List mutability in Python - Step-by-Step Execution

Concept Flow - List mutability

Create list

→Assign to variable

↓

Modify list element

↓

List content changes

↓

Variable still points to same list

↓

Use modified list

This flow shows how a list is created, modified in place, and the variable still points to the same list with updated content.

Execution Sample

Python

my_list = [1, 2, 3]
my_list[1] = 99
print(my_list)

This code creates a list, changes the second element, and prints the updated list.

Execution Table

Step	Action	List Content	Variable Reference	Output
1	Create list [1, 2, 3]	[1, 2, 3]	my_list -> [1, 2, 3]
2	Modify element at index 1 to 99	[1, 99, 3]	my_list -> [1, 99, 3]
3	Print list	[1, 99, 3]	my_list -> [1, 99, 3]	[1, 99, 3]
4	End of program	[1, 99, 3]	my_list -> [1, 99, 3]	Program ends

💡 Program ends after printing the modified list.

Variable Tracker

Variable	Start	After Step 2	Final
my_list	[1, 2, 3]	[1, 99, 3]	[1, 99, 3]

Key Moments - 3 Insights

Why does changing my_list[1] affect the original list?

Because lists are mutable, modifying an element changes the list in place. The variable still points to the same list object, so the change is visible.

Does modifying the list create a new list?

No, the list is changed in place. The variable still references the same list object, just with updated content (see Step 2 in execution_table).

What if we assign my_list = [4, 5, 6] instead of modifying an element?

Then my_list points to a new list object. The original list remains unchanged. This is different from mutating the list content.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the list content after modification?

A [1, 2, 3]

B [1, 99, 3]

C [99, 2, 3]

D [1, 3, 99]

💡 Hint

Check the 'List Content' column at Step 2 in the execution_table.

At which step does the variable my_list point to a different list object?

A Step 3

B Step 2

C None of the steps

D Step 1

💡 Hint

Look at the 'Variable Reference' column; my_list points to the same list throughout.

If we replaced my_list[1] = 99 with my_list = [4, 5, 6], what would happen to the original list?

A It would remain [1, 2, 3]

B It would be deleted

C It would be modified to [4, 5, 6]

D It would become empty

💡 Hint

Assigning a new list to my_list changes the variable's reference, not the original list object.

Concept Snapshot

List mutability means lists can be changed after creation.
Modify elements by index: my_list[1] = new_value.
The variable still points to the same list object.
Changes affect the original list in place.
Assigning a new list changes the variable's reference.

Full Transcript

This example shows how a list in Python can be changed after it is created. First, a list [1, 2, 3] is made and stored in the variable my_list. Then, the second element (index 1) is changed to 99. Because lists are mutable, this change updates the original list. The variable my_list still points to the same list, but now the list content is [1, 99, 3]. When we print my_list, it shows the updated list. This demonstrates that modifying a list element changes the list in place without creating a new list object.