Basic dictionary comprehension in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is dictionary comprehension in Python?

Dictionary comprehension is a concise way to create dictionaries by writing an expression inside curly braces, using a key-value pair format, often with a loop.

beginner

Write a simple dictionary comprehension to create a dictionary with numbers 1 to 3 as keys and their squares as values.

Example: {x: x**2 for x in range(1, 4)} creates {1: 1, 2: 4, 3: 9}.

beginner

What are the parts of a dictionary comprehension?

It has three parts: the key expression, the value expression, and the for loop that generates items.

intermediate

Can dictionary comprehension include conditions? Give a simple example.

Yes, you can add an if condition to filter items. Example: {x: x**2 for x in range(5) if x % 2 == 0} creates {0: 0, 2: 4, 4: 16}.

beginner

Why use dictionary comprehension instead of a for loop?

Dictionary comprehension is shorter, easier to read, and often faster than building a dictionary with a for loop.

What does this dictionary comprehension do? {x: x+1 for x in range(3)}

A Creates {0: 1, 1: 2, 2: 3}

B Creates {1: 0, 2: 1, 3: 2}

C Creates {0: 0, 1: 1, 2: 2}

D Creates {1: 1, 2: 2, 3: 3}

For each x in 0,1,2, the key is x and the value is x+1, so the dictionary is {0:1, 1:2, 2:3}.

Which syntax is correct for dictionary comprehension?

A [key: value for item in iterable]

B (key: value for item in iterable)

C {key: value for item in iterable}

D {key, value for item in iterable}

Dictionary comprehension uses curly braces with key:value pairs and a for loop.

How do you add a condition to a dictionary comprehension?

A You cannot add conditions in dictionary comprehension

B Add if condition before the for loop

C Add if condition inside the key expression

D Add if condition after the for loop

You add an if condition after the for loop to filter items.

What will {x: x*2 for x in range(3) if x != 1} produce?

A {1: 2, 2: 4}

B {0: 0, 2: 4}

C {0: 0, 1: 2, 2: 4}

D {2: 4}

It skips x=1 because of the condition, so keys 0 and 2 with values 0 and 4.

Why might dictionary comprehension be preferred over a for loop?

A It is more concise and readable

B It uses more lines of code

C It is slower

D It cannot include conditions

Dictionary comprehension is shorter and easier to read than a for loop.

Explain how to create a dictionary using dictionary comprehension with an example.

Think about how to turn a list of numbers into a dictionary of squares.

Describe how to add a condition to dictionary comprehension and why it might be useful.

Consider only including even numbers as keys.