Understanding Mutability

Always trace the list and the return value separately. A method can change the list in place while its return value is None.

Programming

The List and the Return Value

When you call a list method, two questions matter: what happens to the list itself, and what value does the method call produce? Python's append, extend, and sort methods modify the existing list in place. Each one returns None. This means the list can change even though the method call does not produce a new list value.

Key point

Always trace the list and the return value separately. A method can change the list in place while its return value is None.

Tracing an In-Place Change

What do you think happens?

Suppose values starts as [3, 1]. After values.append(5), what are the list and the return value?

The list is [3, 1], and the return value is [3, 1, 5].
The list is [3, 1, 5], and the return value is None.
The list is None, and the return value is [3, 1, 5].

Reveal answer

Answer:

The list is [3, 1, 5], and the return value is None.

append modifies the existing list in place and returns None. The changed list and the method's return value are separate results.

What changes when append is called, and what value does the method call return?

The diagram separates two outcomes that are easy to mix up. The list changes from [3, 1] to [3, 1, 5]. The call to append(5), however, evaluates to None. The changed list is the object you continue using; None is the method's return value.

Three Ways to Change Contents

Method	Effect on the list	Return value
append	Adds one item to the list	None
extend	Adds multiple elements to the list	None
sort	Reorders the existing list's contents	None

append, extend, and sort all modify the list in place, but their operations are different.

How does each method change a list's contents: adding one item, adding multiple elements, or reordering existing contents?

append and extend both add elements, but they treat their arguments differently. append adds one item to the list. extend adds multiple elements to the list. This distinction is especially important with nested data or when combining lists. sort does not add elements; it changes the order of the existing list contents. All three methods modify the original list and return None.

Method Calls in Practice

Code walkthrough

numbers = [3, 1]
append_result = numbers.append(5)
print(numbers)
print(append_result)

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

scores = [4, 2, 3]
sort_result = scores.sort()
print(scores)
print(sort_result)

1
numbers starts as [3, 1]. append(5) changes it to [3, 1, 5], while append_result receives None.
2
parts starts as [1, 2]. extend([3, 4]) changes it to [1, 2, 3, 4], while extend_result receives None.
3
scores starts as [4, 2, 3]. sort() changes the order to [2, 3, 4], while sort_result receives None.

Output

[3, 1, 5]
None
[1, 2, 3, 4]
None
[2, 3, 4]
None

Key point

The assignment statements capture None, not a replacement list. The lists themselves have already been modified before the return values are assigned.

The Assignment Trap

What happens when code assigns the result of a method that changes a list in place and returns None?

Common mistake
Assigning the result of an in-place method back to the list variable
What it looks like
```
items = items.sort()
```
Why it fails
sort changes the list in place and returns None, so the assignment replaces the value of items with None.
Do this instead
Call items.sort() without assigning its result.
Common mistake
Treating append and extend as interchangeable
What it looks like
Using append when the intention is to add multiple elements
Why it fails
append and extend both add elements, but they treat the argument differently: append adds one item, while extend adds multiple elements.
Do this instead
Choose append for one item and extend for multiple elements.
Common mistake
Checking only the return value and ignoring the changed list
What it looks like
Assuming that a None return value means the list did not change
Why it fails
These methods modify the list in place even though they return None.
Do this instead
Trace the list after the call and separately record the method's return value.

Predict Before Running

MEDIUM

For each call, predict both the final list and the method's return value.

1. values = [5, 2]
   result = values.append(8)

2. values = [5, 2]
   result = values.extend([8, 9])

3. values = [5, 2]
   result = values.sort()

Then explain why assigning values = values.sort() causes a problem.

Hints

All three methods modify the existing list in place.
All three methods return None.
append adds one item, extend adds multiple elements, and sort reorders the existing contents.

Your attempt (not saved)

Worked example

Tracing extend

Determine the list and return value after values = [5, 2] followed by result = values.extend([8, 9]).

1
Start
The list is [5, 2].
2
Apply the method
extend adds multiple elements to the existing list, so the list becomes [5, 2, 8, 9].
3
Record the return value
extend modifies the list in place and returns None, so result is None.

Result

values is [5, 2, 8, 9], and result is None.

Working Rule

1
append, extend, and sort modify a list in place.
2
append adds one item, extend adds multiple elements, and sort reorders the existing list contents.
3
Each of these methods returns None.
4
Do not assign the result of one of these methods back to the list variable, because that assignment overwrites the variable with None.
5
When debugging, trace the list's state and the method's return value as two separate outcomes.

Key Takeaways

append, extend, and sort all change the existing list rather than producing a new list value.
append adds one item, extend adds multiple elements, and sort changes the order of existing contents.
The return value of each method is None.
Calling a method without assignment preserves the changed list; assigning its result back replaces the variable with None.

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