Question

Can You help me to convert Python list:

[(1, 'a'), (2, 'b'), (2, 'c'), (3, 'd'), (3, 'e')]

so that:
(1, 'a') is index 0

(2, 'b'), (2, 'c') are both index 1

(3, 'd'), (3, 'e') are both index 2

Simply, all tuples which element[0] is equal, have same index.

Thank You,

Answer 1

itertools.groupby to the rescue!:

lst = [(1, 'a'), (2, 'b'), (2, 'c'), (3, 'd'), (3, 'e')]
lst.sort(key=lambda x: x[0])  #Only necessary if your list isn't sorted already.
new_lst = [list(v) for k,v in itertools.groupby(lst,key=lambda x:x[0])]

You could use operator.itemgetter(0) instead of the lambda if you wanted…

demo:

>>> import itertools
>>> lst = [(1, 'a'), (2, 'b'), (2, 'c'), (3, 'd'), (3, 'e')]
>>> lst.sort(key=lambda x: x[0])
>>> new_lst = [list(v) for k,v in itertools.groupby(lst,key=lambda x:x[0])]
>>> new_lst
[[(1, 'a')], [(2, 'b'), (2, 'c')], [(3, 'd'), (3, 'e')]]