python

Reversing a List/Array is very commonly needed and there are three ways to reverse a list or array in Python.

Using the .reverse() method to reverse a list or array

Note: this method does not work for tuples in Python.

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a
(1, 2, 3, 4, 5)
>>> a.reverse()
Traceback (most recent call last):
  File "stdin", line 1, in module
AttributeError: 'tuple' object has no attribute 'reverse'

a
(1, 2, 3, 4, 5)
>>> a.reverse()
Traceback (most recent call last):
  File "stdin", line 1, in module
AttributeError: 'tuple' object has no attribute 'reverse'

The .reverse() will modify the given array/list. See the below:

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a = [1, 2, 3, 4, 5]
a.reverse() # returns None
print(a) # [5, 4, 3, 2, 1]

a = [1, 2, 3, 4, 5]
a.reverse() # returns None
print(a) # [5, 4, 3, 2, 1]

As we can see in the above Python code, the .reverse() does not create a copy, rather, it reverses the array/list in-place. The .reverse() method returns None.

Using the slicing to reverse an array, list or tuples in Python

We can use the [::-1] to returns a copy, without modifying the input. It works for list/array and tuples as well.

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>>> a = (1, 2, 3, 4, 5)
(1, 2, 3, 4, 5)
>>> a[::-1]
(5, 4, 3, 2, 1)
>>> a
(1, 2, 3, 4, 5)
>>> a=[1,2,3,4,5]
>>> a[::-1]
[5, 4, 3, 2, 1]
>>> a
[1, 2, 3, 4, 5]

>>> a = (1, 2, 3, 4, 5)
(1, 2, 3, 4, 5)
>>> a[::-1]
(5, 4, 3, 2, 1)
>>> a
(1, 2, 3, 4, 5)
>>> a=[1,2,3,4,5]
>>> a[::-1]
[5, 4, 3, 2, 1]
>>> a
[1, 2, 3, 4, 5]

This is quite cool and the most Pythonic way to reverse a list or tuple.

Python reversed() method to return a reversed iterator

We can use the reversed() function (inbuilt Python) that allows us to return an iterator. As for the list/array:

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a = [1, 2, 3, 4, 5]
reversed(a) # returns an iterator
list(reversed(a)) converts to list [5, 4, 3, 2, 1]
tuple(reversed(a)) converts to tuple (5, 4, 3, 2, 1)

a = [1, 2, 3, 4, 5]
reversed(a) # returns an iterator
list(reversed(a)) converts to list [5, 4, 3, 2, 1]
tuple(reversed(a)) converts to tuple (5, 4, 3, 2, 1)

It works for tuples as well:

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a = (1, 2, 3, 4, 5)
reversed(a) # returns an iterator
list(reversed(a)) converts to list [5, 4, 3, 2, 1]
tuple(reversed(a)) converts to tuple (5, 4, 3, 2, 1)

a = (1, 2, 3, 4, 5)
reversed(a) # returns an iterator
list(reversed(a)) converts to list [5, 4, 3, 2, 1]
tuple(reversed(a)) converts to tuple (5, 4, 3, 2, 1)

Returning a reversed() iterator does not modify the original array, list or tuple as well. When you call list() or tuple(), you can convert the reversed iterator explicitly to a list or tuple.

Oh, it seems years ago I have written on this topics already: Reverse List/Tuple/String in Python

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