Basic dictionary comprehension in Python - Step-by-Step Execution

Concept Flow - Basic dictionary comprehension

Start with iterable

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Pick each item

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Apply expression for key and value

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Add key:value to dictionary

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Repeat for all items

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Return new dictionary

Dictionary comprehension creates a new dictionary by looping over items and applying expressions for keys and values.

Execution Sample

Python

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

Creates a dictionary where keys are numbers and values are their squares.

Execution Table

Step	x (current item)	Expression for key	Expression for value	Dictionary state
1	1	1	1	{1: 1}
2	2	2	4	{1: 1, 2: 4}
3	3	3	9	{1: 1, 2: 4, 3: 9}
End	-	-	-	All items processed, dictionary complete

💡 All items in the list processed, comprehension ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
x	-	1	2	3	-
squares	{}	{1: 1}	{1: 1, 2: 4}	{1: 1, 2: 4, 3: 9}	{1: 1, 2: 4, 3: 9}

Key Moments - 2 Insights

Why does the dictionary update after each step instead of creating a list?

Because dictionary comprehension uses curly braces { } and key:value pairs, it builds a dictionary step-by-step as shown in the execution_table rows.

What happens if the key expression repeats the same value?

The dictionary will overwrite the previous value for that key, so only the last value for that key remains, as dictionaries cannot have duplicate keys.

Visual Quiz - 3 Questions

Look at the execution table, what is the dictionary state after step 2?

A {1: 1, 2: 4}

B {2: 4}

C {1: 1, 3: 9}

D {}

💡 Hint

Check the 'Dictionary state' column at step 2 in the execution_table.

At which step does the variable x have the value 3?

A Step 1

B Step 2

C Step 3

D End

💡 Hint

Look at the 'x (current item)' column in the execution_table.

If the list numbers was empty, what would the final dictionary be?

A {None: None}

B {}

C Error

D {0: 0}

💡 Hint

An empty iterable means no items to process, so the dictionary stays empty.

Concept Snapshot

Dictionary comprehension syntax:
{key_expression: value_expression for item in iterable}
Creates a new dictionary by looping over iterable.
Each item produces one key:value pair.
Keys must be unique; later duplicates overwrite earlier.
Use curly braces { } to create dictionary comprehension.

Full Transcript

This visual execution shows how basic dictionary comprehension works in Python. We start with a list of numbers. For each number x, we create a key:value pair where the key is x and the value is x squared. Step by step, the dictionary grows by adding these pairs. After processing all items, the final dictionary contains each number and its square. The variable tracker shows how x changes each step and how the dictionary builds up. Key moments clarify why the dictionary updates each step and what happens with duplicate keys. The quiz tests understanding of dictionary state at different steps and behavior with empty input.