Reverse a List in Python

1. The Challenge: Reversing Order

Create a function reverse(lst) that takes a list and returns the elements in reverse order.

2. Why It Matters

Reversing list sequences is crucial for undo operations, stack data processing, and chronological sorting.

3. The Python Solution

Use Python list slicing with step -1 (lst[::-1]):

def reverse(lst):
    return lst[::-1]

# Testing our function
print(reverse([1, 2, 3, 4])) # Output: [4, 3, 2, 1]
print(reverse([9, 9, 2, 3, 4])) # Output: [4, 3, 2, 9, 9]
print(reverse([]))              # Output: []

4. Understanding Slice Syntax [start:stop:step]

• [::-1]: Omitting start and stop defaults to the whole list, while step = -1 steps backward from end to start.
• Non-mutating: Slicing returns a new list without modifying the original source list!

5. Common Mistakes

• Confusing lst.reverse() with lst[::-1]: Calling lst.reverse() reverses the list in-place and returns None! If your function returns lst.reverse(), it will return None to the caller. Always use lst[::-1] when returning a reversed list!

6. Next Steps

Use [::-1] for non-mutating reversed lists. Next, let's filter out strings from a mixed list!