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.

Practice

1. What does the following dictionary comprehension do?
{x: x*x for x in range(3)}

easy

A. Creates a list of squares from 0 to 2

B. Creates a set of squares from 0 to 2

C. Creates a dictionary with numbers 0 to 2 as keys and their squares as values

D. Creates a dictionary with keys as squares and values as numbers

Solution

1. Step 1: Understand the loop and key-value pairs

   The comprehension loops over numbers 0, 1, 2 and uses each number as the key.

2. Step 2: Calculate the value for each key

   Each value is the square of the key (x*x), so values are 0, 1, 4 respectively.

3. Final Answer:

   Creates a dictionary with numbers 0 to 2 as keys and their squares as values -> Option C

4. Quick Check:

   Dictionary comprehension = key: x, value: x*x [OK]

Hint: Look for key:value pairs inside braces with a for loop [OK]

Common Mistakes:

• Confusing dictionary with list or set comprehension
• Mixing keys and values in comprehension
• Thinking it creates a list instead of a dictionary

2. Which of the following is the correct syntax for a dictionary comprehension that creates keys from 1 to 3 and values as their double?

easy

A. [x: 2*x for x in range(1, 4)]

B. {x: 2*x for x in range(1, 4)}

C. {x => 2*x for x in range(1, 4)}

D. (x: 2*x for x in range(1, 4))

Solution

1. Step 1: Identify correct dictionary comprehension syntax

   Dictionary comprehensions use curly braces with key:value pairs and a for loop inside.

2. Step 2: Check each option

   {x: 2*x for x in range(1, 4)} uses curly braces and colon correctly. Options B, C, D use wrong brackets or arrow syntax.

3. Final Answer:

   {x: 2*x for x in range(1, 4)} -> Option B

4. Quick Check:

   Curly braces + key:value + for loop = correct syntax [OK]

Hint: Remember dictionary comprehension uses curly braces and colon [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using arrow (=>) instead of colon for key-value
• Using parentheses which create generator expressions

3. What is the output of this code?

nums = [1, 2, 3]
squares = {n: n**2 for n in nums if n % 2 != 0}
print(squares)

medium

A. {1: 1, 3: 9}

B. {2: 4}

C. {1: 1, 2: 4, 3: 9}

D. {}

Solution

1. Step 1: Understand the filtering condition

   The comprehension includes only numbers where n % 2 != 0, meaning odd numbers only (1 and 3).

2. Step 2: Calculate squares for filtered numbers

   Squares are 1*1=1 and 3*3=9, so dictionary is {1:1, 3:9}.

3. Final Answer:

   {1: 1, 3: 9} -> Option A

4. Quick Check:

   Filter odd numbers and square them = {1:1, 3:9} [OK]

Hint: Check the if condition filters keys before squaring [OK]

Common Mistakes:

• Including even numbers by mistake
• Confusing list comprehension output with dictionary
• Ignoring the if condition in comprehension

4. Find the error in this dictionary comprehension:

result = {x, x*2 for x in range(3)}

medium

A. Missing colon between key and value

B. Using parentheses instead of curly braces

C. Range function is incorrect

D. No error, code is correct

Solution

1. Step 1: Check syntax for key-value pairs

   Dictionary comprehension requires a colon ':' between key and value, but here a comma ',' is used.

2. Step 2: Confirm other parts are correct

   Curly braces and range(3) are correct, so the error is the missing colon.

3. Final Answer:

   Missing colon between key and value -> Option A

4. Quick Check:

   Key:value pairs need colon, not comma [OK]

Hint: Use colon ':' to separate key and value in dict comprehension [OK]

Common Mistakes:

• Using comma instead of colon between key and value
• Confusing set comprehension with dictionary comprehension
• Using wrong brackets for comprehension

5. Given a list of words, create a dictionary comprehension that maps each word to its length but only include words longer than 3 letters.
Which code correctly does this?

words = ['cat', 'house', 'dog', 'elephant']

hard

A. {word: len(word) for word in words where len(word) > 3}

B. {word: len(word) for word in words if word > 3}

C. {word: len(word) if len(word) > 3 for word in words}

D. {word: len(word) for word in words if len(word) > 3}

Solution

1. Step 1: Understand filtering condition syntax

   The if condition must come after the for loop to filter words by length correctly.

2. Step 2: Check each option's syntax

   {word: len(word) for word in words if len(word) > 3} correctly places 'if len(word) > 3' after the for loop. Others have syntax errors or wrong condition placement.

3. Final Answer:

   {word: len(word) for word in words if len(word) > 3} -> Option D

4. Quick Check:

   Filter after for loop with if condition [OK]

Hint: Put if condition after for loop in dictionary comprehension [OK]

Common Mistakes:

• Placing if condition before for loop
• Using invalid syntax like 'where' instead of 'if'
• Comparing word string directly to number