Skip to content

#99: Iterate in Reversed Order Through Your Lists

For a feature I build I needed a simple way to reverse the direction the for-statement iterates through a list. As it turns out, Python has a built-in function called reversed() that does exactly that.

Iterate forward

When we iterate over a list, the iterator gives us the elements in the same order as we inserted them into the list:

1
2
3
numbers = [1, 2, 3, 4, 5]
for x in numbers:
    print(x)

1
2
3
4
5

We could change this behaviour by creating our own iterator. While this may be necessary with a complex requirement, it is not needed to change the direction of the iterator.

Iterate backwards

The built-in function reversed() changes the direction of the iterator without changing the underlying data:

1
2
3
4
5
6
numbers = [1, 2, 3, 4, 5]

for x in reversed(numbers):
    print(x)

print(numbers)


5
4
3
2
1

[1, 2, 3, 4, 5]
 

Conclusion

If you need to run in the opposite direction through an iteratable (like a list), reversed() is the function that does exactly what you need.