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.

Practice

1. What does the following list comprehension do?
[x * 2 for x in range(3)]

easy

A. Creates a list of numbers from 1 to 6

B. Creates a list of numbers from 0 to 3

C. Creates a list of numbers doubled from 0 to 2

D. Creates a list of numbers squared from 0 to 2

Solution

1. Step 1: Understand the range and loop variable

   The range(3) generates numbers 0, 1, 2. The variable x takes these values one by one.

2. Step 2: Apply the expression to each value

   Each x is multiplied by 2, so the list becomes [0*2, 1*2, 2*2] = [0, 2, 4].

3. Final Answer:

   Creates a list of numbers doubled from 0 to 2 -> Option C

4. Quick Check:

   List comprehension doubles each number in range(3) [OK]

Hint: Remember: for x in range(3) gives 0,1,2 only [OK]

Common Mistakes:

• Thinking range(3) includes 3
• Confusing doubling with squaring
• Ignoring the for loop variable

2. Which of the following is the correct syntax for a list comprehension that creates a list of squares of numbers 0 to 4?

easy

A. [x**2 x in range(5)]

B. [for x in range(5) x**2]

C. [x^2 in range(5)]

D. [x**2 for x in range(5)]

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is [expression for variable in iterable]. Here, expression is x**2 and iterable is range(5).

2. Step 2: Check each option

   [x**2 for x in range(5)] matches the correct syntax. Options A, B, and C have syntax errors or wrong order.

3. Final Answer:

   [x**2 for x in range(5)] -> Option D

4. Quick Check:

   Correct syntax is [expr for var in iterable] [OK]

Hint: Remember: list comprehension = [expression for variable in iterable] [OK]

Common Mistakes:

• Placing 'for' after expression
• Using ^ instead of ** for power
• Missing 'in' keyword

3. What is the output of this code?

nums = [1, 2, 3, 4]
squares = [n*n for n in nums if n % 2 == 0]
print(squares)

medium

A. [4, 16]

B. [1, 4, 9, 16]

C. [2, 4]

D. [1, 9]

Solution

1. Step 1: Understand the list and condition

   The list nums is [1, 2, 3, 4]. The condition 'if n % 2 == 0' selects even numbers: 2 and 4.

2. Step 2: Calculate squares of selected numbers

   Squares of 2 and 4 are 4 and 16, so the list squares is [4, 16].

3. Final Answer:

   [4, 16] -> Option A

4. Quick Check:

   Filter even numbers, then square them [OK]

Hint: Filter first, then apply expression in list comprehension [OK]

Common Mistakes:

• Including odd numbers by mistake
• Confusing values with their squares
• Ignoring the if condition

4. Find the error in this list comprehension:

result = [x + 1 for in range(5)]

medium

A. Missing variable name after 'for'

B. Using + instead of *

C. range(5) should be range(1,5)

D. List comprehension cannot use range

Solution

1. Step 1: Check the for loop syntax inside comprehension

   The syntax requires a variable name after 'for', like 'for x in range(5)'. Here, 'x' is missing.

2. Step 2: Identify the error

   Because the variable is missing, Python will raise a syntax error.

3. Final Answer:

   Missing variable name after 'for' -> Option A

4. Quick Check:

   for loop needs variable name [OK]

Hint: Always include variable after 'for' in comprehension [OK]

Common Mistakes:

• Omitting the loop variable
• Misplacing 'in' keyword
• Thinking range needs to start at 1

5. You have a list of words: words = ['apple', 'bee', 'cat', 'dog', 'elephant']. Using list comprehension, how do you create a list of the lengths of words that have more than 3 letters?

hard

A. [w for w in words if len(w) > 3]

B. [len(w) for w in words if len(w) > 3]

C. [len(words) for w in words if len(w) > 3]

D. [len(w) > 3 for w in words]

Solution

1. Step 1: Understand the filtering condition

   We want only words with length greater than 3, so use 'if len(w) > 3' to filter.

2. Step 2: Create list of lengths

   For each filtered word, get its length with len(w). So the comprehension is [len(w) for w in words if len(w) > 3].

3. Final Answer:

   [len(w) for w in words if len(w) > 3] -> Option B

4. Quick Check:

   Filter words by length, then get lengths [OK]

Hint: Filter first, then apply len() in comprehension [OK]

Common Mistakes:

• Returning words instead of lengths
• Using len(words) instead of len(w)
• Returning boolean instead of lengths