Group list by values

Question

Let\'s say I have a list like this:

mylist = [["A",0], ["B",1], ["C",0], ["D",2], ["E",2]]

Answer 1

I don't know about elegant, but it's certainly doable:

oldlist = [["A",0], ["B",1], ["C",0], ["D",2], ["E",2]]
# change into: list = [["A", "C"], ["B"], ["D", "E"]]

order=[]
dic=dict()
for value,key in oldlist:
  try:
    dic[key].append(value)
  except KeyError:
    order.append(key)
    dic[key]=[value]
newlist=map(dic.get, order)

print newlist

This preserves the order of the first occurence of each key, as well as the order of items for each key. It requires the key to be hashable, but does not otherwise assign meaning to it.