How do I iterate over a sequence in reverse order?

Python 2.4 and later has a built-in reversed iterator, which takes an arbitrary sequence, and iterates over it backwards:

for x in reversed(L):
    print x # do something with x

For earlier versions, it’s usually fastest to simply reverse the list during the loop:

L.reverse()
try:
    for x in L:
        print x # do something with x
finally:
    L.reverse()

This has the disadvantage that while you are in the loop, the list is temporarily reversed. If this is a problem, you can make a copy instead:

rev = L[:]
rev.reverse()
for x in rev:
    print x # do something with x

Note that both reverse and the copy operation only copies object references, so they’re a lot faster than they may appear (performance depends on the number of items in the list, not the size of those items).

If the sequence is not a list, but supports extended slicing, you can do:

for x in sequence[::-1]:
    print x # do something with x

For arbitrary sequences, you can do:

for i in xrange(len(sequence)-1, -1, -1):
    x = sequence[i]
    print x # do something with x

Finally, you can define your own version of 2.4’s reversed:

class Reversed:
    def __init__(self, seq):
        self.seq = seq
    def __len__(self):
        return len(self.seq)
    def __getitem__(self, i):
        return self.seq[-(i + 1)]

for x in Reversed(sequence):
    print x # do something with x

However, this solution is slower than the other examples shown here, due to the method call overhead.