To sum up values of same items in a list of tuples while they are string

Question

If I have list of tuples like this:

my_list = [(\'books\', \'$5\'), (\'books\', \'$10\'), (\'ink\', \'$20\'), (\'paper\', \'$15\'), (\'paper\', \'$20\'), (\'pap         


        

Answer 1

If your data is already grouped like you sample input, you can use itertools.groupby, grouping by each first element of the tuples and summing the prices in each group:

from itertools import groupby
from operator import itemgetter

my_list = [('books', '$5'), ('books', '$10'), ('ink', '$20'), ('paper', '$15'), ('paper', '$20'), ('paper', '$15')]

grouped = [(k, "${}".format(sum(int(s[1][1:]) for s in v)))    
               for k ,v in groupby(my_list, itemgetter(0))]

Output:

[('books', '$15'), ('ink', '$20'), ('paper', '$50')]

If it were not ordered you could call sorted groupby(sorted(my_list), itemgetter(0)) but changes the complexity to n log n so a dict option may be better in that case.

You can also do the groupby on data and forget building an intermediary list, csv.reader will also split the data for you:

from itertools import groupby
from operator import itemgetter

from csv import reader
grouped = [(k, "${}".format(sum(int(s[1]) for s in v))) 
              for k, v in groupby(reader(data), itemgetter(0))]

You might be better also leaving the values as ints and formatting when you want to output. Also to handle floats cast to float not int:

from csv import reader
grouped = [(k, sum(float(s[1]) for s in v))
              for k, v in groupby(reader(data), itemgetter(0))]

When you want to output you can add the dollar sign and also pad so you get nicely formatted output:

In [10]: "${:.2f}".format(1.0)
Out[10]: '$1.00'