Implementing an inventory system

In this tutorial we will look at how a simple inventory system can be added to the game. We will then use the contents of the inventory to differentiate between different outcomes for certain events. In particular, possessing the sword from location 2 will allow the player to slay the Ogre, which will in turn allow them to collect the key to the chest in location 1.

Modelling the inventory using a list

We will use a list in Python to represent the player’s inventory. A list represents an ordered collection of items. A variety of operations can be performed on lists, including adding elements to a list, removing elements from a list, determine the size of a list, and checking whether an item is contained in a list. The basic list operations are demonstrated in the following code listing.

New lists are defined within a pair of square brackets. Individual elements of the list are separated by commas.

#define a new list
list1=['a', 'b', 'c']
print(list1)

The number of elements in a list is calculated using the len operator.

#the number of elements in a list
print(len(list1))

To check whether an element is in a list we use the in operator. Because ‘a’ is in list1 the first use of the in operator returns true. The second use of the in operator return false. We will use the in operator later to check whether or not an item is in the player’s inventory.

#check whether elements are in a list
print('a' in list1)
print('d' in list1)

New elements are added to a list using the append operator. Note the usage of the append operation – the name of the variable storing the list is given first, followed by a period, followed by the append operator. The append operator takes a single argument, corresponding to the element to be added, enclosed in brackets. This operator can be used to add new items to the inventory.

#add an element to a list
list1.append('d')
print(list1)
print('d' in list1)

Elements can be removed from a list using the remove operator. The remove operator follows the same syntax as the append operator. The remove operator can be used to remove one time use items from the inventory. An example would be removing a potion once it has been consumed.

#remove an element from a list
list1.remove('a')
print(list1)

The following lines of code demonstrate further the use of the remove operator. It shows that if an element occurs multiple times in a list, then the remove operator only removes the first occurrence of the element. Note that if we attempt to remove an element that does not exist in the list then an error will occur. It is therefore important to test whether the element is present before attempting to remove it.

#remove only removes the first occurrence of an item from the list
list2=['a', 'b', 'a', 'c']
list2.remove('a')
print(list2)

#remove the second occurrence of 'a'
list2.remove('a')
print(list2)

Given that elements can occur multiple times in a list, it is useful to be able to determine how many times an element occurs. The count operator can be used to determine how many times a particular element occurs.

#check how many times an element occurs in a list
list3=['a','a','b', 'a', 'b', 'c']
print(list3.count('a'))
print(list3.count('b'))
print(list3.count('c'))

In some games the player may start off with no items in their inventory. To represent this as a list we can use an empty list. An empty list is represented by a pair of square brackets with nothing in between.

#empty list
list4=[]
list4.append('a')
list4.append('b')
print(list4)

Download source code [List operations]

Implementing the inventory

The code developed in part 2 of the tutorial is updated to make use of an inventory system. There are two items that can be added to the inventory – the sword in location 2 and the key in location 3. The inventory is initialised to an empty list.

inventory=[]

Next we update the playLocation2 function so that the sword is added to the inventory after it is located (line 70). To ensure that the inventory variable can be accessed within the function, we need to define inventory as a global variable (line 55).

def playLocation2():
    global inventory
    print("You stand atop a narrow ledge.")
    print("A rickety rope ladder dangles off the edge.")
    print("You see a pile of rubble at the far end of the ledge.")
    print("You can either (c)limb down the rope ladder or (s)earch the rubble.")
    response=input("> ")
    if (response=="c"):
        print("You carefully climb down the rope ladder.")
        location="location1"
    elif (response=="s"):
        print("You edge your way along the ledge, almost slipping a couple of times.")
        print("You search the pile of rubble, move large rocks until you discover a sword.")
        print("The sword is inscribed with runes and shimmers in the sun.")
        print("You carefully strap the sword to your back.")
        location="location2"
        inventory.append("sword")
    else:
        location="location2"
    return location

We also update the playLocation3 function so that the sword can be used to slay the Ogre if it has added to the inventory. Line 91 checks that the player response is attack and the sword is in the inventory. The two conditions are joined using the and operator, which evaluates to true if both of the conditions are true. Line 97 adds the key to the inventory.

def playLocation3():
    global inventory
    print("You reach what looks like a rudimentary camp site.")
    print("You notice a fire pit in the middle of the camp site.")
    print("Scattered around the fire pit are the many bones picked clean of flesh.")
    print("Ominously some of the bones look human like.")
    print("Just as you finish surveying the scene you here a roar.")
    print("Running towards you, wielding a large club is an Ogre.")
    print("You can either (f)lee to the south, (a)ttack the Ogre or try to (t)alk with the Ogre.")
    response=input("> ")
    if (response=="f"):
        print("You run away from the Ogre at full speed")
        print("The Ogre pursues for a while but eventually gives up.")
        print("You survive another day.")
        print("Once you know you are safe you continue your journey southward at a slower pace.")
        location="location1"
    elif (response=="a" and "sword" in inventory):
        print("As the Ogre draws near you draw your sword.")
        print("With one hefty swing of your sword you slay the Ogre.")
        print("You search the body of the Ogre and find a key.")
        print("You take the key and head back towards the clearing with the chest.")
        location="location1"
        inventory.append("key")
    elif (response=="a"):
        print("You fight valiantly with the Ogre.")
        print("You manage to inflict several telling blows against it but it fights on.")
        print("Having dodged several sweeping blows, the Ogre brings its club down upon you with a crash.")
        print("...")
        location="finish"
    elif (response=="t"):
        print("You attempt to make conversation with the Ogre but this only makes it more angry.")
        location="location3"
    else:
        location="location3"
    return location

The playLocation1 function is updated in a similar way to check whether the player has the key in their inventory when they attempt to open the chest.
The full code listing is given as follows:

location="start"
inventory=[]

def playStart():
    print("You are on a quest to find the lost sippy cup.")
    print("The sippy cup is a priceless treasure that grants\nthe user eternal life")
    print("You come to a junction. You can either go (w)est or (e)ast.")
    print("What do you select?")
    response=input("> ")
    if (response=="w"):
        location="location1"
        print("You travel along the path to the west...")
    elif (response=="e"):
        location="location2"
        print("You travel along the path to the east.")
        print("The path leads gradually upwards until you finally reach a ledge.")
    else:
        location="start"
    return location


def playLocation1():
    global inventory
    print("You are in a large clearing.")
    print("In the centre of the clearing is a chest.")
    print("There is a path leading north.")
    print("To the south you can see a cliff face with a rope ladder leading to a ledge.")
    print("You can go (n)orth, (c)limb the ladder or attempt to (o)pen the chest.")
    print("Enter your selection.")
    response=input("> ")
    
    if (response=="o" and "key" in inventory):
        print("You use the key recovered from the Ogre to unlock the chest.")
        print("As you open the lid of the chest a warm glow flows through your body.")
        print("Inside the chest you see the magical sippy cup. ")
        print("As you bring the sippy cup to your mouth and take a sip you feel instantly invigorated.")
        print("You have completed your quest.")
        location="finish"
    elif (response=="o"):
        print("You are unable to open the chest.")
        print("It has a large padlock which cannot be broken.")
        location="location1"
    elif (response=="n"):
        print("You travel for several hours along a winding path.")
        location="location3"
    elif (response=="c"):
        print("You climb the rope ladder which sways dangerously as you make your way up.")
        print("At the top of a ladder you climb up to a ledge.")
        location="location2"
    else:
        location="location1"
    return location

def playLocation2():
    global inventory
    print("You stand atop a narrow ledge.")
    print("A rickety rope ladder dangles off the edge.")
    print("You see a pile of rubble at the far end of the ledge.")
    print("You can either (c)limb down the rope ladder or (s)earch the rubble.")
    response=input("> ")
    if (response=="c"):
        print("You carefully climb down the rope ladder.")
        location="location1"
    elif (response=="s"):
        print("You edge your way along the ledge, almost slipping a couple of times.")
        print("You search the pile of rubble, move large rocks until you discover a sword.")
        print("The sword is inscribed with runes and shimmers in the sun.")
        print("You carefully strap the sword to your back.")
        location="location2"
        inventory.append("sword")
    else:
        location="location2"
    return location

def playLocation3():
    global inventory
    print("You reach what looks like a rudimentary camp site.")
    print("You notice a fire pit in the middle of the camp site.")
    print("Scattered around the fire pit are the many bones picked clean of flesh.")
    print("Ominously some of the bones look human like.")
    print("Just as you finish surveying the scene you here a roar.")
    print("Running towards you, wielding a large club is an Ogre.")
    print("You can either (f)lee to the south, (a)ttack the Ogre or try to (t)alk with the Ogre.")
    response=input("> ")
    if (response=="f"):
        print("You run away from the Ogre at full speed")
        print("The Ogre pursues for a while but eventually gives up.")
        print("You survive another day.")
        print("Once you know you are safe you continue your journey southward at a slower pace.")
        location="location1"
    elif (response=="a" and "sword" in inventory):
        print("As the Ogre draws near you draw your sword.")
        print("With one hefty swing of your sword you slay the Ogre.")
        print("You search the body of the Ogre and find a key.")
        print("You take the key and head back towards the clearing with the chest.")
        location="location1"
        inventory.append("key")
    elif (response=="a"):
        print("You fight valiantly with the Ogre.")
        print("You manage to inflict several telling blows against it but it fights on.")
        print("Having dodged several sweeping blows, the Ogre brings its club down upon you with a crash.")
        print("...")
        location="finish"
    elif (response=="t"):
        print("You attempt to make conversation with the Ogre but this only makes it more angry.")
        location="location3"
    else:
        location="location3"
    return location

    
while (location!="finish"):
    if (location=="start"):
        location=playStart()
    elif (location=="location1"):
        location=playLocation1()
    elif (location=="location2"):
        location=playLocation2()
    elif (location=="location3"):
        location=playLocation3()
    else:
        break

Download source code [Stage 3 code]

Limitations

When we play through the game we notice that when the player returns to an area we are presented with the same options. For example, when returning to location 3 after having already slaying the Ogre the player encounters a new Ogre. To address this limitation we need some way of tracking the events that have occurred within a particular area.