? Python Dictionary Comprehension

? Quick Overview

Dictionary comprehension in Python is a concise, one-line way to create or transform dictionaries. It is similar to list comprehension, but instead of producing a list, it produces a dictionary with key–value pairs.

General syntax: {key_expression: value_expression for item in iterable}

? Key Concepts

Key–Value Creation: Each iteration generates one key: value pair.
Based on Iterable: You can loop over range(), lists, tuples, or other iterables.
Optional Condition: Add an if at the end to filter which items are included.
Transform Existing Dictionaries: Use dict.items() to update or remap keys and values.
Swap Keys and Values: Create a reversed mapping using {v: k for k, v in dict.items()}.

? Syntax & Theory

A basic dictionary comprehension uses a loop variable to generate keys and values:

{key_expr: value_expr for variable in iterable}

You can also add a condition to include only selected items:

{key_expr: value_expr for variable in iterable if condition}

When transforming an existing dictionary, you typically iterate using for key, value in dict_obj.items(), then build a new dictionary with modified values or swapped key–value positions.

? Code Examples

⚡ Basic Dictionary Comprehension

Create a dictionary of numbers and their squares using a single line of code:

? View Code Example

squares = {x: x**2 for x in range(5)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Here, x is used as the key and x**2 as the value. The dictionary is created for all numbers in the range 0 to 4.

? Dictionary Comprehension with Condition

Use an if condition to include only certain items in the dictionary. This is useful for filtering data while creating the dictionary.

? View Code Example

even_squares = {x: x**2 for x in range(10) if x % 2 == 0}
print(even_squares)  # {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

In this example, only even numbers are included as keys, and their squares as values. The condition if x % 2 == 0 filters out odd numbers.

? Transform Existing Dictionary

Dictionary comprehension can also be used to transform an existing dictionary. You can modify values, filter items, or even swap keys and values.

? View Code Example

prices = {"apple": 2, "banana": 1, "cherry": 3}
discounted = {item: price*0.9 for item, price in prices.items()}
print(discounted)  # {'apple': 1.8, 'banana': 0.9, 'cherry': 2.7}

Here, we created a new dictionary discounted by multiplying each price by 0.9. The items() method provides both key and value for transformation.

? Live Output & Explanation

? Sample Outputs

Basic squares:

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Even squares:

{0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

Discounted prices:

{'apple': 1.8, 'banana': 0.9, 'cherry': 2.7}

In each case, the comprehension builds a complete dictionary in one readable line: the loop decides how many entries you create, the key expression decides the keys, and the value expression decides the values.

✅ Tips & Best Practices

Use dictionary comprehension for concise and readable dictionary creation.
Combine comprehension with an if condition for efficient filtering.
Use items() to iterate over existing dictionaries when transforming values or keys.
Dictionary comprehension can also be used to swap keys and values: {v: k for k, v in some_dict.items()}.
If the logic gets too complex, consider using regular loops for better readability.

? Try It Yourself

Create a dictionary of numbers and their cubes using comprehension: {x: x**3 for x in range(1, 11)}.
Create a dictionary of even numbers and their squares using a condition.
Transform an existing dictionary of item prices by applying a 10% discount.
Swap keys and values of an existing dictionary using comprehension.
Try filtering a dictionary to keep only items above a certain price.