Parallel encounter validation

gistfile1.py

    @requiresMongo                                                                                                 
    @defer.inlineCallbacks                                                                                         
    def getEncountersForUser(self, userID, startTime, endTime, areEncountering=lambda a, b: defer.succeed(True)):  
        user = yield findOne(self.users, col.userID == userID).execute()                                           
        user = ref.user(user)                                                                                      
        def maybeInclude(encountering, u, row):                                                                    
            if encountering:                                                                                       
                return (u, row)                                                                                    
            return ()                                                                                              
        deferreds = []                                                                                             
        for row in (yield self._encountersForUserQuery(user, startTime, endTime).execute()):                       
            otherUser = row['b'] if row['a'] == user else row['a']                                                 
            deferreds.append(areEncountering(user, otherUser).addCallback(maybeInclude, otherUser, row))           
        byUser = {}                                                                                                
        for (k, v) in filter(None, (yield defer.gatherResults(deferreds))):                                        
            byUser.setdefault(k, []).append(v)                                                                     
        defer.returnValue(byUser)