List comprehension with condition in Python - Step-by-Step Execution

Concept Flow - List comprehension with condition

Start with a list

↓

For each item in list

↓

Check condition?

No→Skip item

Yes↓

Apply expression to item

↓

Add result to new list

↓

Repeat for all items

↓

Return new filtered list

This flow shows how list comprehension goes through each item, checks a condition, and includes only those items that meet the condition in the new list.

Execution Sample

Python

numbers = [1, 2, 3, 4, 5]
even_squares = [x*x for x in numbers if x % 2 == 0]
print(even_squares)

This code creates a new list of squares of even numbers from the original list.

Execution Table

Step	Current item (x)	Condition (x % 2 == 0)	Action	New list state
1	1	False	Skip 1	[]
2	2	True	Add 4 (2*2)	[4]
3	3	False	Skip 3	[4]
4	4	True	Add 16 (4*4)	[4, 16]
5	5	False	Skip 5	[4, 16]
End	-	-	Finished iteration	[4, 16]

💡 All items processed; only even numbers squared and added to new list.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
x	-	1	2	3	4	5	-
even_squares	[]	[]	[4]	[4]	[4, 16]	[4, 16]	[4, 16]

Key Moments - 3 Insights

Why are some items skipped and not added to the new list?

Items are skipped when the condition (x % 2 == 0) is False, as shown in steps 1, 3, and 5 in the execution table where the action is 'Skip'.

What does the expression 'x*x' do in the list comprehension?

It calculates the square of the current item x, which is added to the new list only if the condition is True, as seen in steps 2 and 4.

Is the original list changed by the list comprehension?

No, the original list 'numbers' remains unchanged; the comprehension creates a new list 'even_squares' as shown in the variable tracker.

Visual Quiz - 3 Questions

Look at the execution table, what is the new list after processing the item 4?

A [4]

B [16]

C [4, 16]

D []

💡 Hint

Check the 'New list state' column at step 4 in the execution table.

At which step does the condition become False for the first time?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Condition' column in the execution table; step 1 shows False first.

If we change the condition to 'x > 3', what will be the new list after step 5?

A [16, 25]

B [4, 16]

C [9, 16, 25]

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

💡 Hint

Check which numbers are greater than 3 and their squares.

Concept Snapshot

List comprehension with condition syntax:
  [expression for item in iterable if condition]
Creates a new list by including only items that satisfy the condition.
Expression is applied only to items passing the condition.
Original list remains unchanged.
Useful for filtering and transforming lists in one line.

Full Transcript

This visual trace shows how list comprehension with a condition works in Python. We start with a list of numbers. For each number, we check if it is even (condition x % 2 == 0). If it is, we square it and add it to a new list. If not, we skip it. The execution table tracks each step, showing which items are processed, which are skipped, and how the new list grows. The variable tracker shows the values of the current item and the new list after each step. Key moments clarify why some items are skipped, what the expression does, and that the original list is not changed. The quiz questions help reinforce understanding by asking about the list state at specific steps and the effect of changing the condition.