Python summing values in list if it exists in another list

Question

I have a list and a set:

a_list = [[\'1\', 2], [\'2\', 1], [\'1\', 1]]

b_list = {\'1\', \'2\'}

I\'m looking to correspond the items in b_list

Answer 1

Accumulate numbers using a dict, and then gather the results using a list comprehension:

>>> d = dict.fromkeys(b_list, 0)
>>> for k, n in a_list: 
...     if k in d: 
...         d[k] += n 
...
>>> [[k, n] for k, n in d.items()]
[['1', 3], ['2', 1]]