Python defaultdict，计数列表不起作用

Question

So I have a list that I've sorted by date, and the number of records per date. This is working fine, and in my print outs I can see the count. However I need to build up an aggregate data structure from the one shown, and another later, and as such am trying to put together "temp".

Why do I continually get 0 when appending into temp, when just above it in the print out, it shows me 4 ?

created = EmailAddress.objects.all().values_list('email', 'user__date_joined')
temp = defaultdict(list)

for key,group in itertools.groupby(created, key=lambda x: x[1].strftime('%Y%m%d')[:11]):
    print 'Date of signup: ', key
    print '# of signups: ', (sum(1 for _ in group))
    temp[key].append(sum(1 for _ in group))

Output

Date of signup:  20150603
# of signups:  4
defaultdict(<type 'list'>, {'20150603': [0]})

Answer 1

group is like a generator. The first time you iterate over it, you "consume" all the values

You can just store the value in a variable

for key,group in itertools.groupby(created, key=lambda x: x[1].strftime('%Y%m%d')[:11]):
    print 'Date of signup: ', key
    num_signups = (sum(1 for _ in group))
    print '# of signups: ', num_signups
    temp[key].append(num_signups)