I want to group my list of lists by date and sum the values where the dates match in python

Question

I have a list of lists like that:

l = [[\'05-05-2015\', -40], [\'05-05-2015\', 30], [\'07-05-2015\', -75], [\'05-05-2015\', -40], [\'05-05-2015\', 120], [\'07-0