When beginners add items to a Python list, they often mix up append() and +. Both seem to “add something,” but they do different things, especially when the thing you add is another list.

This note focuses on one key difference: append() adds the whole object as one element; + combines two lists into a new list.

Common Mistake

Suppose we have this list:

items = [1, 2]
items.append([3, 4])
print(items)

Many students expect:

[1, 2, 3, 4]

But the actual output is:

[1, 2, [3, 4]]

The list [3, 4] was not unpacked. It was added as one whole element inside items.

Core Idea

append() means: add one element to the end of a list.

The key phrase is “one element.” Whether you pass in a number, a string, or another list, append() treats it as one element.

items = [1, 2]
items.append(3)
print(items)

Output:

[1, 2, 3]

This feels natural because 3 is a single element.

But if you write:

items = [1, 2]
items.append([3, 4])
print(items)

the output is:

[1, 2, [3, 4]]

because the list [3, 4] itself becomes one element.

Use + to Combine Two Lists

If you want [1, 2, 3, 4], use +:

items = [1, 2]
new_items = items + [3, 4]
print(new_items)

Output:

[1, 2, 3, 4]

Here, + connects two lists and creates a new list. It does not put [3, 4] inside as one element; it places 3 and 4 after the original items.

append() Changes the Original List

There is another important difference: append() changes the original list in place.

items = [1, 2]
items.append(3)
print(items)

Output:

[1, 2, 3]

The original items has changed.

But + usually creates a new list:

items = [1, 2]
new_items = items + [3]

print(items)
print(new_items)

Output:

[1, 2]
[1, 2, 3]

This difference will matter later when you learn about list mutability.

Comparison

extend() Can Add Items One by One

If you want to change the original list and add each item from another list, use extend():

items = [1, 2]
items.extend([3, 4])
print(items)

Output:

[1, 2, 3, 4]

A simple rule:

• append() adds one thing.
• extend() adds items from another list one by one.
• + combines two lists and gives a new list.

Practice

1. If items = [1, 2], what is items after items.append([3])?
2. If items = [1, 2], what is the result of items + [3]?
3. Challenge: without using +, add [3, 4] item by item to [1, 2].

One-Sentence Takeaway

append() adds the whole object as one element; + combines two lists into a new list.

Next time you see an extra pair of brackets inside a list, check whether you used append([ ... ]).