] = current
             return orig.commitctx(self, ctx, error=error)
 
         @property
         def topics(self):
             topics = set(['', self.currenttopic])
             for c in self.set('not public()'):
-                topics.add(c.extra().get('topic', ''))
+                topics.add(c.extra().get(constants.extrakey, ''))
             topics.remove('')
             return topics
 
@@ -85,12 +86,12 @@
                     return None
             fixedextra = dict(c.extra())
             newtopic = None if clear else topic
-            if fixedextra.get('topic', None) == topic:
+            if fixedextra.get(constants.extrakey, None) == topic:
                 continue
-            if clear and 'topic' in fixedextra:
-                del fixedextra['topic']
+            if clear and constants.extrakey in fixedextra:
+                del fixedextra[constants.extrakey]
             else:
-                fixedextra['topic'] = topic
+                fixedextra[constants.extrakey] = topic
             c.parents()
             mc = context.memctx(
                 repo, (c.p1().node(), c.p2().node()), c.description(),
@@ -125,7 +126,7 @@
         repo.vfs.unlink('topic')
     else:
         # inherit the topic of the parent revision
-        t = pctx.extra().get('topic', '')
+        t = pctx.extra().get(constants.extrakey, '')
         if t and pctx.phase() > phases.public:
             with repo.vfs.open('topic', 'w') as f:
                 f.write(t)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/topic/constants.py	Wed Jun 10 16:13:20 2015 -0400
@@ -0,0 +1,1 @@
+extrakey = 'topic'

--- a/src/topic/revset.py	Wed Jun 10 15:03:39 2015 -0400
+++ b/src/topic/revset.py	Wed Jun 10 16:13:20 2015 -0400
@@ -1,5 +1,7 @@
 from mercurial import revset
 
+from . import constants
+
 def topicset(repo, subset, x):
     """`topic([topic])`
     Specified topic or all changes with any topic specified.
@@ -17,7 +19,8 @@
     else:
         matcher = lambda t: bool(t)
     drafts = subset.filter(lambda r: repo[r].mutable())
-    return drafts.filter(lambda r: matcher(repo[r].extra().get('topic', '')))
+    return drafts.filter(
+        lambda r: matcher(repo[r].extra().get(constants.extrakey, '')))
 
 def modsetup():
     revset.symbols.update({'topic': topicset})