Question

While working on a problem from Google Python class, I formulated following result by using 2-3 examples from Stack overflow-

def sort_last(tuples):
    return [b for a,b in sorted((tup[1], tup) for tup in tuples)]

print sort_last([(1, 3), (3, 2), (2, 1)])

I learned List comprehension yesterday, so know a little about list comprehension but I am confused how this solution is working overall. Please help me to understand this (2nd line in function).

Answer 1

That pattern is called decorate-sort-undecorate.

1. You turn each (1, 3) into (3, (1, 3)), wrapping each tuple in a new tuple, with the item you want to sort by first.
2. You sort, with the outer tuple ensuring that the second item in the original tuple is sorted on first.
3. You go back from (3, (1, 3)) to (1, 3) while maintaining the order of the list.

In Python, explicitly decorating is almost always unnecessary. Instead, use the key argument of sorted:

sorted(list_of_tuples, key=lambda tup: tup[1]) # or key=operator.itemgetter(1)

Or, if you want to sort on the reversed version of the tuple, no matter its length:

sorted(list_of_tuples, key=lambda tup: tup[::-1]) 
                              # or key=operator.itemgetter(slice(None, None, -1))