def get_inputs(self):
start_p = [p for p in self.postorder() if not self.outgoing_edges(p)]<EOL>self.log.debug('<STR_LIT>' % (repr(self), start_p))<EOL>return start_p<EOL>
Returns ``Piper`` instances, which are inputs to the pipeline i.e. have 
no incoming pipes (outgoing dependency edges).