Python Reverse String

Python String doesn’t have a built-in reverse() function. However, there are various ways to reverse a string in Python.

Python Reverse String

Some of the common ways to reverse a string are:

• Using Slicing to create a reverse copy of the string.
• Using for loop and appending characters in reverse order
• Using while loop to iterate string characters in reverse order and append them
• Using string join() function with reversed() iterator
• Creating a list from the string and then calling its reverse() function
• Using Recursion

Let’s look at these algorithms implementations to reverse a string in Python.

Python Reverse String using Slicing

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def reverse_slicing(s):
    return s[::-1]

input_str = 'ABç∂EF'

if __name__ == "__main__":
    print('Reverse String using slicing =', reverse_slicing(input_str))

If you run above Python script, the output will be:

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Reverse String using slicing = FE∂çBA

Python Reverse String using For Loop

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def reverse_for_loop(s):
    s1 = ''
    for c in s:
        s1 = c + s1  # appending chars in reverse order
    return s1

input_str = 'ABç∂EF'

if __name__ == "__main__":
    print('Reverse String using for loop =', reverse_for_loop(input_str))

Output: Reverse String using for loop = FE∂çBA

Python Reverse String using While Loop

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def reverse_while_loop(s):
    s1 = ''
    length = len(s) - 1
    while length >= 0:
        s1 = s1 + s[length]
        length = length - 1
    return s1

input_str = 'ABç∂EF'

if __name__ == "__main__":
    print('Reverse String using while loop =', reverse_while_loop(input_str))

Python Reverse String using join() and reversed()

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def reverse_join_reversed_iter(s):
    s1 = ''.join(reversed(s))
    return s1

Python Reverse String using List reverse()

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def reverse_list(s):
    temp_list = list(s)
    temp_list.reverse()
    return ''.join(temp_list)

Python Reverse String using Recursion

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

Best Way to Reverse a String in Python

We can reverse a string through multiple algorithms. We have already seen six of them. But which of them you should choose to reverse a string.

We can use timeit module to run multiple iterations of these functions and get the average time required to run them.

All the above functions are stored in a python script named string_reverse.py. I executed all these functions one by one for 1,00,000 times using the timeit module and got the average of best 5 runs.

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$ python3.7 -m timeit --number 100000 --unit usec 'import string_reverse' 'string_reverse.reverse_slicing("ABç∂EF"*10)'
100000 loops, best of 5: 0.449 usec per loop

$ python3.7 -m timeit --number 100000 --unit usec 'import string_reverse' 'string_reverse.reverse_list("ABç∂EF"*10)'
100000 loops, best of 5: 2.46 usec per loop

$ python3.7 -m timeit --number 100000 --unit usec 'import string_reverse' 'string_reverse.reverse_join_reversed_iter("ABç∂EF"*10)'
100000 loops, best of 5: 2.49 usec per loop

$ python3.7 -m timeit --number 100000 --unit usec 'import string_reverse' 'string_reverse.reverse_for_loop("ABç∂EF"*10)'
100000 loops, best of 5: 5.5 usec per loop

$ python3.7 -m timeit --number 100000 --unit usec 'import string_reverse' 'string_reverse.reverse_while_loop("ABç∂EF"*10)'
100000 loops, best of 5: 9.4 usec per loop

$ python3.7 -m timeit --number 100000 --unit usec 'import string_reverse' 'string_reverse.reverse_recursion("ABç∂EF"*10)'
100000 loops, best of 5: 24.3 usec per loop

Below table presents the results and slowness of an algorithm from the best one.

Summary

We should use slicing to reverse a string in Python. Its code is very simple and small and we don’t need to write our own logic to reverse the string. Also, it’s the fastest way to reverse a string as identified by the above test executions.

About Pankaj

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