Reversing a String in Python

Question

I have a simple function that takes a string and reverses it:

def reverse(string): 
    begin = 0 
    end = len(string) - 1 
    strlist = [i for i in string] 
    while(begin < end): 
        temp = strlist[begin] 
        strlist[begin] = strlist[end] 
        strlist[end] = temp 
        begin += 1 
        end -= 1 
    return ''.join(strlist)

print reverse('flash')

I know it's only a simple function but I'm keen to see where my thought process could be improved (even in such a simple context) for future projects.

Answer 1

Well, this is technically a way to invalidate your function, but either the built in slicing operator or the reversed function would do this for you in one line:

"flash"[::-1]
>>> "hsalf"

The slice operator takes three parameters, where it starts and ends, but also what step to take as it increments. You could pass in 2 to get every second letter, but you can also pass in a negative value to start at the end and work backwards, hence reversing your string.

The reversed function can take any iterator and return it in reverse as a generator object. This means that it wont strictly print properly, ie. this is what you would get when printing it:

reversed("flash")
>>> <reversed object at 0x0000000002F50898>

But calling join with it will give the result you need:

''.join(reversed("flash"))
>>> "hsalf"

---

Note that as erip points out this can have trouble with unicode characters in Python 2. In 3 all strings are unicode, but not before then. So reversing non unicode strings actually messes up the byte order, causing erratic behaviour like this:

>>> print("mañana"[::-1])
ana±am

To safely avoid this in 2, make sure to declare it as unicode with a u before the string u"mañana". This will make either method work fine. Alternatively from __future__ import unicode_literals will make all strings unicode like they are in Python 3.

---

But for your approach I'll still give suggestions. You use a slightly complicated approach. Swapping the values at opposite ends is not entirely obvious so I'd recommend adding comments to elaborate a bit on what the intent is as code is not always readable on its own.

Also you could use Python's multiple assignment to clean up some code. A more simple example is your begin and end could be set at the same time:

    begin, end = 0, len(string) - 1 

This doesn't add a lot but can look a little neater. It essentially lets you assign two variables at once. It might be unnecessary, but the same principle can be used to eliminate the need for your temp.

    strlist[end], strlist[begin] = strlist[begin], strlist[end]

This allows you to safely assign both at the same time. The right hand side of the expression is fully evaluated before any assignment, so you wont lose either value.

Answer 2

Python encourages the use of generator expressions. Taken directly from Functional Programming, this language construct condenses a lot of meaning in just a line. It is also very easy to read.

def reverse(string):
    return ''.join(string[i] for i in range(len(string) - 1, -1, -1))

It reads almost as english, join all the i-th-s elements of the string where i goes from the len of the string to zero.

Answer 3

Other option for reverse string is use a deque object.

from collections import deque

def reverse_deque(string):
    d = deque(string)
    d.reverse()
    return ''.join(d)

In [245]: reverse_deque('reverse')
Out[245]: 'esrever'

comparison

1. string slice ([::-1]):

   In [243]: %timeit "reverse"[::-1]
   1000000 loops, best of 3: 310 ns per loop
   

2. deque reverse:

   In [244]: %timeit reverse_deque("reverse")
   100000 loops, best of 3: 2.61 µs per loop
   

3. `reverse' function from Caridorc:

   In [241]: %timeit reverse_caridorc("reverse")
   100000 loops, best of 3: 3.98 µs per loop
   

4. function in question of Codingo

   In [250]: %timeit reverse_codingo("reverse")
   100000 loops, best of 3: 3.61 µs per loop
   

Clearly the slice option is the winner.

Answer 4

Building on Caridorc's answer, there's a more Pythonic way to approach the problem which has an even better readability:

def reverse(string):
    return ''.join(string[i] for i in reversed(range(len(string))

Here's an explanation from furthest depth outward:

1. range(len(string)): Create an iterator object which iterates from 0 to len(string) - 1 (ie: the parameter is non-inclusive).
2. reversed(range(len(string))): Reverse this iterator so you're starting at len(string) - 1 and going backward one-by-one to 0.
3. [string[i] for i in ...]: This is a list comprehension, basically a way to create a list with a single line.
4. ''.join(...): Join each element together into a string from the list, using an empty string as a separator.

Documentation can be found for each of these concepts here:

1. PEP 281: loop counters with range and xrange.
2. PEP 322: reversed iteration.
3. PEP 202: list comprehensions.

The hope is that the above code will be fairly readable to someone nominally proficient in Python.

Answer 5

Similar to Naftuli Tzvi Kay, I immediately thought of the reversed builtin. reversed returns an iterator, so you need to force evaluation of it, which is why it needs to be fed into ''.join().

''.join(letter for letter in reversed('foobar'))