Sorting and reversing lists in Python - Step-by-Step Execution

Concept Flow - Sorting and reversing lists

Start with list

↓

Choose operation

↓

Sort list

↓

Sorted list

↓

End

We start with a list, then choose to either sort it or reverse it, resulting in a new list order.

Execution Sample

Python

numbers = [4, 2, 9, 1]
numbers.sort()
print(numbers)
numbers.reverse()
print(numbers)

This code sorts the list in ascending order, prints it, then reverses the list and prints it again.

Execution Table

Step	Operation	List Before	Action	List After	Output
1	Start	[4, 2, 9, 1]	None	[4, 2, 9, 1]
2	sort()	[4, 2, 9, 1]	Sort ascending	[1, 2, 4, 9]
3	print()	[1, 2, 4, 9]	Print list	[1, 2, 4, 9]	[1, 2, 4, 9]
4	reverse()	[1, 2, 4, 9]	Reverse order	[9, 4, 2, 1]
5	print()	[9, 4, 2, 1]	Print list	[9, 4, 2, 1]	[9, 4, 2, 1]
6	End	[9, 4, 2, 1]	None	[9, 4, 2, 1]

💡 All operations done, list sorted then reversed and printed twice.

Variable Tracker

Variable	Start	After sort()	After reverse()	Final
numbers	[4, 2, 9, 1]	[1, 2, 4, 9]	[9, 4, 2, 1]	[9, 4, 2, 1]

Key Moments - 3 Insights

Why does the list change after calling sort()?

Because sort() changes the list in place, it rearranges the original list instead of creating a new one. See execution_table step 2 where the list changes from [4, 2, 9, 1] to [1, 2, 4, 9].

Does reverse() sort the list?

No, reverse() only flips the order of elements. It does not sort. Look at step 4 in execution_table: the list [1, 2, 4, 9] becomes [9, 4, 2, 1], which is reversed but not sorted.

What if I want a sorted list but keep the original unchanged?

Use the sorted() function which returns a new sorted list without changing the original. This example uses sort() which changes the list itself, as shown in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the list output?

A [4, 2, 9, 1]

B [1, 2, 4, 9]

C [9, 4, 2, 1]

D [1, 9, 2, 4]

💡 Hint

Check the Output column at step 3 in execution_table.

At which step does the list get reversed?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the Operation and Action columns in execution_table to find when reverse() is called.

If we replace numbers.sort() with sorted(numbers), what changes in variable_tracker?

A numbers list stays the same after sorting

B numbers list changes after sorting

C numbers list becomes empty

D numbers list reverses automatically

💡 Hint

Recall that sorted() returns a new list and does not modify the original list, unlike sort().

Concept Snapshot

Sorting and reversing lists in Python:
- Use list.sort() to sort the list in place (changes original).
- Use list.reverse() to reverse the list order in place.
- Use sorted(list) to get a new sorted list without changing original.
- Both sort() and reverse() do not return a new list.
- Print to see the current list state after operations.

Full Transcript

We start with a list of numbers. First, we call sort() which rearranges the list in ascending order. Then we print the sorted list. Next, we call reverse() which flips the order of the list elements. We print the reversed list. The variable 'numbers' changes after each operation because sort() and reverse() modify the list in place. If you want to keep the original list unchanged, use the sorted() function instead of sort().