Stage 6: Use Items¶

In this lesson we will learn

why a list is a good way to store several pieces of data
how to create and update a list to store and manage items
how player commands trigger different branches of code
how to add commands that use a list to decide which actions are allowed

Terminology

collection – a data type that stores a group of values together, such as a list, tuple, set or dictionary.
list – a collection that stores values in order, can be changed, and is made with square brackets [].
tuple – a collection like a list that can't be changed once it's made, written with brackets ().
set – a collection that stores values with no duplicates and in no particular order.
array – a collection that stores lots of values of the same type.
queue – a collection where the first item put in is the first item taken out.
stack – a collection where the last item put in is the first item taken out.
nested if statement – an if statement placed inside another if statement, which creates more paths that each need testing.

Introduction¶

Our game is coming along well. We can move around, meet different characters and find items in the rooms. Now we want the player to pick up items and use them.

Pseudocode¶

Stage 6 pseudocode

- Make a backpack variable to hold the items we collect
- Add a take command to pick up items
- Add a backpack command to see what we're carrying
- Only allow fighting with items that are in the backpack

Class diagram¶

The class diagram hasn't changed. Everything in this stage happens in main.py, so we won't need to edit any classes.

The class diagram, unchanged from Stage 5

Create a backpack¶

First, we need a backpack variable to keep the items we pick up. It needs to store more than one thing at a time. In Python, these types are called collections. We've already used one collection type, the dictionary. Now we'll use a list.

Collections in Python

A collection is a way to store a group of things together. Different collection types work in different ways, and choosing the right one makes our code easier to understand and faster to run.

Python's main built-in collection types are:

lists: store things in order and can hold any type of value. We make them with square brackets [].
tuples: like lists, but they can't be changed once they're made. We make them with brackets ().
sets: store values with no duplicates, in no particular order. We make them with {} or set().
dictionaries: store pairs of information using a key and a value. We make them with {key: value}.

Python also has modules for other collection types, such as arrays (lots of values of the same type), queues (first in, first out) and stacks (last in, first out).

Lists work really well for our backpack:

they can start empty, and we can use append() to add items when the player picks them up
we can check whether the backpack has a certain item using the in operator
we can get items by their position
we can use pop() or remove() to take an item out

Open main.py and add the highlighted code below.

# main.py

from room import Room
from character import Enemy, Friend
from item import Item

# create rooms
cavern = Room("Cavern")
cavern.description = ("A room so big that the light of your torch doesn’t reach the walls.")

armoury = Room("Armoury")
armoury.description = ("The walls are lined with racks that once held weapons and armour.")

lab = Room("Laboratory")
lab.description = ("A strange odour hangs in a room filled with unknowable contraptions.")

# link rooms
cavern.link_rooms(armoury,"south")
armoury.link_rooms(cavern,"north")
armoury.link_rooms(lab,"east")
lab.link_rooms(armoury,"west")

# create characters
ugine = Enemy("Ugine")
ugine.description = "a huge troll with rotting teeth."
ugine.weakness = "cheese"

nigel = Friend("Nigel")
nigel.description = "a burly dwarf with golden beads woven through his beard."
nigel.conversation = "Well youngun, what are you doing here?"

# add characters to rooms
armoury.character = ugine
lab.character = nigel

# create items
cheese = Item("Cheese")
cheese.description = "super smelly"

chair = Item("Chair")
chair.description = "designed to be sat on"

elmo = Item("Elmo")
elmo.description = "wanting to be tickled"

# add items to rooms
cavern.item = chair
armoury.item = elmo
lab.item = cheese

'''
# describe the rooms
cavern.describe()
armoury.describe()
lab.describe()
'''

# initialise variables
running = True
current_room = cavern
backpack = []

# ----- MAIN LOOP -----
while running:
    current_room.describe()

    command = input("> ").lower()

    if command in ["north", "south", "east", "west"]:
        current_room = current_room.move(command)
    elif command == "talk":
        if current_room.character is not None:
            current_room.character.talk()
        else:
            print("There is no one here to talk to")
    elif command == "hug":
        if current_room.character is not None:
            current_room.character.hug()
        else:
            print("There is no one here to hug")
    elif command == "fight":
        if current_room.character is not None:
            weapon = input("What will you fight with? > ").lower()
            if current_room.character.fight(weapon):
                if isinstance(current_room.character, Enemy):
                    current_room.character = None
            else:
                running = False
        else:
            print("There is no one here to fight")
    elif command == "quit":
        running = False
    else:
        print("I don't understand.")

Code explanation

line 61 → creates an empty list called backpack.

Add the take command¶

Now we need a command that lets the player pick up an item from the room and put it in the backpack. We'll call it take.

Still in main.py, add the highlighted code below.

# main.py

from room import Room
from character import Enemy, Friend
from item import Item

# create rooms
cavern = Room("Cavern")
cavern.description = ("A room so big that the light of your torch doesn’t reach the walls.")

armoury = Room("Armoury")
armoury.description = ("The walls are lined with racks that once held weapons and armour.")

lab = Room("Laboratory")
lab.description = ("A strange odour hangs in a room filled with unknowable contraptions.")

# link rooms
cavern.link_rooms(armoury,"south")
armoury.link_rooms(cavern,"north")
armoury.link_rooms(lab,"east")
lab.link_rooms(armoury,"west")

# create characters
ugine = Enemy("Ugine")
ugine.description = "a huge troll with rotting teeth."
ugine.weakness = "cheese"

nigel = Friend("Nigel")
nigel.description = "a burly dwarf with golden beads woven through his beard."
nigel.conversation = "Well youngun, what are you doing here?"

# add characters to rooms
armoury.character = ugine
lab.character = nigel

# create items
cheese = Item("Cheese")
cheese.description = "super smelly"

chair = Item("Chair")
chair.description = "designed to be sat on"

elmo = Item("Elmo")
elmo.description = "wanting to be tickled"

# add items to rooms
cavern.item = chair
armoury.item = elmo
lab.item = cheese

'''
# describe the rooms
cavern.describe()
armoury.describe()
lab.describe()
'''

# initialise variables
running = True
current_room = cavern
backpack = []

# ----- MAIN LOOP -----
while running:
    current_room.describe()

    command = input("> ").lower()

    if command in ["north", "south", "east", "west"]:
        current_room = current_room.move(command)
    elif command == "talk":
        if current_room.character is not None:
            current_room.character.talk()
        else:
            print("There is no one here to talk to")
    elif command == "hug":
        if current_room.character is not None:
            current_room.character.hug()
        else:
            print("There is no one here to hug")
    elif command == "fight":
        if current_room.character is not None:
            weapon = input("What will you fight with? > ").lower()
            if current_room.character.fight(weapon):
                if isinstance(current_room.character, Enemy):
                    current_room.character = None
            else:
                running = False
        else:
            print("There is no one here to fight")
    elif command == "take":
        if current_room.item is not None:
            backpack.append(current_room.item)
            print(f"You put {current_room.item.name} into your backpack")
            current_room.item = None
        else:
            print("There is nothing here to take")
    elif command == "quit":
        running = False
    else:
        print("I don't understand.")

PRIMM

Predict what you think will happen. Be specific. What happens to the room description after we take an item?
Run the program.
Time to investigate the code. What does each line do?

Code explanation

line 91 → checks whether the player's command was take.
line 92 → checks whether there is an item in the current room.
line 93 → adds the room's item to the backpack list.
line 94 → tells the player which item they picked up.
line 95 → removes the item from the room by setting item to None.
lines 96–97 → otherwise tells the player there's nothing to take.

Add the backpack command¶

Now we need a way for the player to check what they're carrying. Let's add a backpack command that shows every item in the backpack.

Still in main.py, add the highlighted code below.

# main.py

from room import Room
from character import Enemy, Friend
from item import Item

# create rooms
cavern = Room("Cavern")
cavern.description = ("A room so big that the light of your torch doesn’t reach the walls.")

armoury = Room("Armoury")
armoury.description = ("The walls are lined with racks that once held weapons and armour.")

lab = Room("Laboratory")
lab.description = ("A strange odour hangs in a room filled with unknowable contraptions.")

# link rooms
cavern.link_rooms(armoury,"south")
armoury.link_rooms(cavern,"north")
armoury.link_rooms(lab,"east")
lab.link_rooms(armoury,"west")

# create characters
ugine = Enemy("Ugine")
ugine.description = "a huge troll with rotting teeth."
ugine.weakness = "cheese"

nigel = Friend("Nigel")
nigel.description = "a burly dwarf with golden beads woven through his beard."
nigel.conversation = "Well youngun, what are you doing here?"

# add characters to rooms
armoury.character = ugine
lab.character = nigel

# create items
cheese = Item("Cheese")
cheese.description = "super smelly"

chair = Item("Chair")
chair.description = "designed to be sat on"

elmo = Item("Elmo")
elmo.description = "wanting to be tickled"

# add items to rooms
cavern.item = chair
armoury.item = elmo
lab.item = cheese

'''
# describe the rooms
cavern.describe()
armoury.describe()
lab.describe()
'''

# initialise variables
running = True
current_room = cavern
backpack = []

# ----- MAIN LOOP -----
while running:
    current_room.describe()

    command = input("> ").lower()

    if command in ["north", "south", "east", "west"]:
        current_room = current_room.move(command)
    elif command == "talk":
        if current_room.character is not None:
            current_room.character.talk()
        else:
            print("There is no one here to talk to")
    elif command == "hug":
        if current_room.character is not None:
            current_room.character.hug()
        else:
            print("There is no one here to hug")
    elif command == "fight":
        if current_room.character is not None:
            weapon = input("What will you fight with? > ").lower()
            if current_room.character.fight(weapon):
                if isinstance(current_room.character, Enemy):
                    current_room.character = None
            else:
                running = False
        else:
            print("There is no one here to fight")
    elif command == "take":
        if current_room.item is not None:
            backpack.append(current_room.item)
            print(f"You put {current_room.item.name} into your backpack")
            current_room.item = None
        else:
            print("There is nothing here to take")
    elif command == "backpack":
        if backpack == []:
            print("It is empty")
        else:
            print("You have:")
            for item in backpack:
                print(f"- {item.name.capitalize()}")
    elif command == "quit":
        running = False
    else:
        print("I don't understand.")

PRIMM

Predict what you think will happen. Be specific.
Run the program.
Time to investigate the code. What does each line do?

Code explanation

line 98 → checks whether the player's command was backpack.
line 99 → checks whether the backpack is an empty list…
line 100 → …and if so, tells the player it's empty.
line 101 → runs if the backpack has at least one item.
line 102 → prints a heading before the list of items.
line 103 → loops through each item in the backpack.
line 104 → prints each item's name, starting with a capital letter.

This time we need to do some serious testing. We need to make sure that:

we can pick up every item in every room
each item is added to the backpack when it's picked up
each item is removed from the room when it's picked up

Here's an example testing table for the cavern. Make one for each room.

Room	Command	Expected result
Cavern	backpack	It is empty
Cavern	take	You put chair into your backpack
Cavern	backpack	Chair
Cavern	take	There is nothing here to take

Adjust the fight command¶

Lastly, let's change the fight command so the player can only use items that are in their backpack.

Still in main.py, change the highlighted code below.

# main.py

from room import Room
from character import Enemy, Friend
from item import Item

# create rooms
cavern = Room("Cavern")
cavern.description = ("A room so big that the light of your torch doesn’t reach the walls.")

armoury = Room("Armoury")
armoury.description = ("The walls are lined with racks that once held weapons and armour.")

lab = Room("Laboratory")
lab.description = ("A strange odour hangs in a room filled with unknowable contraptions.")

# link rooms
cavern.link_rooms(armoury,"south")
armoury.link_rooms(cavern,"north")
armoury.link_rooms(lab,"east")
lab.link_rooms(armoury,"west")

# create characters
ugine = Enemy("Ugine")
ugine.description = "a huge troll with rotting teeth."
ugine.weakness = "cheese"

nigel = Friend("Nigel")
nigel.description = "a burly dwarf with golden beads woven through his beard."
nigel.conversation = "Well youngun, what are you doing here?"

# add characters to rooms
armoury.character = ugine
lab.character = nigel

# create items
cheese = Item("Cheese")
cheese.description = "super smelly"

chair = Item("Chair")
chair.description = "designed to be sat on"

elmo = Item("Elmo")
elmo.description = "wanting to be tickled"

# add items to rooms
cavern.item = chair
armoury.item = elmo
lab.item = cheese

'''
# describe the rooms
cavern.describe()
armoury.describe()
lab.describe()
'''

# initialise variables
running = True
current_room = cavern
backpack = []

# ----- MAIN LOOP -----
while running:
    current_room.describe()

    command = input("> ").lower()

    if command in ["north", "south", "east", "west"]:
        current_room = current_room.move(command)
    elif command == "talk":
        if current_room.character is not None:
            current_room.character.talk()
        else:
            print("There is no one here to talk to")
    elif command == "hug":
        if current_room.character is not None:
            current_room.character.hug()
        else:
            print("There is no one here to hug")
    elif command== "fight":
        if current_room.character is not None:
            weapon = input("What will you fight with? > ").lower()
            available_weapons = []
            for item in backpack:
                available_weapons.append(item.name)
            if weapon in available_weapons:
                if current_room.character.fight(weapon):
                    if isinstance(current_room.character, Enemy):
                        current_room.character = None
                else:
                    running = False
            else:
                print(f"You don't have {weapon}")
                print(f"{current_room.character.name} strikes you down.")
                running = False
        else:
            print("There is no one here to fight")
    elif command == "take":
        if current_room.item is not None:
            backpack.append(current_room.item)
            print(f"You put {current_room.item.name} into your backpack")
            current_room.item = None
        else:
            print("There is nothing here to take")
    elif command == "backpack":
        if backpack == []:
            print("It is empty")
        else:
            print("You have:")
            for item in backpack:
                print(f"- {item.name.capitalize()}")
    elif command == "quit":
        running = False
    else:
        print("I don't understand.")

Indenting several lines

Lines 88–92 are the same as before, just indented one more level. To indent them quickly, highlight the lines in Thonny and press Tab once.

PRIMM

Predict what you think will happen. Be specific.
Run the program.
Time to investigate the code. What does each line do?

Code explanation

line 84 → creates an empty list to hold the names of the items in the backpack. The backpack stores Item objects, but the player types a weapon as a string, so we need a list of names to compare with.
line 85 → loops through each item in the backpack…
line 86 → …and adds its name to available_weapons.
line 87 → checks whether the player has the weapon they typed.
lines 88–92 → fight with the weapon, exactly as before.
line 93 → runs when the weapon isn't in available_weapons.
line 94 → tells the player they don't have that item.
line 95 → tells the player the character strikes them down.
line 96 → ends the game by stopping the main loop.

Testing¶

This code has nested if statements, so we need to test every possible path. We'll test with Ugine, because we know his weakness is cheese.

Collected cheese	Fought Ugine	Weapon used	Expected result
Yes	Yes	cheese	You strike Ugine down with cheese.
Yes	Yes	elmo	You don't have elmo…
Yes, and elmo	Yes	elmo	Ugine crushes you. Puny adventurer
No	Yes	cheese	You don't have cheese…

Exercises¶

Now it's time to make.

Exercise 1¶

Can you make sure these changes work for the characters you've added? If you've added another enemy, make sure the player can collect its weakness and use it against them. Test each path with a testing table.