Lists

A list of 5 items has indexes 0 to 4, so menu[4] is the last item and menu[5] stops the program with IndexError.

One variable can hold a whole menu

A school lunch menu has several dishes, and a shopping list has several items. You could make one variable per dish, dish1, dish2, dish3, but that breaks the moment the menu grows. A list keeps any number of values in one variable, in order. Write the values between square brackets, separated by commas.

menu = ["rice", "seaweed soup", "bulgogi", "kimchi", "milk"]
print(menu)
print(len(menu))
print(menu[0], menu[-1])

['rice', 'seaweed soup', 'bulgogi', 'kimchi', 'milk']
5
rice milk

Lists use the same index numbers as strings: 0 for the first item, -1 for the last, and slices such as menu[1:3]. len() gives the number of items. When you print a whole list, Python shows the brackets and puts each string in single quotes, so you can see exactly where each item starts and ends.

Unlike strings, lists can change

A string cannot be edited in place, but a list can. Assign to an index to replace an item. append() adds an item at the end, remove() deletes the first matching value, and pop() takes the last item off and gives it back to you. The keyword in checks whether a value is in the list. These methods change the list itself, which is the big difference from string methods.

cart = ["eggs", "milk"]
cart.append("tofu")
cart[0] = "ramen"
print(cart)
cart.remove("milk")
last = cart.pop()
print(cart, last)
print("tofu" in cart)

['ramen', 'milk', 'tofu']
['ramen'] tofu
False

Lists of numbers come with ready-made tools

For a list of numbers, sum(), min() and max() do what their names say. sorted() gives back a new list in order and leaves the original alone, while the method sort() reorders the list itself and returns nothing. The average is the sum divided by the count, which is a nice use of len().

scores = [72, 95, 88, 64, 81]
print(sum(scores), min(scores), max(scores))
print(sum(scores) / len(scores))
print(sorted(scores))
print(scores)

400 64 95
80.0
[64, 72, 81, 88, 95]
[72, 95, 88, 64, 81]

Two names can point at the same list

Here is the part that catches almost everyone. After b = a, there are not two lists. There is one list with two names. Change it through b, and a sees the change too, because a and b are the same list. With numbers and strings you never notice this, since they cannot be changed in place. With lists you do. If you want a separate copy, ask for one with a.copy() or the slice a[:].

a = ["rice", "soup"]
b = a
b.append("kimchi")
print(a)
c = a.copy()
c.append("egg")
print(a)
print(c)

['rice', 'soup', 'kimchi']
['rice', 'soup', 'kimchi']
['rice', 'soup', 'kimchi', 'egg']

Lists and loops belong together. A list of 30 test scores is only useful if a program can visit every one of them without writing 30 separate lines, and that is what the for loop in the unit on loops does. Until then, sum(), min(), max() and len() do the visiting for you, one call each.