Pack Your Backpack

A variable holds one thing. But what if you want to remember a whole bunch of things, like everything in your backpack? You could make a hundred variables, but that would be silly. Python has a better idea: the list.

What you need

• Python and IDLE.
• A new file called backpack.py.

Making a list

A list is written between square brackets [ ], with commas between the items:

backpack = ["torch", "map", "sandwich"]
print(backpack)
print(len(backpack))

['torch', 'map', 'sandwich']
3

When Python prints a whole list, it shows the square brackets and the quote marks too. The word len is short for length, and len(backpack) tells you how many items are in the list. There are three.

Picking out one item

Each item has a number called its index, which tells you its place in the list. And here is the surprise: the counting starts at 0, not 1.

To get one item, write the list name and then the index in square brackets:

backpack = ["torch", "map", "sandwich"]
print(backpack[0])
print(backpack[1])
print(backpack[2])
print(backpack[-1])

torch
map
sandwich
sandwich

The last line shows a handy trick. A negative index counts from the end, so -1 is always the last item, however long the list is.

Changing a list

Use append to add a new item to the end of the list. The word means “stick on the end”.

backpack = ["torch", "map"]
backpack.append("compass")
backpack.append("apple")
print(backpack)
print("Items:", len(backpack))

['torch', 'map', 'compass', 'apple']
Items: 4

You can swap one item for another by assigning to its index, and remove an item by name:

backpack = ["torch", "map", "sandwich"]
backpack[1] = "telescope"
backpack.remove("sandwich")
print(backpack)

['torch', 'telescope']

You can also ask whether something is in the list, using the word in. It gives you a yes or no answer, so it works perfectly with if:

backpack = ["torch", "map", "sandwich"]
if "map" in backpack:
    print("Great, we will not get lost!")
else:
    print("Oh no, we forgot the map!")

Great, we will not get lost!

Looping over a list

Remember from the last lesson that a for loop can walk through a list? Now you can use it on your own lists:

backpack = ["torch", "map", "sandwich", "compass"]
for item in backpack:
    print("In my backpack:", item)

In my backpack: torch
In my backpack: map
In my backpack: sandwich
In my backpack: compass

If you want a numbered list, loop over the positions with range(len(...)). The + 1 makes the numbers start from 1, which is how people usually count:

backpack = ["torch", "map", "sandwich", "compass"]
for position in range(len(backpack)):
    print(position + 1, backpack[position])

1 torch
2 map
3 sandwich
4 compass

Fill the backpack with input

Lists and input work well together. This program asks for three things and packs them one by one. We start with an empty list, written [].

backpack = []
for turn in range(3):
    thing = input("What will you pack? ")
    backpack.append(thing)

print("Your backpack has", len(backpack), "things:")
for thing in backpack:
    print("-", thing)

If you typed banana, kite and rope, the whole conversation would look like this:

What will you pack? banana
What will you pack? kite
What will you pack? rope
Your backpack has 3 things:
- banana
- kite
- rope

Challenges

foods = ["pizza", "pancakes", "grapes", "popcorn"]
print(foods[0])
print(foods[-1])

pizza
popcorn

pets = ["cat", "dog"]
pets.append("hamster")
print("Number of pets:", len(pets))
for pet in pets:
    print(pet)

Number of pets: 3
cat
dog
hamster

What you learned

• A list holds many items in one variable. You write it with [ ] and commas.
• Each item has an index, and the first index is 0.
• len(list) counts the items, append adds one at the end, and remove takes one out.
• item in list checks whether something is there.
• A for loop can walk through every item in a list.