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02. Comprehensions Revisited

Comprehensions are a concise way to create new lists, sets, and dictionaries in Python. They are more Pythonic than using loops for building collections, and often faster as well.

Why Comprehensions?​

  • Conciseness: Replace multiple lines of loops with one clear expression.
  • Readability: Easier to understand intent at a glance.
  • Performance: Comprehensions are implemented in C under the hood, often faster than explicit loops.

List Comprehensions​

General form:

[expression for item in iterable if condition]

Example:

numbers = [1, 2, 3, 4, 5, 6]
squares = [n*n for n in numbers if n % 2 == 0]
print(squares)  # [4, 16, 36]

Equivalent loop:

squares = []
for n in numbers:
    if n % 2 == 0:
        squares.append(n*n)

Nested List Comprehensions​

You can nest comprehensions for multiple loops:

pairs = [(x, y) for x in range(3) for y in range(3)]
print(pairs)
# [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

Set Comprehensions​

Set comprehensions look like list comprehensions but use curly braces:

unique_squares = {n*n for n in numbers}
print(unique_squares)

Dictionary Comprehensions​

General form:

{key_expr: value_expr for item in iterable if condition}

Example:

fruits = ["apple", "banana", "cherry"]
fruit_lengths = {fruit: len(fruit) for fruit in fruits}
print(fruit_lengths)
# {'apple': 5, 'banana': 6, 'cherry': 6}

Conditional Expressions in Comprehensions​

You can use if...else directly inside expressions:

result = ["even" if n % 2 == 0 else "odd" for n in range(5)]
print(result)  # ['even', 'odd', 'even', 'odd', 'even']

Practice Challenge​

Rewrite this loop as a comprehension:

words = ["python", "is", "fun"]
uppercase_words = []
for word in words:
    if len(word) > 2:
        uppercase_words.append(word.upper())
print(uppercase_words)

👉 Hint: Use a list comprehension with a condition.

Wrap-Up​

  • Use list, set, and dictionary comprehensions for concise, readable code.
  • Avoid deeply nested comprehensions; if they get too complex, use loops instead.
  • Remember: readability counts, so choose clarity over cleverness.