⚡️ append() vs extend()

These two methods look similar, but they do completely different things.

numbers = [1, 2, 3]

numbers.append([4, 5])

print(numbers)

Output:

[1, 2, 3, [4, 5]]

Now compare it with:

numbers = [1, 2, 3]

numbers.extend([4, 5])

print(numbers)

Output:

[1, 2, 3, 4, 5]

👉 append() adds one object.
👉 extend() adds every element.

This small difference causes countless beginner bugs.