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Conditional and Nested Comprehensions

Learn advanced comprehension techniques including conditional expressions and complex nested patterns.

Ali Berro

By Ali Berro

5 min read Section 4
From: Python Fundamentals: From Zero to Hero
python comprehension conditional nested

Table of Contents

Conditional and Nested Comprehensions

This section covers advanced comprehension techniques, including conditional expressions (ternary operators) within comprehensions and complex nested patterns.

Conditional Expressions in Comprehensions

You can use conditional expressions (ternary operators) in comprehensions to apply different expressions based on conditions:

conditional-expression.py
numbers = [n if n % 2 == 0 else n * 2 for n in range(5)]
print(numbers)  # [0, 2, 2, 6, 4]

This keeps even numbers as-is and doubles odd numbers.

Combining Conditions and Conditional Expressions

You can combine filtering conditions with conditional expressions:

combined-conditional.py
result = [n * 2 if n > 5 else n for n in range(10) if n % 2 == 0]
print(result)  # [0, 2, 4, 12, 16]

This:

  1. Filters to only even numbers (if n % 2 == 0)
  2. Doubles numbers greater than 5, otherwise keeps them as-is

Nested Comprehensions with Conditions

You can have multiple levels of nesting with conditions at each level:

nested-with-conditions.py
matrix = [[i * j for j in range(5) if j > 0] for i in range(3) if i > 0]
print(matrix)  # [[1, 2, 3, 4], [2, 4, 6, 8]]

This creates a matrix where:

  • Outer loop: i from 0 to 2, but only when i > 0
  • Inner loop: j from 0 to 4, but only when j > 0
  • Value: i * j

Complex Nested Patterns

You can create complex data structures with multiple levels of nesting:

complex-nested.py
# Create a list of dictionaries
data = [{f"key_{i}": i * j for j in range(3)} for i in range(3)]
print(data)
# [{'key_0': 0, 'key_0': 0, 'key_0': 0}, {'key_1': 0, 'key_1': 1, 'key_1': 2}, ...]

Multiple Conditions in Nested Comprehensions

You can apply conditions at multiple levels:

multiple-conditions-nested.py
result = [
    (i, j, k)
    for i in range(5) if i % 2 == 0
    for j in range(5) if j % 2 == 1
    for k in range(5) if k > 2
]
print(result[:5])  # [(0, 1, 3), (0, 1, 4), (0, 3, 3), ...]

Exercises

Exercise 1: Conditional Expression

Use list comprehension with a conditional expression to create a list where even numbers are kept as-is and odd numbers are squared.

Conditional Expression

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Answer:
numbers = [n if n % 2 == 0 else n * n for n in range(10)]
print(numbers)

Exercise 2: Multiple Conditions

Use list comprehension to create a list of numbers from 1 to 20, but replace numbers divisible by 3 with “Fizz”, numbers divisible by 5 with “Buzz”, and numbers divisible by both with “FizzBuzz”.

Multiple Conditions

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Answer:
result = [
    "FizzBuzz" if n % 15 == 0
    else "Fizz" if n % 3 == 0
    else "Buzz" if n % 5 == 0
    else n
    for n in range(1, 21)
]
print(result)

Exercise 3: Nested Comprehension with Conditions

Use nested list comprehension to create a list of lists representing a multiplication table for numbers 1 to 5, but only include products greater than 10.

Nested Comprehension with Conditions

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Answer:
table = [[i * j for j in range(1, 6) if i * j > 10] for i in range(1, 6) if any(i * j > 10 for j in range(1, 6))]
print(table)

Exercise 4: Dictionary with Conditional Values

Use dictionary comprehension to create a dictionary mapping numbers from 1 to 10, where values are “even” for even numbers and “odd” for odd numbers.

Dictionary with Conditional Values

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Answer:
result = {n: "even" if n % 2 == 0 else "odd" for n in range(1, 11)}
print(result)

Exercise 5: Complex Nested Structure

Use nested comprehension to create a list of dictionaries, where each dictionary maps numbers 1 to 3 to their squares, but only for even numbers.

Complex Nested Structure

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Answer:
result = [{n: n * n for n in range(1, 4) if n % 2 == 0} for i in range(3)]
print(result)

Course Progress

Section 44 of 61

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