Use Python Slice Notation To Reverse A List Easily

The aim of this explainer💡 analyzes the reversion of a Python list using slicing [::-1] notation combining notes from StackOverflow and LeetCode.

1. instructions

• slicing syntax, in general, utilizing half-open range

[ <first element to include> : <first element to exclude> : <step> ]

• positive int in <steps> is the size of increment from the left to the right of a list
• negative int in <steps> is the size of increment from the right the left of a list (reversed iteration)
• only the first colon in [:]is really required, all of the other fields can be empty
• therefore, to make a copy of the list (aka “colon copy”)

>>> l = [1,2,3,4]
>>> l[:]
[1, 2, 3, 4]

• to iterate in a reverse order

>>> l[::-1]
[4, 3, 2, 1]

• When you use a negative index as either <first element to include> or <first element to exclude> it is indexing from the back of the list, so -1 is the last element, -2 is the second to last element, etc.

>>> l[-1:]
[4]

• to iterate over 2 items in a reversed order:

>>> l[::-2]
[4, 2]

• therefore to reverse an integer, including the validation method against overflow is easy

class Solution:
    def reverse(self, x: int) -> int:  
        if x > 0:  # handle positive numbers  
            a =  int(str(x)[::-1])  
        if x <=0:  # handle negative numbers  
            a = -1 * int(str(x*-1)[::-1])  
        # handle 32 bit overflow  
        mina = -2**31  
        maxa = 2**31 - 1  
        if a not in range(mina, maxa):  
            return 0  
        else:  
            return a

2. sources

• https://leetcode.com/problems/reverse-integer/solution/258298
• https://stackoverflow.com/a/5877008/11082684