Best way to loop over a python string backwards

Question

What is the best way to loop over a python string backwards?

The following seems a little awkward for all the need of -1 offset:

string = \"trick or          


        

Answer 1

reversed takes an iterable and and returns an iterator that moves backwards. string[::-1] is fine, but it creates a new, reversed string instead. If you just want to iterate, then this will probably better:

for c in reversed(string):
    print c

If you want to use the reversed string afterwards, creating it once will be better.

Answer 2

Yes, the second syntax shortcut creates an intermediate string and has an associated performance penalty.

The first version is better written as:

for index, char in enumerate(reversed(s)):
   print "pos %d: %s" % (index, char)

Which is easy to comprehend. Neither reversed nor enumerate` need to make a copy of the string.

Also be careful about using string as a variable name, as it is also the name of a module in the standard library.

Answer 3

Try the reversed builtin:

for c in reversed(string):
     print c

The reversed() call will make an iterator rather than copying the entire string.

PEP 322 details the motivation for reversed() and its advantages over other approaches.

Answer 4

Python 3 with enumerate and reversed methods:

string = "trick or treat"
for i, c in enumerate(reversed(string)):
    print(i, c)

You can use print(c) just for retrieving each character without the index.

Answer 5

Less code is usually faster in Python. Luckily, you don't have to guess:

python -mtimeit -s"s='x'*100000" "for x in s[::-1]: pass"
100 loops, best of 3: 1.99 msec per loop

python -mtimeit -s"s='x'*100000" "for x in reversed(s): pass"
1000 loops, best of 3: 1.97 msec per loop

python -mtimeit -s"s='x'*100000" "for i in xrange(len(s)-1, 0-1, -1): s[i]"
100 loops, best of 3: 4.95 msec per loop

So the shorter code is a bit faster, but it comes with a memory overhead.