Creating Set of objects of user defined class in python

Question

table = set([])  class GlobeLearningTable(object):     def __init__(self,mac,port,dpid):          self.mac = mac         self.port = port         self.dpid = dpid      def add(self):          global table         if self not in table:             table.add(self)  class LearningSwitch(object):     def __init__ (self, connection, transparent):        self.connection = connection        self.transparent = transparent        self.macToPort = {}        connection.addListeners(self)        self.hold_down_expired = _flood_delay == 0      def _handle_PacketIn (self, event):        packet = event.parsed        self.macToPort[packet.src] = event.port # 1        packet_src = str(packet.src)        packet_mac = packet_src.upper()        entry = GlobeLearningTable(packet_mac, event.port, dpid_to_str(self.connection.dpid))        entry.add() 

Problem : entry.add() method adds new object every time it is called and increments the items in the table.

This should not happen because

1. In the add method I am checking that is that object in the table or not , then I am adding that particular object.
2. Table is a set which is unordered list, which should not have duplicate objects.

Help: is there any way in this set up I can add the object only when it's not in the table.

Answer 1

You need to implement __eq__ and __hash__ methods to teach Python about how to recognise unique GlobeLearningTable instances.

class GlobeLearningTable(object):     def __init__(self,mac,port,dpid):         self.mac = mac         self.port = port         self.dpid = dpid      def __hash__(self):         return hash((self.mac, self.port, self.dpid))      def __eq__(self, other):         if not isinstance(other, type(self)): return NotImplemented         return self.mac == other.mac and self.port == other.port and self.dpid == other.dpid 

Now your object is comparable, and equal objects will also return equal values for __hash__. This lets set and dict objects store your objects efficiently and detect if it is already present:

>>> demo = set([GlobeLearningTable('a', 10, 'b')]) >>> GlobeLearningTable('a', 10, 'b') in demo True