Dragon Realm

Experimenting with the not Operator

The third boolean operator is not. The not operator is different from every other operator we've seen before, because it only works on one value, not two. There is only value on the right side of the not keyword, and none on the left. The not operator will evaluate to True as False and will evaluate False as True.

Try typing the following into the interactive shell:

>>> not True
False
>>> not False
True
>>> True not
SyntaxError: invalid syntax (<pyshell#0>, line 1)

Notice that if we put the boolean value on the left side of the not operator results in a syntax error.

We can use both the and and not operators in a single expression. Try typing True and not False into the shell:

>>> True and not False True

Normally the expression True and False would evaluate to False. But the True and not False expression evaluates to True. This is because not False evaluates to True, which turns the expression into True and True, which evaluates to True.

Truth Tables

If you ever forget how the boolean operators work, you can look at these charts, which are called truth tables:

Getting the Player's Input

14 . print('Which cave will cave = input ()
15 . you go into? (1 or 2)')

Here, the player is asked to enter which cave they chose to enter by typing in 1 or 2 and hitting Enter. Whatever string the player typed will be stored in cave. After this code is executed, we jump back to the top of the while statement and recheck the condition. Remember that the line was:

13 . while cave != '1' and cave != '2':

If this condition evaluates to True, we will enter the while-block again and ask the player for a cave number to enter. But if the player typed in 1 or 2, then the cave value will either be '1' or '2'. This causes the condition to evaluate to False, and the program execution will continue on past the while loop.

The reason we have a loop here is because the player may have typed in 3 or 4 or HELLO. Our program doesn't make sense of this, so if the player did not enter 1 or 2, then the program loops back and asks the player again. In fact, the computer will patiently ask the player for the cave number over and over again until the player types in 1 or 2. When the player does that, the while-block's condition will be False, and we will jump down past the while-block and continue with the program.

Return Values

17.     return cave

This is the return keyword, which only appears inside def-blocks. Remember how the input() function returns the string value that the player typed in? Or how the randint () function will return a random integer value? Our function will also return a value. It returns the string that is stored in cave.

This means that if we had a line of code like spam = chooseCave(), the code inside chooseCave() would be executed and the function call will evaluate to chooseCave()'s return value. The return value will either be the string '1' or the string '2'. (Our while loop guarantees that chooseCave() will only return either '1' or '2'.)

The return keyword is only found inside def-blocks. Once the return statement is executed, we immediately jump out of the def-block. (This is like how the break statement will make us jump out of a while-block.) The program execution moves back to the line that had called the function.

You can also use the return keyword by itself just to break out of the function, just like the break keyword will break out of a while loop.

Variable Scope

Just like the values in our program's variables are forgotten after the program ends, variables created inside the function are forgotten after the execution leaves the function. Not only that, but when execution is inside the function, we cannot change the variables outside of the function, or variables inside other functions. The variable's scope is this range that variables can be modified in. The only variables that we can use inside a function are the ones we create inside of the function (or the parameter variables, described later). That is, the scope of the variable is inside in the function's block. The scope of variables created outside of functions is outside of all functions in the program.

Not only that, but if we have a variable named spam created outside of a function, and we create a variable named spam inside of the function, the Python interpreter will consider them to be two separate variables. That means we can change the value of spam inside the function, and this will not change the spam variable that is outside of the function. This is because these variables have different scopes, the global scope and the local scope.

Global Scope and Local Scope

We have names for these scopes. The scope outside of all functions is called the global scope. The scope inside of a function is called the local scope. The entire program has only one global scope, and each function has a local scope of its own.

Variables defined in the global scope can be read outside and inside functions, but can only be modified outside of all functions. Variables defined in a function's local scope can only be read or modified inside that function.

Specifically, we can read the value of global variables from the local scope, but attempting to change the value in a global variable from the local scope will leave the global variable unchanged. What Python actually does is create a local variable with the same name as the global variable. But Python will consider these to be two different variables.

Look at this example to see what happens when you try to change a global variable from inside a local scope. Remember that the code in the funky() function isn't run until the funky() function is called. The comments explain what is going on:

# This block doesn't run until funky() is called: 
def funky():
     # We read the global variable's value: 
     print(spam)   # 42
     # We create a local variable named "spam"
     # instead of changing the value of the global
     # variable "spam": 
     spam = 99
     # The name "spam" now refers to the local
     # variable only for the rest of this
     # function: 
     print(spam) # 99

# A global variable named "spam": 
spam = 42

# Call the funky() function: 
funky()
# The global variable was not changed in funky(): 
print(spam)  # 42

It is important to know when a variable is defined because that is how we know the variable's scope. A variable is defined the first time we use it in an assignment statement. When the program first executes the line:

12 .    cave = ' '

...the variable cave is defined.

If we call the chooseCave() function twice, the value stored in the variable the first time won't be remember the second time around. This is because when the execution left the chooseCave() function (that is, left chooseCave()'s local scope), the cave variable was forgotten and destroyed. But it will be defined again when we call the function a second time because line 12 will be executed again.

The important thing to remember is that the value of a variable in the local scope is not remembered in between function calls.

Defining the checkCave() Function

19. def checkCave(chosenCave):

Now we are defining yet another function named checkCave(). Notice that we put the text chosenCave in between the parentheses. The variable names in between the parentheses are called parameters.

Remember, for some functions like for the str() or randint(), we would pass an argument in between the parentheses:

>>> str(5)
'5'
>>> random.randint(1, 20)
14

When we call checkCave(), we will also pass one value to it as an argument. When execution moves inside the checkCave() function, a new variable named chosenCave will be assigned this value. This is how we pass variable values to functions since functions cannot read variables outside of the function (that is, outside of the function's local scope). Parameters are local variables that get defined when a function is called. The value stored in the parameter is the argument that was passed in the function call.

Parameters

For example, here is a short program that demonstrates parameters. Imagine we had a short program that looked like this:

def sayHello(name):
print('Hello, ' + name)

print('Say hello to Alice.')
fizzy = 'Alice'
sayHello(fizzy)
print('Do not forget to say hello to Bob.')
sayHello('Bob')

If we run this program, it would look like this:

Say hello to Alice.
Hello, Alice
Do not forget to say hello to Bob.
Hello, Bob

This program calls a function we have created, csayHello() and first passes the value in the fizzy variable as an argument to it. (We stored the string 'Alice' in fizzy.) Later, the program calls the sayHello() function again, passing the string 'Bob' as an argument.

The value in the fizzy variable and the string 'Bob' are arguments. The name variable in sayHello() is a parameter. The difference between arguments and parameters is that arguments are the values passed in a function call, and parameters are the local variables that store the arguments. It might be easier to just remember that the thing in between the parentheses in the def statement is an parameter, and the thing in between the parentheses in the function call is an argument.

We could have just used the fizzy variable inside the sayHello() function instead of using a parameter. (This is because the local scope can still see variables in the global scope.) But then we would have to remember to assign the fizzy variable a string each time before we call the sayHello() function. Parameters make our programs simpler. Look at this code:

def sayHello():
    print('Hello, ' + fizzy)

print('Say hello to Alice.')
fizzy = 'Alice'
sayHello()
print('Do not forget to say hello to Bob.')
sayHello()

When we run this code, it looks like this:

Say hello to Alice.
Hello, Alice
Do not forget to say hello to Bob.
Hello, Alice

This program's sayHello() function does not have a parameter, but uses the global variable fizzy directly. Remember that you can read global variables inside of functions, you just can't modify the value stored in the variable.

Without parameters, we have to remember to set the fizzy variable before calling sayHello(). In this program, we forgot to do so, so the second time we called sayHello() the value of fizzy was still 'Alice'. Using parameters makes function calling simpler to do, especially when our programs are very big and have many functions.

Local Variables and Global Variables with the Same Name

Now look at the following program, which is a bit different. To make it clear to see, the global variable has been bordered with a line, and the local variable has been bordered with dots;.

def spam(ImyName!):
    print('Hello, ' + ImyName!)
    ImyName! = 'Waffles'
    print('Your new name is ' + ImyName!)

myName = 'Albert'
spam(myName)
print('Howdy, ' + myName)

If we run this program, it would look like this:

Hello, Albert
Your new name is Waffles
Howdy, Albert

This program defines a new variable called myName and stores the string 'Albert' in it. Then the program calls the spam() function, passing the value in myName as an argument. The execution moves to the spam() function. The parameter in spam() is also named myName, and has the argument value assigned to it. Remember, the myName inside the spam() function (the local scope) is considered a different variable than the myName variable outside the function (the global scope).

The function then prints 'Hello, Albert', and then on the next line changes the value in myName to 'Waffles'. Remember, this only changes the local myName variable that is inside the function. The global myName variable that is outside the function still has the value 'Albert' stored in it.

The function now prints out 'Your new name is Waffles', because the myName variable in the local scope has changed to 'Waffles'. The execution has reached the end of the function, so it jumps back down to where the function call was. The local myName is destroyed and forgotten. The next line after that is print('Howdy, ' + myName), which will display Howdy, Albert.

Remember, the myName outside of functions (that is, in the global scope) still has the value 'Albert', not 'Waffles'. This is because the myName in the global scope and the myName in spam()'s local scope are different variables, even though they have the same name.

Where to Put Function Definitions

A function's definition (where we put the def statement and the def-block) has to come before you call the function. This is like how you must assign a value to a variable before you can use the variable. If you put the function call before the function definition, you will get an error. Look at this code:

sayGoodBye()
def sayGoodBye():
print('Good bye!')

If you try to run it, Python will give you an error message that looks like this:

Traceback (most recent call last):
File "C:\Python31\foo.py", line 1, in <module> sayGoodBye()
NameError: name 'sayGoodBye' is not defined

To fix this, put the function definition before the function call:

def sayGoodBye():
print('Good bye!')
sayGoodBye()

Displaying the Game Results

Back to the game's source code:

20.     print('You approach the cave...') 
21.    time.sleep(2)

We display some text to the player, and then call the time.sleep() function. Remember how in our call to randint(), the function randint() is inside the random module? In the Dragon Realm game, we also imported the time module. The time module has a function called sleep() that will pause the program for a few seconds. We pass the integer value 2 as an argument to the time.sleep() function to tell it to pause for exactly 2 seconds.

22 .     print('It is dark and spooky...') 
23.    time.sleep(2)

Here we print some more text and wait again for another 2 seconds. These short pauses add suspense to the game, instead of displaying all the text all at once. In our jokes program, we called the input() function to wait until the player pressed the Enter key. Here, the player doesn't have to do anything at all except wait.

24.     print('A large dragon jumps out in front of you! He
opens his jaws and...') 
25 .     print() 
26.    time.sleep(2)

What happens next? And how does the program decide what happens?

Deciding Which Cave has the Friendly Dragon

28.     friendlyCave = random.randint(1, 2)

Now we are going to have the program randomly chose which cave had the friendly dragon in it. Our call to the random.randint() function will return either the integer 1 or the integer 2, and store this value in a variable called friendlyCave.

30.      if chosenCave == str(friendlyCave):
31.         print('Gives you his treasure!')

Here we check if the integer of the cave we chose ('1' or '2') is equal to the cave randomly selected to have the friendly dragon. But wait, the value in chosenCave was a string (because input() returns strings) and the value in friendlyCave is an integer (because random.randint() returns integers). We can't compare strings and integers with the == sign, because they will always be different ('1' does not equal 1).

Comparing values of different data types with the == operator will always evaluate to False.

So we are passing friendlyCave to the str() function, which returns the string value of friendlyCave.

What the condition in this if statement is really comparing is the string in chosenCave and the string returned by the str() function. We could have also had this line instead:

if int(chosenCave) == friendlyCave:

Then the if statement's condition would compare the integer value returned by the int () function to the integer value in friendlyCave. The return value of the int() function is the integer form of the string stored in chosenCave.

If the if statement's condition evaluates to True, we tell the player they have won the treasure.

32.     else:
33.         print('Gobbles you down in one bite!')

Line 32 has a new keyword. The else keyword always comes after the if-block. The else-block that follows the else keyword executes if the condition in the if statement was False. Think of it as the program's way of saying, "If this condition is true then execute the if-block or else execute the else-block."

Remember to put the colon (the : sign) after the else keyword.

The Colon :

You may have noticed that we always place a colon at the end of if, else, while, and def statements. The colon marks the end of the statement, and tells us that the next line should be the beginning of a new block.

Where the Program Really Begins

35. playAgain = 'yes'

This is the first line that is not a def statement or inside a def-block. This line is where our program really begins. The previous def statements merely defined the functions, it did not run the code inside of the functions. Programs must always define functions before the function can be called. This is exactly like how variables must be defined with an assignment statement before the variable can be used in the program.

36. while playAgain == 'yes' or playAgain == 'y':

Here is the beginning of a while loop. We enter the loop if playAgain is equal to either 'yes' or 'y'. The first time we come to this while statement, we have just assigned the string value 'yes' to the playAgain variable. That means this condition will be True.

Calling the Functions in Our Program

38.     displayIntro()

Here we call the displayIntro() function. This isn't a Python function, it is our function that we defined earlier in our program. When this function is called, the program execution jumps to the first line in the displayIntro() function on line 5. When all the lines in the function are done, the execution jumps back down to the line after this one.

40.    caveNumber = chooseCave()

This line also calls a function that we created. Remember that the chooseCave() function lets the player type in the cave they choose to go into. When the return cave line in this function executes, the program execution jumps back down here, and the local variable cave's value is the return value of this function. The return value is stored in a new variable named caveNumber. Then the execution moves to the next line.

42.    checkCave(caveNumber)

This line calls our checkCave() function with the argument of caveNumber's value. Not only does execution jump to line 20, but the value stored in caveNumber is copied to the parameter chosenCave inside the checkCave() function. This is the function that will display either 'Gives you his treasure!' or 'Gobbles you down in one bite!', depending on the cave the player chose to go in.

Asking the Player to Play Again

44.     print('Do you want to play again? (yes or no)')
45.     playAgain = input()

After the game has been played, the player is asked if they would like to play again. The variable playAgain stores the string that the user typed in. Then we reach the end of the while-block, so the program rechecks the while statement's condition: while playAgain == 'yes' or playAgain == 'y'

The difference is, now the value of playAgain is equal to whatever string the player typed in. If the player typed in the string 'yes' or 'y', then we would enter the loop again at line 38.

If the player typed in 'no' or 'n' or something silly like 'Abraham Lincoln', then the while statement's condition would be False, and we would go to the next line after the while-block. But since there are no more lines after the while-block, the program terminates.

But remember, the string 'YES' is different from the string 'yes'. If the player typed in the string 'YES', then the while statement's condition would evaluate to False and the program would still terminate.

We've just completed our second game! In our Dragon Realm game, we used a lot of what we learned in the "Guess the Number" game and picked up a few new tricks as well. If you didn't understand some of the concepts in this program, then read the summary at the end of this chapter, or go over each line of the source code again, or try changing the source code and see how the program changes. In the next chapter we won't create a game, but a computer program that will create secret codes out of ordinary messages and also decode the secret code back to the original message.

We went through the source code from top to bottom. If you would like to go through the source code in the order that the execution flows, then check out the online tracing web site for this program at the URL http://inventwithpython.com/traces/dragon.html.

Designing the Program

Dragon Realm was a pretty simple game. The other games in this book will be a bit more complicated. It sometimes helps to write down everything you want your game or program to do before you start writing code. This is called "designing the program."

For example, it may help to draw a flow chart. A flow chart is a picture that shows every possible action that can happen in our game, and in what order. Normally we would create a flow chart before writing our program, so that we remember to write code for each thing that happens in the game. Figure 6.2 is a flow chart for Dragon Realm.

To see what happens in the game, put your finger on the "Start" box and follow one arrow from the box to another box. Your finger is kind of like the program execution. Your finger will trace out a path from box to box, until finally your finger lands on the "End" box. As you can see, when you get to the "Check for friendly or hungry dragon" box, the program could either go to the "Player wins" box or the "Player loses" box. Either way, both paths will end up at the "Ask to play again" box, and from there the program will either end or show the introduction to the player again.

Summary

In the "Dragon Realm" game, we created our own functions that the main section of the program called. You can think of functions as mini-programs within our program. The code inside the function is run when our program calls that function. By breaking up our code into functions, we can organize our code into smaller and easier to understand sections. We can also run the same code by placing it inside of a function, instead of typing it out each time we want to run that code.

The inputs for functions are the arguments we pass when we make a function call. The function call itself evaluates to a value called the return value. The return value is the output of the function.

We also learned about variable scopes. Variables that are created inside of a function exist in the local scope, and variables created outside of all functions exist in the global scope. Code in the global scope can not make use of local variables. If a local variable has the same name as a variable in the global scope, Python considers it to be a separate variable and assigning new values to the local variable will not change the value in the global variable.

Variable scopes might seem complicated, but they are very useful for organizing functions as pieces of code that are separate from the rest of the function. Because each function has it's own local scope, we can be sure that the code in one function will not cause bugs in other functions.

All nontrivial programs use functions because they are so useful, including the rest of the games in this book. By understanding how functions work, we can save ourselves a lot of typing and make our programs easier to read later on.