reducer = Code("""function(obj, prev){prev.count++;}""")
keys = {'event':True,'type':True}
conditions = {'event':'video_start'} 
a = posts.group(key=keys,condition=conditions, initial={"count": 0}, reduce=reducer)
for i in a:
    print i['type']
    keys = {'sn':True,'clip':True,'type':True}
    conditions = {'event':'video_play_blockend','type':i['type']}
    a = posts.group(key=keys,condition=conditions, initial={"count": 0}, reduce=reducer)
    for t in a:
        if t['count']>=3:
            print t
    print 1-round(len(a)/i['count'],2),i['type']
    print len(a),i['count'],i['type']

Traceback (most recent call last):
File "read.py", line 24, in <module>
a = posts.group(key=keys,condition=conditions, initial={"count": 0}, reduce=reducer)
File "/usr/lib64/python2.6/site-packages/pymongo/collection.py", line 1941, in group
collation=collation)["retval"]
File "/usr/lib64/python2.6/site-packages/pymongo/collection.py", line 232, in _command
collation=collation)
File "/usr/lib64/python2.6/site-packages/pymongo/pool.py", line 419, in command
collation=collation)
File "/usr/lib64/python2.6/site-packages/pymongo/network.py", line 116, in command
parse_write_concern_error=parse_write_concern_error)
File "/usr/lib64/python2.6/site-packages/pymongo/helpers.py", line 210, in _check_command_response
raise OperationFailure(msg % errmsg, code, response)
pymongo.errors.OperationFailure: errmsg: "group() can't handle more than 20000 unique keys"