--- a/buttercup/source.py	Mon May 30 13:41:14 2011 -0700
+++ b/buttercup/source.py	Mon May 30 13:49:51 2011 -0700
@@ -1,5 +1,8 @@
 import os
-import subprocess
+try:
+    from subprocess import check_call as call
+except:
+    from subprocess import call
 
 class Source(object):
     """abstract base class for VCS source"""
@@ -20,6 +23,12 @@
 
     def update(self):
         """updates a checkout or does one if it does not exist"""
+        if os.path.exists(self.directory()):
+            assert os.path.exists(os.path.join(self.directory(), '.hg'))
+            call(['hg', 'pull'], cwd=self.directory)
+            call(['hg', 'update'], cwd=self.directory)
+        else:
+            pass
 
 class GitSource(Source):