Lists

A list holds many values in order. You write one with square brackets and commas.

items = ["apple", "bread", "milk"]
print(items)
print(len(items))

A list can hold numbers, strings, booleans, even other lists. Usually all the values in a list are the same kind of thing, but Python does not insist.

prices = [40, 25, 2.5]
mixed = ["bread", 25, True]
empty = []
print(prices)
print(mixed)
print(len(empty))

Indexing and slicing work exactly like strings

Positions start at 0, negatives count from the end, and slices use [start:stop] with the stop left out.

items = ["apple", "bread", "milk", "eggs", "flour"]
print(items[0])
print(items[-1])
print(items[1:3])
print(items[:2])
print(items[2:])

A slice of a list is a new, smaller list. All the string tricks carry over: items[:-1] is everything but the last, items[::2] is every second item, and items[::-1] is the list reversed.

items = ["apple", "bread", "milk", "eggs", "flour"]
print(items[:-1])
print(items[::2])
print(items[::-1])

Is it in there?

in checks whether a value is one of the items.

items = ["apple", "bread", "milk"]
print("milk" in items)
print("eggs" in items)

Lists can change

Unlike strings, lists can be changed after you make them, without making a new list. append adds an item at the end. Assigning to a position replaces what is there.

items = ["apple", "bread"]
items.append("milk")
print(items)

items[0] = "pear"
print(items)

append returns nothing, which Python shows as None; it changes the list it was called on. A common mistake is items = items.append("milk"), which sets items to None.

Try it

Last item

Write last_item(items) that returns the last value in the list items. The list always has at least one item.

last_item(["apple", "bread", "milk"])  ->  "milk"
last_item([7])                         ->  7

def last_item(items):
    ...

Add to the list

Write add_item(items, item) that adds item to the end of the list items and returns it. Change the list you are given rather than building a new one; the checks look at the original list afterwards.

add_item(["apple"], "bread")  ->  ["apple", "bread"]
add_item([], "milk")          ->  ["milk"]

def add_item(items, item):
    ...

Is it on the list?

Write on_list(items, item) that returns True if item is one of the values in items, and False otherwise.

on_list(["apple", "bread"], "bread")  ->  True
on_list(["apple", "bread"], "milk")   ->  False

def on_list(items, item):
    ...

Next: For loops.