default dict is your friend

session.py

from collections import defaultdict, namedtuple

class Session:
    def __init__(self):
        self.seconds = 0
        self.session_id = None
        self.events_by_type = defaultdict(int)

    def add_event(self, event):
        if self.session_id and event.session_id != self.session_id:
            raise Exception("Argh mixing sessions!")
        self.session_id = event.session_id
        self.seconds += event.seconds
        self.events_by_type[event.type_name] += 1

Event = namedtuple('Event', 'session_id type_name seconds')

sessions = defaultdict(Session)

events = (
        Event('session_1', 'watched', 20),
        Event('session_1', 'paused',  20),
        Event('session_2', 'watched', 20),
        Event('session_2', 'paused',  20),
        Event('session_1', 'watched', 20),
        )

# creating Sessions and added events
for event in events:
    sessions[event.session_id].add_event(event)


# result
for session in sessions.itervalues():
    print vars(session)


# OUTPUT
# {'seconds': 60, 'events_by_type': defaultdict(<type 'int'>, {'paused': 1, 'watched': 2}), 'session_id': 'session_1'}
# {'seconds': 40, 'events_by_type': defaultdict(<type 'int'>, {'paused': 1, 'watched': 1}), 'session_id': 'session_2'}