how to aggregate elements of a list of tuples if the tuples have the same first element?

Question

I have a list in which each value is a list of tuples. for example this is the value which I extract for a key :

     [('1998-01-20',8) , ('1998-01-22',4) , ('1998-06-18',8 ) , ('1999-07-15' , 7), ('1999-07-21',1) ]

I have also sorted the list. now I want to aggregate the values like this :

    [('1998-01' , 12 ) , ('1998-06' ,8 ) , ('1999-07',8 )]

in some sense I want to group my tuples in terms of month , to sum the ints for each month together , I have read about groupby and I think it can't help me with my data structure because I have no idea what I'll be facing in my list so I'm trying to find a way to say : start from the first elements of the tuples if i[0][:6] are equal : sum i[1] . but I'm facing difficulty to implement this idea .

    for i in List :
        if i[0][:6] # *problem* I don't know how to say my condition :
        s=sum(i[1]) #?

I would appreciate any advices as I'm a new user of python!

Answer 1

Try using itertools.groupby to aggregate values by month:

from itertools import groupby
a = [('1998-01-20', 8), ('1998-01-22', 4), ('1998-06-18', 8), 
     ('1999-07-15', 7), ('1999-07-21', 1)]

for key, group in groupby(a, key=lambda x: x[0][:7]):
    print key, sum(j for i, j in group)

# Output

1998-01 12
1998-06 8
1999-07 8

Here's a one-liner version:

print [(key, sum(j for i, j in group)) for key, group in groupby(a, key=lambda x: x[0][:7])]

# Output

[('1998-01', 12), ('1998-06', 8), ('1999-07', 8)]

Answer 2

Just use defaultdict:

from collections import defaultdict


DATA = [
    ('1998-01-20', 8),
    ('1998-01-22', 4),
    ('1998-06-18', 8),
    ('1999-07-15', 7),
    ('1999-07-21', 1),
]


groups = defaultdict(int)
for date, value in DATA:
    groups[date[:7]] += value


from pprint import pprint
pprint(groups)