Reverse a string in Python

Question

There is no built in reverse function for Python's str object. What is the best way of implementing this method?

If supplying a very concise answer, please elaborate on its efficiency. For example, whether the str object is converted to a different object, etc.

Answer 1

Quick Answer (TL;DR)

Example

### example01 -------------------
mystring  =   'coup_ate_grouping'
backwards =   mystring[::-1]
print backwards

### ... or even ...
mystring  =   'coup_ate_grouping'[::-1]
print mystring

### result01 -------------------
'''
gnipuorg_eta_puoc
'''

Detailed Answer

Background

This answer is provided to address the following concern from @odigity:

Wow. I was horrified at first by the solution Paolo proposed, but that took a back seat to the horror I felt upon reading the first comment: "That's very pythonic. Good job!" I'm so disturbed that such a bright community thinks using such cryptic methods for something so basic is a good idea. Why isn't it just s.reverse()?

Problem

• Context
  • Python 2.x
  • Python 3.x
• Scenario:
  • Developer wants to transform a string
  • Transformation is to reverse order of all the characters

Solution

• example01 produces the desired result, using extended slice notation.

Pitfalls

• Developer might expect something like string.reverse()
• The native idiomatic (aka "pythonic") solution may not be readable to newer developers
• Developer may be tempted to implement his or her own version of string.reverse() to avoid slice notation.
• The output of slice notation may be counter-intuitive in some cases:
  • see e.g., example02
    • print 'coup_ate_grouping'[-4:] ## => 'ping'
    • compared to
    • print 'coup_ate_grouping'[-4:-1] ## => 'pin'
    • compared to
    • print 'coup_ate_grouping'[-1] ## => 'g'
  • the different outcomes of indexing on [-1] may throw some developers off

Rationale

Python has a special circumstance to be aware of: a string is an iterable type.

One rationale for excluding a string.reverse() method is to give python developers incentive to leverage the power of this special circumstance.

In simplified terms, this simply means each individual character in a string can be easily operated on as a part of a sequential arrangement of elements, just like arrays in other programming languages.

To understand how this works, reviewing example02 can provide a good overview.

Example02

### example02 -------------------
## start (with positive integers)
print 'coup_ate_grouping'[0]  ## => 'c'
print 'coup_ate_grouping'[1]  ## => 'o' 
print 'coup_ate_grouping'[2]  ## => 'u' 

## start (with negative integers)
print 'coup_ate_grouping'[-1]  ## => 'g'
print 'coup_ate_grouping'[-2]  ## => 'n' 
print 'coup_ate_grouping'[-3]  ## => 'i' 

## start:end 
print 'coup_ate_grouping'[0:4]    ## => 'coup'    
print 'coup_ate_grouping'[4:8]    ## => '_ate'    
print 'coup_ate_grouping'[8:12]   ## => '_gro'    

## start:end 
print 'coup_ate_grouping'[-4:]    ## => 'ping' (counter-intuitive)
print 'coup_ate_grouping'[-4:-1]  ## => 'pin'
print 'coup_ate_grouping'[-4:-2]  ## => 'pi'
print 'coup_ate_grouping'[-4:-3]  ## => 'p'
print 'coup_ate_grouping'[-4:-4]  ## => ''
print 'coup_ate_grouping'[0:-1]   ## => 'coup_ate_groupin'
print 'coup_ate_grouping'[0:]     ## => 'coup_ate_grouping' (counter-intuitive)

## start:end:step (or start:end:stride)
print 'coup_ate_grouping'[-1::1]  ## => 'g'   
print 'coup_ate_grouping'[-1::-1] ## => 'gnipuorg_eta_puoc'

## combinations
print 'coup_ate_grouping'[-1::-1][-4:] ## => 'puoc'

Conclusion

The cognitive load associated with understanding how slice notation works in python may indeed be too much for some adopters and developers who do not wish to invest much time in learning the language.

Nevertheless, once the basic principles are understood, the power of this approach over fixed string manipulation methods can be quite favorable.

For those who think otherwise, there are alternate approaches, such as lambda functions, iterators, or simple one-off function declarations.

If desired, a developer can implement her own string.reverse() method, however it is good to understand the rationale behind this aspect of python.

See also

• alternate simple approach
• alternate simple approach
• alternate explanation of slice notation

Answer 2

All of the above solutions are perfect but if we are trying to reverse a string using for loop in python will became a little bit tricky so here is how we can reverse a string using for loop

string ="hello,world"
for i in range(-1,-len(string)-1,-1):
    print (string[i],end=(" ")) 

I hope this one will be helpful for someone.

Answer 3

Recursive method:

def reverse(s): return s[0] if len(s)==1 else s[len(s)-1] + reverse(s[0:len(s)-1])

example:

print(reverse("Hello!"))    #!olleH

Answer 4

original = "string"

rev_index = original[::-1]
rev_func = list(reversed(list(original))) #nsfw

print(original)
print(rev_index)
print(''.join(rev_func))

Answer 5

You can use the reversed function with a list comprehesive. But I don't understand why this method was eliminated in python 3, was unnecessarily.

string = [ char for char in reversed(string)]

Answer 6

def reverse(input):
    return reduce(lambda x,y : y+x, input)