Grouping Python tuple list

Question

I have a list of (label, count) tuples like this:

[('grape', 100), ('grape', 3), ('apple', 15), ('apple', 10), ('apple', 4), ('banana', 3)] 

From that I want to sum all values with the same label (same labels always adjacent) and return a list in the same label order:

[('grape', 103), ('apple', 29), ('banana', 3)] 

I know I could solve it with something like:

def group(l):     result = []     if l:         this_label = l[0][0]         this_count = 0         for label, count in l:             if label != this_label:                 result.append((this_label, this_count))                 this_label = label                 this_count = 0             this_count += count         result.append((this_label, this_count))     return result 

But is there a more Pythonic / elegant / efficient way to do this?

Answer 1

itertools.groupby can do what you want:

import itertools import operator  L = [('grape', 100), ('grape', 3), ('apple', 15), ('apple', 10),      ('apple', 4), ('banana', 3)]  def accumulate(l):     it = itertools.groupby(l, operator.itemgetter(0))     for key, subiter in it:        yield key, sum(item[1] for item in subiter)   print(list(accumulate(L))) # [('grape', 103), ('apple', 29), ('banana', 3)]