--- a/hgext3rd/evolve/obscache.py	Fri May 12 18:56:42 2017 +0200
+++ b/hgext3rd/evolve/obscache.py	Fri May 12 18:56:56 2017 +0200
@@ -86,8 +86,6 @@
 
     return obsstore
 
-emptykey = (node.nullrev, node.nullid, 0, 0, node.nullid)
-
 # XXX copied as is from Mercurial 4.2 and added the "offset" parameters
 @util.nogc
 def _readmarkers(data, offset=None):
@@ -151,6 +149,8 @@
     _filepath = 'cache/evoext-obscache-00'
     _headerformat = '>q20sQQ20s'
 
+    emptykey = (node.nullrev, node.nullid, 0, 0, node.nullid)
+
     def __init__(self, repo):
         self._vfs = repo.vfs
         # cache key covering the changesets and obsmarkers content
@@ -176,7 +176,7 @@
 
     def clear(self, reset=False):
         """invalidate the cache content"""
-        self._cachekey = emptykey if reset else None
+        self._cachekey = self.emptykey if reset else None
         self._data = bytearray()
 
     def _checkkey(self, changelog, obsstore):
@@ -229,7 +229,7 @@
         status = self._checkkey(cl, obsstore)
         if status is None:
             reset = True
-            key = emptykey
+            key = self.emptykey
             obssize, obskey = obsstore.cachekey()
             tiprev = len(cl) - 1
         else:
@@ -353,7 +353,7 @@
 
         data = repo.vfs.tryread(self._filepath)
         if not data:
-            self._cachekey = emptykey
+            self._cachekey = self.emptykey
             self._data = bytearray()
         else:
             headersize = struct.calcsize(self._headerformat)