Basic list comprehension syntax in Python - Step-by-Step Execution

Concept Flow - Basic list comprehension syntax

Start with iterable

↓

Pick each item

↓

Apply expression to item

↓

Add result to new list

↓

Repeat for all items

↓

Return new list

The flow shows how list comprehension takes each item from an iterable, applies an expression, and collects results into a new list.

Execution Sample

Python

numbers = [1, 2, 3, 4]
squares = [x * x for x in numbers]
print(squares)

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

Execution Table

Step	Current item (x)	Expression (x * x)	Action	New list state
1	1	1	Add 1 to list	[1]
2	2	4	Add 4 to list	[1, 4]
3	3	9	Add 9 to list	[1, 4, 9]
4	4	16	Add 16 to list	[1, 4, 9, 16]
End	-	-	All items processed	[1, 4, 9, 16]

💡 All items in 'numbers' list processed, comprehension ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
x	-	1	2	3	4	-
squares	[]	[1]	[1, 4]	[1, 4, 9]	[1, 4, 9, 16]	[1, 4, 9, 16]

Key Moments - 3 Insights

Why does the variable 'x' change each step inside the comprehension?

Because the comprehension picks each item from the original list one by one, as shown in execution_table steps 1 to 4.

Is the original list 'numbers' changed by the comprehension?

No, the original list stays the same; the comprehension creates a new list 'squares' as shown in variable_tracker.

What happens if the original list is empty?

The comprehension runs zero times and returns an empty list immediately, similar to the 'End' row in execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'x' at step 3?

A 9

B 3

C 4

D 16

💡 Hint

Check the 'Current item (x)' column at step 3 in the execution_table.

At which step does the new list first contain the value 4?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'New list state' column in execution_table rows.

If the original list 'numbers' was empty, what would the final 'squares' list be?

A [0]

B [1]

C []

D None

💡 Hint

Refer to the 'End' row in execution_table and the key_moments about empty input.

Concept Snapshot

Basic list comprehension syntax:
[new_list] = [expression for item in iterable]
- Iterates over each item
- Applies expression
- Collects results in new list
- Original list unchanged
- Fast and concise way to create lists

Full Transcript

This visual trace shows how basic list comprehension works in Python. We start with an original list called 'numbers'. The comprehension picks each item 'x' from this list one by one. For each 'x', it calculates 'x * x' and adds the result to a new list called 'squares'. This process repeats until all items are processed. The original list remains unchanged. The final output is a list of squares of the original numbers. Key points include how 'x' changes each step, the new list growing step by step, and what happens if the original list is empty.