python - List Comprehension returns empty list -

i'm trying query mongodb database , throw 2 sets of results ('_id' , 'team') 2 separate lists.

import pymongo  client = pymongo.mongoclient('localhost:27017') db = client['db_name']  query = {'team': {'$exists': 1}} projection = {'_id': 1, 'team': 1}  data = db['collection_name'].find(query, projection)  # line 9  id_list = [value dict in data key, value in dict.iteritems() if key == '_id'] teams_list = [value dict in data key, value in dict.iteritems() if key == 'team']  print id_list print teams_list  client.close() 

for code above, 'id_list' expected 'teams_list' empty. when put 'teams_list' before 'id_list' expected 'teams_list' output , 'id_list' empty. , when repeat data call (line 9) in between 2 list comprehensions expected output both lists.

any idea why happening?

you need define data as:

data = list(db['collection_name'].find(query, projection)) 

as find() returns generator. once iterate values, lost. need store them list. here list() i.e. stores items returns generator list.

instead of iterating list twice, better way 2 single loop as:

id_list, teams_list = [], []  #   v  `dict` in-built data type, should not using variable d in data:     key, value in d.iteritems():         if key == '_id':             id_list.append(value)         elif key == 'team':             teams_list.append(value) 

refer generator wiki more information related generators