Two Ways to Reverse a List in Python

An easy way to reverse a list in Python is to use the reverse() list function:

l = [1, 2, 3]
l.reverse()
print(l) # [3, 2, 1]

This is nice and declarative, but reverse() doesn't return the list. If you want to have the list implicitly returned so that you can use it in, say, a list comprehension, then using the [::-1] syntax is better.

class Solution:
  def flipAndInvertImage(self, A: List[List[int]]) -> List[List[int]]:
    return [[int(not bool(n)) for n in r[::-1]] for r in A]

Here's a solution to a LeetCode problem that uses the [::-1] reversing method to immediately return the reverse list for use.

This takes A, which is a list of lists of integers, and loops through each row r. It then takes each of these rows and gets the reverse of that row with r[::-1], which is immediately returned for use. It then takes each number n from this reversed row and gets the boolean inverse of this value using int(not bool(n)).

rpivo/index.md

Two Ways to Reverse a List in Python