Object-oriented design is a powerful programming paradigm that can be intimidating to anyone trying to learn how to use it. I worked through a number of excellent tutorials and books on the subject, but struggled to apply the concepts to my own code. I decided to convert a working program that didn’t use OOP and wrap it in a single class.

The program I chose was Guess the Number written by Al Sweigart for his book Invent Your Own Computer Games with Python. This book is an excellent way to learn Python 3 (and also a fun way!)

After you’ve looked at the code, you’ll notice that the game doesn’t include any options. It runs only once. There are no functions, only global variables. It does well for its purpose as a program that teaches beginning concepts, but it’s not extendable as-is. You could certainly wrap it all in a single “main” function, then wrap that function in a while loop, and that would solve a few problems, but I wanted to go beyond those simple additions.

The full code for the class-extended program is here. Below, I will explain the additions and changes from adding object-orientation.

class Game:
    def __init__(self, high_num, myName):
        self.high_num = high_num          #the highest possible number that can be chosen
        self.myName = myName              #the user's name
        self.guessesTaken = 0             #the initial number of guesses taken
        self.number = random.randint(1, self.high_num)  #the number the computer chooses
        self.guess = None                 #the user's guess

To initiate the class, we use the __init__ method. Adding high_num and myName parameters allowed me to add some configurations to the game. high_num allows me to set a game to go up to 20 (as in the original) or lower (to make the game easier) or higher (to make the game more difficult). [I’ll discuss this further when we get to the main() block.] A huge advantage from the standpoint of code-organization was being able to set most of the variables in the game at the outset. These include the number of guesses taken, the number that the user tries to guess, and the user’s guess

class Game:

    def get_guess(self):
        print('Take a guess.')
            self.guess = int(input())   # the state of this variable is changed of the class instance!
                                        # see more below.
        except ValueError:
            print('Not a valid guess.')
            return False

        return True

get_guess() is one of the two methods in the class. It modifies the self.guess if the user inputs an integer, then returns True. If not, the function returns False after an error message. It doesn’t add a guess to self.guessesTaken.

class Game:

    def play(self):
        print('Well, {},  I am thinking of a number between 1 and {}.'
            .format(self.myName, self.high_num))
        while self.guessesTaken < 6:
            if not self.get_guess():
            # else: self.guess gets changed in get_guess function

            self.guessesTaken += 1

            if self.guess < self.number:
                print('Your guess is too low.')

            if self.guess > self.number:
                print('Your guess is too high.')

            if self.guess == self.number:

        if self.guess == self.number:
            print('Good job, {}! You guessed my number in {} guesses!'
                .format(self.myName, self.guessesTaken))
            print('Nope. The number I was thinking of was', self.number)

I find it slightly cumbersome to put self in front of so many variables, but it’s only necessary if the variables determine the state of the game. (If I wanted to put a non-state-changing variable in, there’s no need for self in front of it, but I wouldn’t be able to access the variable outside of the function.) The great thing about self is that you can modify the variable in multiple functions without returning it. This is excellent from a design perspective - there’s no need to pass a parameter from a function return to another function’s input.

def main():
    print('Hello! What is your name?')
    myName = input()
    print("Right on, {}! We've got 2 brands of game here.".format(myName))

    while True:
        print('Type 1 for easy or 2 for difficult guessing game. Type q to quit.')
        user_choice = input()

        if user_choice.lower().startswith('q'):

            user_choice = int(user_choice)
            if user_choice not in [1,2]:
        except ValueError:

        if user_choice == 1:
            # make easy game
            easy_game = Game(20, myName)
            # play easy game

        elif user_choice == 2:
            # make difficult game
            diff_game = Game(30, myName)
            # play difficult game

        print("\nHow about another?")

The main() function is the main entry point of the program. We greet the user, get his/her name in myName, then give them the choice of an easy or hard game (with the user_choice variable). After this, you can see that if user_choice == 1, we instantiate an “easy” game with easy_game = Game(20, myName). The hard game is instantiated if user_choice == 2, with diff_game = Game(30, myName). Each instance uses the play() method to run a single version of the game. The end of the main() function displays “How about another?” and the while loop starts over.

# if __name__ == 'main':
#     main()


To run the program, you may choose the commented out module method, or simply use main(). Don’t forget to comment out one or the other.

More Resources on OOP in Python

Some favorite videos / tutorials: