Merge pull request #1631 from Avira/master

help the user in the rare case this assertion actually fails

Author: nicoddemus

_pytest/python.py

@@ -1987,7 +1987,7 @@ def parsefactories(self, node_or_obj, nodeid=NOTSET, unittest=False):
                 # fixture attribute
                 continue
             else:
-                assert not name.startswith(self._argprefix)
+                assert not name.startswith(self._argprefix), name
             fixturedef = FixtureDef(self, nodeid, name, obj,
                                     marker.scope, marker.params,
                                     yieldctx=marker.yieldctx,