Dictionary comprehension with condition 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 loop and optionally a condition.

beginner

How do you add a condition in a dictionary comprehension?

You add an if statement after the for loop inside the comprehension to filter which items are included.

beginner

Example: What does this code do?

{x: x*x for x in range(5) if x % 2 == 0}

It creates a dictionary with keys as even numbers from 0 to 4, and values as their squares. Result: {0: 0, 2: 4, 4: 16}

intermediate

Can dictionary comprehension with condition replace loops?

Yes, it can replace loops for creating dictionaries when you want to filter items, making code shorter and easier to read.

beginner

What happens if the condition in dictionary comprehension is always false?

The resulting dictionary will be empty because no items meet the condition to be included.

Which part of this dictionary comprehension is the condition?

{k: v for k, v in data.items() if v > 10}

A if v > 10

B for k, v in data.items()

C k: v

D data.items()

The condition filters items where the value is greater than 10.

What will this code output?

{i: i+1 for i in range(3) if i == 5}

A {}

B {5: 6}

C {0: 1, 1: 2, 2: 3}

D Error

No number in range(3) equals 5, so the dictionary is empty.

How do you write a dictionary comprehension that includes only keys that start with 'a' from a dictionary d?

A {k: v for v in d.values() if v.startswith('a')}

B {k: v for k, v in d.items() if v.startswith('a')}

C {k: v for k, v in d.items() if k.startswith('a')}

D {k: v for k in d if k == 'a'}

The condition filters keys starting with 'a'.

Which of these is NOT a valid use of condition in dictionary comprehension?

A Filtering items to include

B Changing keys based on condition

C Adding multiple conditions with 'and' or 'or'

D Using condition outside the comprehension braces

Conditions must be inside the comprehension braces, after the for loop.

What does this dictionary comprehension do?

{x: x**2 for x in range(6) if x % 2 != 0}

A Creates squares of even numbers from 0 to 5

B Creates squares of odd numbers from 0 to 5

C Creates squares of all numbers from 0 to 5

D Creates squares of numbers divisible by 2

The condition x % 2 != 0 selects odd numbers.

Explain how to create a dictionary comprehension with a condition to include only items where the value is greater than 10.

Think about filtering items while creating the dictionary.

Describe what happens if the condition in a dictionary comprehension never matches any item.

What does an empty filter produce?

Practice

1.

What does the following dictionary comprehension do?
{k: v for k, v in {'a': 1, 'b': 2, 'c': 3}.items() if v > 1}

easy

A. {'a': 1, 'b': 2, 'c': 3} - keeps all items

B. {'a': 1} - keeps items with values equal to 1

C. {'b': 2, 'c': 3} - keeps items with values greater than 1

D. {} - removes all items

Solution

1. Step 1: Understand the dictionary comprehension structure

   The comprehension loops over each key-value pair in the original dictionary.

2. Step 2: Apply the condition if v > 1

   Only pairs where the value is greater than 1 are included in the new dictionary.

3. Final Answer:

   {'b': 2, 'c': 3} - keeps items with values greater than 1 -> Option C

4. Quick Check:

   Filter values > 1 = {'b': 2, 'c': 3} [OK]

Hint: Look for the condition after the for loop to filter items [OK]

Common Mistakes:

• Ignoring the condition and including all items
• Confusing keys and values in the condition
• Using wrong comparison operator

2.

Which of the following is the correct syntax for a dictionary comprehension with a condition?

?

easy

A. {k: v for k, v in d.items() if v % 2 == 0}

B. {k: v if v % 2 == 0 for k, v in d.items()}

C. {k: v for k, v in d.items() where v % 2 == 0}

D. {k: v for k, v in d.items() when v % 2 == 0}

Solution

1. Step 1: Recall dictionary comprehension syntax

   The correct syntax is {key: value for key, value in iterable if condition}.

2. Step 2: Identify the correct option

   {k: v for k, v in d.items() if v % 2 == 0} matches the correct syntax with if after the loop. Others use invalid keywords or wrong order.

3. Final Answer:

   {k: v for k, v in d.items() if v % 2 == 0} -> Option A

4. Quick Check:

   Correct syntax uses 'if' after for loop [OK]

Hint: Remember: condition goes after the for clause in comprehension [OK]

Common Mistakes:

• Placing 'if' before the for loop
• Using 'where' or 'when' instead of 'if'
• Incorrect order of clauses

3.

What is the output of this code?

nums = {'x': 10, 'y': 5, 'z': 0}
filtered = {k: v for k, v in nums.items() if v}
print(filtered)

medium

A. {'x': 10, 'y': 5}

B. {}

C. {'z': 0}

D. {'x': 10, 'y': 5, 'z': 0}

Solution

1. Step 1: Understand the condition if v

   This condition filters out values that are 'falsy' in Python, like 0, None, or empty.

2. Step 2: Apply the condition to each item

   Values 10 and 5 are truthy, so included; 0 is falsy, so excluded.

3. Final Answer:

   {'x': 10, 'y': 5} -> Option A

4. Quick Check:

   Falsy values excluded = {'x': 10, 'y': 5} [OK]

Hint: Falsy values like 0 are skipped when condition is just 'if v' [OK]

Common Mistakes:

• Including zero as truthy
• Confusing keys and values
• Expecting all items to be included

4.

Find the error in this dictionary comprehension:

data = {'a': 1, 'b': 2, 'c': 3}
result = {k: v for k, v in data.items() if v > 1 else 0}
print(result)

medium

A. No error, outputs {'b': 2, 'c': 3}

B. SyntaxError due to incorrect use of else in comprehension

C. Outputs {'a': 0, 'b': 2, 'c': 3}

D. KeyError because of else clause

Solution

1. Step 1: Check the use of else in comprehension condition

   Dictionary comprehensions cannot have an else clause directly after the if condition.

2. Step 2: Identify correct syntax

   To use else, it must be inside a value expression with a ternary operator, not after if.

3. Final Answer:

   SyntaxError due to incorrect use of else in comprehension -> Option B

4. Quick Check:

   else must be inside value expression, not after if [OK]

Hint: Use ternary inside value, not else after if in comprehension [OK]

Common Mistakes:

• Placing else after if in comprehension
• Confusing ternary operator syntax
• Expecting else to filter keys

5.

You have a dictionary of student scores:
scores = {'Alice': 85, 'Bob': 42, 'Charlie': 73, 'David': 58}
Use dictionary comprehension with a condition to create a new dictionary passed containing only students who scored 60 or more, but store their scores as 'Pass' instead of the number.
Which code correctly does this?

hard

A. {k: 'Pass' for k, v in scores.items() if v > 60}

B. {k: 'Pass' if v >= 60 else 'Fail' for k, v in scores.items()}

C. {k: v for k, v in scores.items() if v >= 60}

D. {k: 'Pass' for k, v in scores.items() if v >= 60}

Solution

1. Step 1: Understand the requirement

   We want only students with scores 60 or more, and their values replaced by 'Pass'.

2. Step 2: Check each option

   {k: 'Pass' for k, v in scores.items() if v >= 60} filters with if v >= 60 and sets value to 'Pass'. {k: 'Pass' if v >= 60 else 'Fail' for k, v in scores.items()} includes all students with ternary but no filtering. {k: v for k, v in scores.items() if v >= 60} keeps original scores. {k: 'Pass' for k, v in scores.items() if v > 60} excludes 60 exactly.

3. Final Answer:

   {k: 'Pass' for k, v in scores.items() if v >= 60} -> Option D

4. Quick Check:

   Filter and replace value with 'Pass' = {k: 'Pass' for k, v in scores.items() if v >= 60} [OK]

Hint: Filter with if, set value directly to 'Pass' in comprehension [OK]

Common Mistakes:

• Using else without filtering
• Not including scores equal to 60
• Keeping original scores instead of 'Pass'