Comprehensions

A comprehension builds a new collection from an old one in a single readable line. Compare the long way with the short way:

# long way
squares = []
for n in range(5):
    squares.append(n * n)

# list comprehension
squares = [n * n for n in range(5)]
print(squares)   # [0, 1, 4, 9, 16]

Add an if at the end to keep only some items:

evens = [n for n in range(10) if n % 2 == 0]
print(evens)   # [0, 2, 4, 6, 8]

A dict comprehension uses key: value inside braces:

squared = {n: n * n for n in range(4)}
print(squared)   # {0: 0, 1: 1, 2: 4, 3: 9}

A set comprehension looks the same but builds a set of unique values:

lengths = {len(w) for w in ["hi", "yo", "hey"]}
print(lengths)   # {2, 3}

Swap the brackets for parentheses and you get a generator expression: it produces values lazily, one at a time, which is perfect to feed straight into sum() without building a list:

total = sum(n * n for n in range(5))   # 30