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)

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)

Answer 3

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 4

Yet another answer different from the ones given already. You can simpy create a new dictionary where the keys are the year-month combinations. A loop over the dates in your list + using dictionary.get(key, defaultvalue) should do the trick. IT adds the current value to the value in the new dictionary, if the key did not yet exist, it returns the default value 0 and creates the key.

data = [('1998-01-20',8) , ('1998-01-22',4) , ('1998-06-18',8 ) , ('1999-07-15' , 7), ('1999-07-21',1)]
dictionary = dict()
for (mydate, val) in data: #
    ym = mydate[0:7]    # the key is only the year month combination (i.e. '1998-01' for example)
    dictionary[ym] = dictionary.get(ym, 0) + val  # return the value for that key or return default 0 (and create key)

data_aggregated = [(key, val) for (key, val) in dictionary.iteritems()] # if you need it back in old format