[dataformat] Remove use of Prefix

New beat.cmdline tests don't need it anymore

beat/backend/python/dataformat.py

@@ -516,9 +516,9 @@ class DataFormat(object):
         if isinstance(prefix, six.string_types):
             prefix = utils.Prefix(prefix)
 
-        if prefix.paths[0] in self.prefix.paths:
-            raise RuntimeError("Cannot export dataformat to the same prefix (%s in " \
-                    "%s)" % (prefix.paths[0], self.prefix.paths))
+        if prefix == self.prefix:
+            raise RuntimeError("Cannot export dataformat to the same prefix ("
+                               "%s)" % prefix)
 
         for k in self.referenced.values():
             k.export(prefix)