Dictionary comprehension with condition in Python

Introduction

Dictionary comprehension with condition helps you create a new dictionary by picking only the items you want from another dictionary or sequence.

You want to filter a dictionary to keep only certain keys or values.

You need to create a smaller dictionary from a bigger one based on some rule.

You want to quickly transform data but only include items that meet a condition.

Syntax

Python

{key_expression: value_expression for item in iterable if condition}

The if condition part filters items before adding them to the new dictionary.

You can use any expression for keys and values, not just simple variables.

Examples

This keeps only items where the value is greater than 1.

Python

numbers = {'a': 1, 'b': 2, 'c': 3}
new_dict = {k: v for k, v in numbers.items() if v > 1}

This creates a dictionary of words with their lengths, but only for words longer than 5 letters.

Python

words = ['apple', 'banana', 'cherry']
lengths = {word: len(word) for word in words if len(word) > 5}

Sample Program

This program filters the fruits dictionary to keep only those with quantity greater than 3.

Python

fruits = {'apple': 5, 'banana': 3, 'cherry': 7, 'date': 2}
# Keep fruits with quantity more than 3
filtered = {fruit: qty for fruit, qty in fruits.items() if qty > 3}
print(filtered)

Important Notes

Remember, the condition comes after the for part in the comprehension.

You can use multiple conditions by combining them with and or or.

Summary

Dictionary comprehension with condition lets you build dictionaries by filtering items easily.

Use it to create smaller dictionaries from bigger ones based on rules.

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'