Comprehensions

Big idea

A comprehension builds a new list (or set, or dict) from an existing sequence in one readable line: [expression for item in sequence if condition].

From loop to comprehension

This loop builds a list of squares:

squares = []
for n in range(1, 6):
    squares.append(n * n)

The comprehension says the same thing in one line:

squares = [n * n for n in range(1, 6)]
print(squares)

output
[1, 4, 9, 16, 25]

Read it as: "n × n, for each n in 1 to 5".

Filtering with if

Add if at the end to keep only some items:

nums = [4, 7, 10, 13, 16]
evens = [n for n in nums if n % 2 == 0]
halves = [n // 2 for n in nums if n > 8]
print(evens)
print(halves)

[4, 10, 16]
[5, 6, 8]

Working with strings

raw = ["  ada ", "GRACE", "linus  "]
clean = [name.strip().title() for name in raw]
print(clean)
lengths = [len(name) for name in clean]
print(lengths)

output
['Ada', 'Grace', 'Linus']
[3, 5, 5]

Set and dict comprehensions

Curly braces make a set; key: value makes a dict:

words = ["apple", "avocado", "banana", "blueberry", "cherry"]
first_letters = {w[0] for w in words}
print(sorted(first_letters))
lengths = {w: len(w) for w in words if w.startswith("b")}
print(lengths)

output
['a', 'b', 'c']
{'banana': 6, 'blueberry': 9}

Keep them readable

A comprehension should fit comfortably on one line. If you need nested loops, several conditions, or a print inside, write a normal for loop — clarity beats cleverness.

words = ["hi", "hello", "yo", "world", "hey"]
long_words = [w for w in words if len(w) < 3]
print(long_words)

output
['hello', 'world']

Watch it

Think of it like

An instruction to a helper: "for every apple in the basket, if it isn't bruised, slice it and put it on the plate." One sentence, and you get a plate of slices.

Key terms

• List comprehension — [expr for x in seq if cond]: builds a list in one line.
• Filter — The if part, which keeps only some items.
• Set / dict comprehension — The same idea with {x ...} or {k: v ...}.