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

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)]