sorting messages in vector timestamp sim -not working, think i just mixed something up

message_sorting.py

from collections import namedtuple
import pprint

## parsing user input ##

# named tuples to make things more organized?
Event = namedtuple('Event','process event')
Message = namedtuple('Message', 'sender receiver')

# raw input, ostensibly from the user
raw_msgs = ['1-1->2-2','1-3->3-4','2-3->3-1','3-2->1-2']

# list to hold processed user input
messages = []

# create message tuples
for msg in raw_msgs:
    messages.append(
        Message(
            *[Event(*map(int,params.split('-'))) for params in msg.split('->')]
        )
    )

# set of process ids
proc_ids = set([])

# add process id from each message to set (sets do not hold duplicate values)
for msg in messages:
    proc_ids.update(set([event.process for event in msg]))

## sort messages sent and received message by process ##

# create two dictionaries (aka hash tables) to store sorted lists of messages
# by process. The first dictionary (sent) will hold messages in the order in 
# which they were sent by each process. The second list (received) will hold 
# messages in the order in which they were received by each process.

sent = dict.fromkeys(proc_ids)
received = dict.fromkeys(proc_ids)

# populate dictionaries with sorted lists of messages
for proc_id in proc_ids:
    # sent message
    sent_msgs = filter(
        lambda msg, proc_id=proc_id: msg.sender.process == proc_id, 
        messages
    )
    sent[proc_id] = sorted(sent_msgs, key=lambda message: message.sender.event)


    # received messages
    recvd_msgs = filter(
        lambda msg, proc_id=proc_id: msg.receiver.process == proc_id, 
        messages
    )
    received[proc_id] = sorted(recvd_msgs, key=lambda message: message.receiver.event)

## create overall sorting of  messages ##

sorted_messages = []

# use our message list like a queue. pop messages off the queue, if the message
# is the first in the received list for the receiving process, and the first
# in the sent list for the sending process, add message to queue, and remove
# messages from the sent and received lists

print '\nsent\n'
pprint.pprint(sent)
print '\nreceived\n'
pprint.pprint(received)

while messages:
    message = messages.pop()
    
    first_msg_from_sender = next(iter(sent[message.sender.process]), None)
    if not first_msg_from_sender:
        raise(Exception('Message mismatch'))

    first_msg_to_receiver = next(iter(received[message.receiver.process]), None)
    if not first_msg_to_receiver:
        raise(Exception('Message mismatch'))

    print '--------'
    print '\nmessage: \n', message
    print '\nfirst sender: \n', first_msg_from_sender
    print '\nfirst receiver: \n', first_msg_to_receiver

    if message == first_msg_to_receiver and message == first_msg_from_sender:
        # add to final ordering
        sorted_messages.insert(0, message)

        # remove message from the sent and received collections
        sent[message.sender.process].remove(message)
        received[message.receiver.process].remove(message)
    else:
        # add back into the queue (to the opposite end from where we pop)
        messages.insert(0, message)

pprint.pprint(sorted_messages)