How do I reverse a list or loop over it backwards?

Question

How do I iterate over a list in reverse in Python?

_{See also: How can I get a reversed copy of a list (avoid a separate statement when chaining a method after .reverse)?}

user_6501141 · Answer

Summary of Reverse Methods

There are three different built-in ways to reverse a list. Which method is best depends on whether you need to:

Reverse an existing list in-place (altering the original list variable)
- Best solution is object.reverse() method
Create an iterator of the reversed list (because you are going to feed it to a for-loop, a generator, etc.)
- Best solution is reversed(object) which creates the iterator
Create a copy of the list, just in the reverse order (to preserve the original list)
- Best solution is using slices with a -1 step size: object[::-1]

From a speed perspective, it is best to use the above built-in functions to reverse a list. For reversing, they are 2 to 8 times faster on short lists (10 items), and up to ~300+ times faster on long lists compared to a manually-created loop or generator. This makes sense - they are written in a native language (i.e. C), have experts creating them, scrutiny, and optimization. They are also less prone to defects and more likely to handle edge and corner cases.

Test Script

Put all the code snippets in this answer together to make a script that will run the different ways of reversing a list that are described below. It will time each method while running it 100,000 times. The results are shown in the last section for lists of length 2, 10, and 1000 items.

from timeit import timeit
from copy import copy

def time_str_ms(t):
    return '{0:8.2f} ms'.format(t * 1000)

Method 1: Reverse in place with obj.reverse()

If the goal is just to reverse the order of the items in an existing list, without looping over them or getting a copy to work with, use the .reverse() function. Run this directly on a list object, and the order of all items will be reversed:

Note that the following will reverse the original variable that is given, even though it also returns the reversed list back. i.e. you can create a copy by using this function output. Typically, you wouldn't make a function for this, but the timing script requires it.

We test the performance of this two ways - first just reversing a list in-place (changes the original list), and then copying the list and reversing it afterward to see if that is the fastest way to create a reversed copy compared to the other methods.

def rev_in_place(mylist):
    mylist.reverse()
    return mylist

def rev_copy_reverse(mylist):
    a = copy(mylist)
    a.reverse()
    return a

Method 2: Reverse a list using slices `obj[::-1]`

The built-in index slicing method allows you to make a copy of part of any indexed object.

It does not affect the original object
It builds a full list, not an iterator

The generic syntax is:

user_1668274 · Answer

For reversing the same list use:

array.reverse()

To assign reversed list into some other list use:

newArray = array[::-1]

user_451710 · Answer

With reversed and list:

>>> list1 = [1,2,3]
>>> reversed_list = list(reversed(list1))
>>> reversed_list
>>> [3, 2, 1]

user_419863 · Answer

Another solution would be to use numpy.flip for this

import numpy as np
array = [0, 10, 20, 40]
list(np.flip(array))
[40, 20, 10, 0]

user_7081302 · Answer

Using some logic

Using some old school logic to practice for interviews.

Swapping numbers front to back. Using two pointers index[0] and index[last]

def reverse(array):
    n = array
    first = 0
    last = len(array) - 1
    while first < last:
      holder = n[first]
      n[first] = n[last]
      n[last] = holder
      first += 1
      last -= 1
    return n

input -> [-1 ,1, 2, 3, 4, 5, 6]
output -> [6, 5, 4, 3, 2, 1, -1]

user_2117922 · Answer

ORGANIZING VALUES:

In Python, lists' order too can be manipulated with sort, organizing your variables in numerical/alphabetical order: Temporarily:

print(sorted(my_list))

Permanent:

my_list.sort(), print(my_list)

You can sort with the flag "reverse=True":

print(sorted(my_list, reverse=True))

or

my_list.sort(reverse=True), print(my_list)

WITHOUT ORGANIZING

Maybe you do not want to sort values, but only reverse the values. Then we can do it like this:

print(list(reversed(my_list)))

**Numbers have priority over alphabet in listing order. The Python values' organization is awesome.

Edit 1: a mistaken moderator claimed that my answer was a copy and deleted my old post.

user_5561203 · Answer

With minimum amount of built-in functions, assuming it's interview settings

array = [1, 2, 3, 4, 5, 6,7, 8]
inverse = [] #create container for inverse array
length = len(array)  #to iterate later, returns 8 
counter = length - 1  #because the 8th element is on position 7 (as python starts from 0)

for i in range(length): 
   inverse.append(array[counter])
   counter -= 1
print(inverse)

user_6657567 · Answer

There are 3 methods to get the reversed list:

Slicing Method 1: reversed_array = array[-1::-1]
Slicing Method 2: reversed_array2 = array[::-1]
Using the builtin function: reversed_array = array.reverse()

The third function actually reversed the list object in place. That means no copy of pristine data is maintained. This is a good approach if you don't want to maintain the old version. But doesn't seem to be a solution if you do want the pristine and reversed version.

Admin · Answer

Reversing in-place by switching references of opposite indices:

>>> l = [1,2,3,4,5,6,7]    
>>> for i in range(len(l)//2):
...     l[i], l[-1-i] = l[-1-i], l[i]
...
>>> l
[7, 6, 5, 4, 3, 2, 1]

user_11037648 · Answer

use

print(list(reversed(list_name)))

user_8614314 · Answer

Reverse of a user input values in one line code:

for i in input()[::-1]: print(i,end='')

user_11741338 · Answer

Here's a way to lazily evaluate the reverse using a generator:

def reverse(seq):
    for x in range(len(seq), -1, -1): #Iterate through a sequence starting from -1 and increasing by -1.
        yield seq[x] #Yield a value to the generator

Now iterate through like this:

for x in reverse([1, 2, 3]):
    print(x)

If you need a list:

l = list(reverse([1, 2, 3]))

user_6553328 · Answer

This class uses Python magic methods and iterators for reversing, and reverses a list:

class Reverse(object):
    """ Builds a reverse method using magic methods """

    def __init__(self, data):
        self.data = data
        self.index = len(data)


    def __iter__(self):
        return self

    def __next__(self):
        if self.index == 0:
            raise StopIteration

        self.index = self.index - 1
        return self.data[self.index]


REV_INSTANCE = Reverse([0, 10, 20, 40])

iter(REV_INSTANCE)

rev_list = []
for i in REV_INSTANCE:
    rev_list.append(i)

print(rev_list)

Output

[40, 20, 10, 0]

user_15140984 · Answer

A clean simple class object to solve your issue.

class lister():
    def reverse(self):
        return  [self[len(self)-e]for e,x in enumerate(self,start=1)]
print(lister.reverse([0, 10, 20, 40]))

user_9932834 · Answer

alternative way using list comprehension and abs

array = [0, 10, 20, 40]

reversed_array = [array[abs(indx)] for indx in range(abs(len(array)-1),1)]
            
reversed_array
[40, 20, 10, 0]

user_17017474 · Answer

The cute Solution


class List_reverse():
    def reverse_list(self):
        list = [1, 2, 3, 4, 5, 6]
        list1 = []
        for i in range(1, len(list)+1):
            list1.append(list[-i])
        print(list1)


if __name__ == "__main__":
    obj = List_reverse()
    obj.reverse_list()
    enter code here

user_11833198 · Answer

I had this question come up during a python code test for a job interview. The below is my answer. Note it works for any value any length

def get_reverse(list_check, count_num):
    final_list =[]
    for index in range(list_length):
        value = list_check[count_num]
        final_list.append(value)
        count_num = count_num -1

    return final_list

new_list = ['A', 'GOAT', 'C', 'D', 'Mac']

list_length = len(new_list)
x = list_length -1

print(get_reverse(new_list, x))

How do I reverse a list or loop over it backwards?

37 Answers

Summary of Reverse Methods

Test Script

Method 1: Reverse in place with obj.reverse()

Method 2: Reverse a list using slices `obj[::-1]`

Method 3: Reverse a list with the `reversed(obj)` iterator function

Method 4: Reverse list with Custom/Manual indexing

Timing each method

Timing Results

Using some logic

Output

How do I reverse a list or loop over it backwards?

37 Answers

Summary of Reverse Methods

Test Script

Method 1: Reverse in place with obj.reverse()

Method 2: Reverse a list using slices obj[::-1]

Method 3: Reverse a list with the reversed(obj) iterator function

Method 4: Reverse list with Custom/Manual indexing

Timing each method

Timing Results

Using some logic

Output

Method 2: Reverse a list using slices `obj[::-1]`

Method 3: Reverse a list with the `reversed(obj)` iterator function