List Comprehensions in Python: build a list in one line
Building a list with a loop takes three lines: make an empty list, loop, append. A list comprehension does the same thing in one clear line.
The big idea
A list comprehension builds a list from a loop in a single expression: [expr for item in iterable].
See it in code
The shape is [expr for item in iterable]. Here we build the x-positions of a six-wide wave — one value per column — in a single line:
xs = [55 + col * 74 for col in range(6)]
print(xs)[55, 129, 203, 277, 351, 425]
Read it as a sentence: '55 + col * 74 for each col in range(6).' No empty list, no append — the whole list appears at once.
Add an if at the end to filter. From those positions, keep only the ones on the left half of the screen:
xs = [55 + col * 74 for col in range(6)]
left = [x for x in xs if x < 250]
print(left)[55, 129, 203]
The if x < 250 kept three of the six. Now let's turn each position into a real Enemy — same one-line pattern.
The long way to build a wave is a loop that appends. The comprehension [Enemy(x, 90) for x in range(40, 480, 74)] says the same thing left to right: make an `Enemy(x, 90)` for each `x` in that range. It produces the whole list at once:
from game import Stage, Enemy
screen = Stage.create()
Stage.clear(screen)
wave = [Enemy(x, 90) for x in range(40, 480, 74)]
print("Built", len(wave), "enemies in one line")
for enemy in wave:
enemy.draw(screen)Built 6 enemies in one line
Read it as the sentence it is: 'an Enemy at (x, 90) for each x in the range.' Same pattern as the warm-ups — and you can add the same if filter to keep only some, transforming each item as you go.
Comprehensions are the Pythonic way to transform data: [p * 1.1 for p in prices] to adjust a series, [c.upper() for c in bases] to normalize DNA, [w for w in words if len(w) > 3] to filter. They're shorter and usually faster than the append loop — and dictionaries and sets have their own comprehension forms too.
Try it yourself
Add a filter to keep only even columns: [Enemy(x, 90) for x in range(40, 480, 74) if x < 300]. Then build a list of squares, [n * n for n in range(6)], and print it.
The common mistake
Cramming too much in, or reaching for a comprehension when you only need side effects. If the body is complex or you're just calling draw() on each item, a plain for loop reads better. Comprehensions are for building a list, not for doing work and throwing the result away.
What it unlocks
Comprehensions build on for loops, range(), and lists, and make wave-building and data prep concise.