List comprehension vs loop in Python - Visual Side-by-Side Comparison

Concept Flow - List comprehension vs loop

Start with empty list

↓

For each item in input

↓

Apply expression to item

↓

Add result to list

↓

Repeat for all items

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Return final list

Both list comprehension and loop build a new list by processing each item one by one and adding the result.

Execution Sample

Python

numbers = [1, 2, 3, 4]
squares_loop = []
for n in numbers:
    squares_loop.append(n*n)
squares_comp = [n*n for n in numbers]

This code creates a list of squares of numbers using a loop and a list comprehension.

Execution Table

Step	Variable	Value	Action	Output List
1	n	1	Append 1*1=1	[1]
2	n	2	Append 2*2=4	[1, 4]
3	n	3	Append 3*3=9	[1, 4, 9]
4	n	4	Append 4*4=16	[1, 4, 9, 16]
5	squares_comp	[1, 4, 9, 16]	List comprehension creates list	[1, 4, 9, 16]
6	-	-	End of processing	[1, 4, 9, 16]

💡 All numbers processed, both methods produce the same list of squares.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
n	-	1	2	3	4	-
squares_loop	[]	[1]	[1, 4]	[1, 4, 9]	[1, 4, 9, 16]	[1, 4, 9, 16]
squares_comp	-	-	-	-	-	[1, 4, 9, 16]

Key Moments - 3 Insights

Why does the list comprehension create the whole list at once?

Because list comprehension runs the expression for each item internally and collects results before assigning to the variable, unlike the loop which appends step-by-step (see execution_table row 5).

Is the output list the same for both methods?

Yes, both methods produce the same final list of squares as shown in execution_table rows 4 and 5.

Can list comprehension replace any loop that builds a list?

Yes, if the loop only appends transformed items, list comprehension is a shorter and clearer way to write it (see code in execution_sample).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of squares_loop after step 3?

A [1, 4, 9, 16]

B [1, 4]

C [1, 4, 9]

D []

💡 Hint

Check the Output List column at step 3 in execution_table.

At which step does the list comprehension create the full list?

A Step 5

B Step 4

C Step 6

D Step 3

💡 Hint

Look for the row where squares_comp is assigned in execution_table.

If we change the expression to n+1, how would the output list change?

A [1, 4, 9, 16]

B [2, 3, 4, 5]

C [1, 2, 3, 4]

D [0, 1, 2, 3]

💡 Hint

Think about applying n+1 to each number in variable_tracker for n.

Concept Snapshot

List comprehension is a concise way to create lists.
Syntax: [expression for item in iterable]
Equivalent to a loop that appends each expression result.
Runs faster and looks cleaner for simple transformations.
Use loops for complex logic or multiple statements.

Full Transcript

This visual shows how list comprehension and loops both create lists by processing each item. The loop appends each squared number step-by-step, updating the list after each iteration. The list comprehension runs internally and creates the full list at once. Both methods produce the same final list of squares. Beginners often wonder why list comprehension looks shorter and how it works internally. The execution table and variable tracker clarify these steps clearly.