Category Archives: Python

Developing a text-based adventure game (Part 4)

Developing a battle system

In this tutorial we will look at the development of a simple battle system. We restrict our attention to a one-on-one battle.
We will implement a Dungeons and Dragons like battle system, where the player and the monster they are facing have a number of hitpoints. The player and monster will take it in turns to attack the other entity. Each attack causes damage that reduces the hitpoints of the entity that was hit. The damage caused by an attack will be randomly determined, but must lie between a predetermined minimum and maximum damage. The battle ends when the hitpoints of either the player or monster are reduced to zero or below.

Initial implementation

The battle mechanics will be encapsulated within the battle function.
The function takes seven inputs. The first three inputs are used for player information, describing the number of hitpoints that they had at the beginning of the combat, the minimum damage that one of their attacks does, and the maximum damage done by their attack. The remaining inputs are used for the title/name of the monster, the monster’s hitpoints and the minimum and maximum damage done by their attack.

The code for the battle mechanics is shown below. The battle function is tested by calling the function at the end of the code listing.

import random

def battle(hitpoints, minDamage, maxDamage,
           monsterTitle, monsterHitpoints,
           monsterMinDamage, monsterMaxDamage):
    playerAttack=True
    while (hitpoints>0 and monsterHitpoints>0):
        if (playerAttack):
            damage=random.randint(minDamage, maxDamage)
            monsterHitpoints=monsterHitpoints-damage
            print("You did {0:d} hitpoints damage".format(damage))
            if (monsterHitpoints <= 0):
                print("The {0} is dead".format(monsterTitle))
            playerAttack=False
        else:
            damage=random.randint(monsterMinDamage, monsterMaxDamage)
            hitpoints=hitpoints-damage
            print("You took {0:d} hitpoints damage".format(damage))
            if (hitpoints <= 0):
                print("You have been killed".format(monsterTitle))
            playerAttack=True

battle(20, 1, 8, "Ogre", 25, 1, 12)

Download source code [Battle mechanics (version 1)].

To randomise the amount of damage done we need to import the random module. From this module we use the randint function, which generates a random integer between the lower and upper bounds which are passed as inputs to the function. To keep track of whether it is the player’s turn or the monster’s turn we use a boolean-valued variable called playerAttack (line 6). This variable can be set to either true or false. A while loop is used to provide a sequence of attacks that continue until either the player or monster is reduced to zero or less hitpoints (line 7). Lines 8-14 of the code implement the player’s attack. The randint function is used to determine the amount of damage done, this is then subtracted from the monster’s hitpoints. If the monster’s hitpoints are reduced below zero, then the monster is declared dead. Lines 15-21 use a similar sequence of code to implement the monster’s attack. The boolean-valued playerAttack variable is updated after each attack to ensure that the player and monster have alternate attacks.

Determining if attacks hit

In the first version of the battle mechanics code all attacks hit. However it is more realistic that only some attacks hit – the attacker may have just swung wildly and missed their target, or the defender may have managed to dodge the attack, or perhaps even parry the attack with their shield. The improve the battle mechanics we adapt the code so that only some of the attacks hit their target. In the code shown below we assume that the player has a 50% chance of hitting their opponent, while the monster only has a 25% chance of hitting.

def battle(hitpoints, minDamage, maxDamage,
           monsterTitle, monsterHitpoints,
           monsterMinDamage, monsterMaxDamage):
    playerAttack=True
    while (hitpoints>0 and monsterHitpoints>0):
        if (playerAttack):
            hitRoll=random.randint(1,20)
            if (hitRoll>=11):
                damage=random.randint(minDamage, maxDamage)
                monsterHitpoints=monsterHitpoints-damage
                print("You hit!")
                print("You did {0:d} hitpoints damage".format(damage))
                if (monsterHitpoints <= 0):
                    print("The {0} is dead".format(monsterTitle))
            else:
                print("You missed")
            playerAttack=False
        else:
            hitRoll=random.randint(1,20)
            if (hitRoll>=16):
                damage=random.randint(monsterMinDamage, monsterMaxDamage)
                hitpoints=hitpoints-damage
                print("{0} hit".format(monsterTitle))
                print("You took {0:d} hitpoints damage".format(damage))
                if (hitpoints <= 0):
                    print("You have been killed".format(monsterTitle))
            else:
                print("{0} missed".format(monsterTitle))
            playerAttack=True

Download source code [Battle mechanics (version 2)].

Lines 7 and 8 of the above code are used to determine whether or not the player hits the monster. A random integer between 1 and 20 is generated and if this number if 11 or larger the player hits the monster, otherwise the attack results in a miss. Similar code is given on lines 19-20 to determine whether or not the monster hits.

Improvements

We improve the code by firstly generalising the hit chances of the player and monster by adding these as variables. We also add a return statement at the end of the code showing how many hitpoints the player has at the end of the battle. This will be used when we call the function in the game to keep track of the ongoing health of the player as well as to determine whether or not the game continues.

def battle(hitpoints, minDamage, maxDamage, toHit,
           monsterTitle, monsterHitpoints,
           monsterMinDamage, monsterMaxDamage, monsterToHit):
    playerAttack=True
    while (hitpoints>0 and monsterHitpoints>0):
        if (playerAttack):
            hitRoll=random.randint(1,20)
            if (hitRoll>=toHit):
                damage=random.randint(minDamage, maxDamage)
                monsterHitpoints=monsterHitpoints-damage
                print("You hit!")
                print("You did {0:d} hitpoints damage".format(damage))
                if (monsterHitpoints <= 0):
                    print("The {0} is dead".format(monsterTitle))
            else:
                print("You missed")
            playerAttack=False
        else:
            hitRoll=random.randint(1,20)
            if (hitRoll>=monsterToHit):
                damage=random.randint(monsterMinDamage, monsterMaxDamage)
                hitpoints=hitpoints-damage
                print("{0} hit".format(monsterTitle))
                print("You took {0:d} hitpoints damage".format(damage))
                if (hitpoints <= 0):
                    print("You have been killed".format(monsterTitle))
            else:
                print("{0} missed".format(monsterTitle))
            playerAttack=True
    return hitpoints

Testing the balance of the battle mechanics

Before we add the battle mechanics to the game we need to test it to make sure it has the right balance. We don’t want to always die when they battle a monster, but at the same time the monster should not be a pushover. Furthermore, if the player acquires an item that is supposed to give them an edge in combat then it should indeed give them and edge. To test the battle function we need to call it many times and then analyse the number of player wins versus the number of player losses. The best way to do this to use a for loop as shown in the code below.

wins=0
for i in range(0,1000):
    hps=battle(20, 1, 8, 11, "Ogre", 25, 1, 12, 16)
    if (hps>0):
        wins=wins+1

print(wins)

In this case the player has 20 hitpoints and a to-hit-roll of 11 (meaning they will need at least an 11 on a roll between 1 and 20 to hit). The monster has 25 hitpoints, does more damage, but is only half as likely to hit. Running the battle mechanics 1000 times with the given configuration results in the player winning 60.5% of the battles. This is probably too high in the situation when the player has not found the sword, but too low if they have found the sword. This suggests we need to adjust the player statistics for these two scenarios. We will use trial an error to get a 20% win chance if they don’t have the sword and an 80% win chance if they do have the sword.

Modifying the player’s damage to 1-4 hps and increase the to-hit-roll to 12 gives the player a win percentage of approximately 20%. On the other hand, if we change the player’s damage to 4-10 and decrease the to-hit-roll to 11, the player will have a win percentage of 79%.

Integrating battle mechanics into the game

To integrate the battle mechanics into the game we modify the playLocation3 function. When the player selects the attack option, the code will call the battle function. The values passed to the battle function depend on whether or not the player has picked up the sword earlier in the game, remembering that possessing the sword gives the player a much better chance of defeating the Ogre (compare lines 20 and 23).

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"):
        if ("sword" in inventory):
            print("As the Ogre draws near you draw your sword.")
            hps=battle(20, 4, 10, 11, "Ogre", 25, 1, 12, 16)
        else:
            print("You pick up a fallen branch from the ground and prepare to battle the Ogre")
            hps=battle(20, 1, 4, 12, "Ogre", 25, 1, 12, 16)
        if (hps >0):
            print("You have felled the evil beast.")
            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")
        else:
            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

Download source code [Stage 4 code]

Developing a text-based adventure game (Part 3)

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.

Developing a text-based adventure game (Part 2)

Adding and linking multiple locations

In the first part of the text-based adventure game development tutorial we described how print, input and conditional statements can be used to implement an interactive encounter. We also noted that although nested conditional statements could be used to link multiple encounters, this approach will quickly lead to the development of spaghetti code.

In this tutorial we investigate a solution to developing linked encounters using functions and loops.

Game synopsis

We begin by providing a brief synopsis for our proposed game.

The player quests to retrieve the magical sippy cup, which is believed to grant eternal life whoever has it in their possession. However the sippy cup has been stolen, and
it is the responsibility of our brave adventurer to rescue it. The game consists of three location.

  • Location 1 contains a locked chest. The chest cannot be opened without the key which is not in this location.
  • Location 2 contains a magical sword hidden on a ledge at the top of a steep cliff face.
  • Location 3 contains an evil Ogre who happens to have the key to the chest in his possession. The Ogre can only be slain if the player has found the magical sword.

Game layout

Having come up with an overall synopsis for the game, the next step is to consider the game layout and how the locations connect to each other. This can be done with pencil and paper, or a simple digital tool such as OneNote or Paint.

In the layout design shown below there are three locations in addition to a start and finish location. Directed arcs show the possible transitions between locations; for example the start location links to location 1 and location 2.

The diagram shows the layout for a simple non-linear text based adventure game. The nodes of the diagram represent the game locations, while the edges represent connections between these locations.

Initial layout for a simple non-linear text based adventure game

Location details

Having designed the layout of the adventure, we next need to define the narrative for each location. This should include the contextual information as well as the options available to the player. For example, the narrative for the start location would be as follows.

You are on a quest to retrieve the find the lost sippy cup. The sippy cup is a priceless treasure that grants the user eternal life. You come to a junction. You can either go west or east.

Linking locations

The final step before we can begin code is to define the links between the different locations. We need to provide details of the actions or events that need to occur to move from one location to another.

Shows how locations can be linked with events.

Game layout with linking events

Initial coding step

To code the flowchart shown earlier we firstly add a variable to represent the location that the player is currently in. Line 1 of the code listing below
initialises the location variable to the start location. Lines 20-24 contain a while loop which is run repeatedly until the condition is true.
In this case, the code inside the while loop is repeated until the player is in the finish location. The != operator checks that the first argument is not equal to the second argument. A conditional statement is used to check the current location and call a function corresponding to the location the player is currently in. The else branch at the end uses a break statement to exit the loop. This is used to ensure that the game does not get stuck in an infinite loop.

location="start"

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

while (location!="finish"):
    if (location=="start"):
        location=playStart()
    else:
        break

Download source code [Stage 1 code]

Defining functions

To avoid monolithic code and to aid with the understandability of the code we employ functions to break up the code. In lines 3-18 a function that handles the starting location is defined. Line 3 of the function declares the name of the function, in this case playStart, together with any inputs that are passed to the function. The first line of a function definition must begin with the def keyword and end with a colon. Input arguments are specified inside of brackets – note that if the function does not have any inputs then an empty of brackets are used. Lines 4-18 contain the body of the function – this is the code that runs when the function is called. Lines 4-7 output the narrative text using print statements. Line 8 prompts the user for their action and assigns the value to the response variable. If the player enters ‘w’, then “location1” is assigned to a local variable used to store the location. Otherwise, if the player enters ‘e’, then “location2” is stored in the local variable. In all other cases the local variable is assigned the “start” location. On line 18 the value stored in the local variable is returned as the result of the function.

Looking at line 22 of the code again, on this line the playStart function is called. Since this function has no inputs we do not pass any values, however we still need to include the empty pair of brackets. The result returned by the function is assigned to the location variable, which will in turn be used in the next iteration of the loop to determine the next function to call.

Coding the other locations

Once we have got the first location coded we follow a similar process to add the additional locations. A function needs to be defined for each of the additional locations and extra cases need to be added in the while loop.

location="start"

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():
    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"):
        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():
    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"
    else:
        location="location2"
    return location

def playLocation3():
    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"):
        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 2 code]

Limitations

If you download and run the code you may notice that collecting the sword in location 2 still does not allow you to kill the Ogre. We will address this issue in the next tutorial using a simple inventory solution.

Developing a text-based adventure in Python (Part 1)

Getting started with basic commands

In this series we will develop a text-based adventure game using the Python programming language. Although various development environments exist that can be used to help design such games (e.g. Twine and ADRIFT) we will strip it down to the bare bones, relying only on the basic Python IDLE programming environment for programming and pencil and paper for design.

It is assumed that you have already setup Python with IDLE and are familiar with the basics of creating, saving and running a new program. Download an appropriate version of Python if you do not have it installed already.

Print and input statements

We begin by using print and input statements to tell part of the story to the player and to get their action. This is shown in the code snippet below.

print("You have reached a cross road. You can go north,\nwest or east, or return south from where you came.")
print("Enter 'n', 's', 'e' or 'w'")
direction=input("> ")

Line 1 of the code uses a print statement to provide contextual (narrative) information to the player. Note that the text within the print statement must be enclosed in quotes.
In line 2 the player options are presented using a print statement.
An input statement is used in line 3 of the code to get the responses from the player. The prompt for the input statement is a ‘>’ symbol followed by a space. The response entered by the player is assigned to a variable called direction.

Gotchas

  1. Make sure you the text used in print statements are enclosed in a pair of quotes.
  2. An open bracket, ‘(‘, must be placed immediately after the print and input keywords. A matching closed bracket, ‘)’ must be placed at the end of the the statement.
  3. Remember to use a backslash, not a forward slash, for special commands like the newline command \n.
  4. When assigning a value to a variable, as done on line 3, use a single = symbol.

Conditional statements

The next step in the development is to process the player’s response.
The code shown below takes the player’s response and determines an appropriate course of action using a conditional statement.

if (direction=="n" or direction=="N"):
    print("You meet a Bugbear")
elif (direction=="e" or direction=="E"):
    print("You discover a treasure chest. It is locked!!")
elif (direction=="w" or direction=="W"):
    print("You have come a dead end.")
elif (direction=="s" or direction=="S"):
    print("Running away are you? You big chicken!")
else:
    print("This is not an option")

Lines 4 and 5 represent the first case of the conditional statement. It tests whether the player inputted the direction ‘n’ or ‘N’ to move north. If the player did
enter this direction then the code uses a print statement to indicate that they have encountered a Bugbear. Lines 6-11 of the code handle the three other directions that the player may have selected. To do this elif statements are used, which is a shorthand for else if. The elif statements include a test condition that evaluates to either true or false – the code on the line following the condition is only executed when the condition is true. Lines 12 and 13 of the code handle all other inputs from the user. This is done using an else statement, which does not include condition since it covers all remaining cases not covered by the previous conditions.

Gotchas

There are several common mistakes to try avoid when using conditional statements:

  1. When testing equality in conditions, make sure you use double equal symbols, ==.
  2. A colon must be placed at the end of an if or elif condition.
  3. The statements that follow each of the if, elif and else lines of code must be indented by a full tab.
  4. You must not include a condition after the else keyword.

Nested if statements

The code shown above implements one step in the text adventure – the user is presented with a narrative, they select an option, a brief description of what they observe is given, then the game ends. This game is clearly not going to engage the player – the game needs to continue until some end resolution is achieved.

Consider the encounter with Bugbear that occurs when the player opts to move north. When this encounter occurs we want to again present the player with some options. One way to do this in code is to use a nested conditional statement, i.e. placing an additional conditional statement inside of the original conditional statement. This is shown in the code below where a nested conditional statement is used to handle the encounter with the Bugbear.

print("You have reached a cross road. You can go north,\nwest or east, or return south from where you came.")
print("Enter 'n', 's', 'e' or 'w'")
direction=input("> ")

if (direction=="n" or direction=="N"):
    print("You meet a Bugbear")
    print("You have the following options")
    print("[A]ttack the Bugbear")
    print("[T]alk to the Bugbear")
    print("[R]un away from the Bugbear")
    response=input("> ")
    if (response=="A" or response=="a"):
        print("Bad move - the Bugbear is too strong for you.")
        print("You are badly wounded. This is the end of your adventure.")
    elif (response=="T" or response=="t"):
        print("The Bugbear is very wise.")
        print("He imparts great wisdom upon you.")
        print("Your intelligence is increased.")
    elif (response=="R" or response=="r"):
        print("You manage to escape the Bugbear.")
        print("You have returned to the crossroad")
elif (direction=="e" or direction=="E"):
    print("You discover a treasure chest. It is locked!!")
elif (direction=="w" or direction=="W"):
    print("You have come a dead end.")
elif (direction=="s" or direction=="S"):
    print("Running away are you? You big chicken!")
else:
    print("This is not an option")

Download source code [conditionals.py].

Lines 6-21 are the updated portion of the code. Additional print statements are added to present options to the player. In this case the options are single word commands with the key letter highlighted, indicating the command that the player would need to enter. Line 11 uses an input statement to get the player’s response and store it in a variable. Lines 12-21 contain a conditional statement that selects an appropriate action based on the response given by the player. Note the indentation of commands used in the nested conditional statements. Indentation is critical in Python – it is used to determine where blocks of code start and end.

Limitations

Using nested conditional statements works in this case, but quickly becomes very messy, resulting in what is referred to as spaghetti code. Furthermore, if we want to be able to return to areas/encounters that have already been visited then this code solution will not work at all.