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) ,         


        

Answer 1

I like to use defaultdict for counting:

from collections import defaultdict

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

result = defaultdict(int)

for date, cnt in lst:
    year, month, day = date.split('-')
    result['-'.join([year, month])] += cnt

print(result)