Exercise - Model and scaffold your game

  • 10 minutes

A game is no different from the invoice system mentioned in a previous unit. It still has the same parts in the world of object-oriented programming (OOP). These parts are objects, data, and behavior. Just like we did with the invoice system, you can model a game like rock, paper, scissors by first describing its domain, and then try listing what's what. Modeling your game is what you're going to do next. You'll also write some code that you can build on later.

Analyze rock, paper, scissors for OOP modeling

This exercise is a modeling exercise. You'll be given a problem domain description. You'll then be asked to take out the important keywords from the text and arrange them in a table.

Tip

If you need to be reminded how to model, see the "Find objects, data, and behavior" section in the second unit. Remember, modeling starts with asking the questions who interacts with whom or who does what to whom.

Problem description

Rock, paper, scissors is a game played by two participants. The game consists of rounds. In each round, a participant chooses a symbol from rock, paper, or scissors, and the other participant does the same. Then a winner of the round is determined by comparing the chosen symbols. The rules of the game state that rock wins over scissors, scissors beats (cuts) paper, and paper beats (covers) rock. The winner of the round is awarded a point. The game goes on for as many rounds as the participants agree on. The winner is the participant with the most points.

Model the game

  1. Copy and paste the preceding text into a document. Highlight the important keywords by making them bold or italic.

    Tip

    Spend a few minutes trying to underline what you think are the important keywords. After you're finished, scroll down to text that highlights the important parts.

    Here's the problem description text with highlights:

    Rock, paper, scissors is a game played by two participants. The game consists of rounds. In each round, a participant chooses a symbol from rock, paper, or scissors, and the other participant does the same. Then a winner of the round is determined by comparing the chosen symbols. The rules of the game state that rock wins over scissors, scissors beats (cuts) paper, and paper beats (covers) rock. The winner of the round is awarded a point. The game goes on for as many rounds as the participants agree on. The winner is the participant with the most points.

  2. Next, create a table with the columns Phase, Actor, Behavior, and Data. Arrange the highlighted words where you think they should be placed.

    Tip

    Spend a few minutes thinking this through, and scroll down after you've given it some thought.

    Here's what a resulting table can look like:

    Phase Actor Behavior Data
    Input Participant Chooses symbol Symbol saved as choice on Participant(choice)
    Processing GameRound Compares choices against game rules Result inspected
    Processing GameRound Awards points based on result value Points added to winning Participant(point)
    Processing Game Check continue answer Answer is true, continue, else quit
    Output Game New game round or game end credit

Create classes and state

The preceding table tells the story of how the game progresses through different phases. By focusing on the two columns Behavior and Data, you can scaffold some initial code as a foundation on which to build the game.

  1. In the Cloud Shell window on the right side of the screen, select the More icon (...), then select Settings > Go to Classic version.

  2. Copy the code below into the terminal and press Enter, to create a file rock-paper-scissor.py and open the editor:

    touch rock-paper-scissor.py
code .

    Tip

    Use the contextual menu (mouse right-click) to paste code into the terminal.

  3. Give it the following content and save the file (Ctrl+S or Command+S on macOS):

    class Participant:
class GameRound:
class Game:

    Tip

    Use Ctrl+V to paste code into the editor.

    You have the needed classes created for your game. Next, you need to think about what data you have and what class to place it on.

  4. Keep working with the same file, and expand the code like so, then save the file:

    class Participant:
    def __init__(self):
        self.points = 0
        self.choice = ""

class GameRound:

class Game:
    def __init__(self):
        self.endGame = False
        self.participant = Participant()
        self.secondParticipant = Participant()

    Tip

    • You can drag the separator between the editor and the terminal to adjust the space available.
    • You can close the editor from the ... menu.
    • You can type code . in the terminal to reopen the editor.

    We gave the Participant class the attributes points and choice as indicated by the first and third line of our table.

    We gave the Game the field endGame because of the fourth line. Additionally, the Game class has two participants, participant and secondParticipant. There were two roles a variable on an object could have. The roles could be a state, like the floor of an elevator, or a descriptive attribute. The points and choice attributes are state variables in this context. The participants on the Game class are descriptive attributes because a game has participants.

    Congratulations! You've added classes to your game and created data-attributes that you've assigned to the created classes. At this point, you have a good starting code. It doesn't do much yet because it needs behavior. You'll add behavior in the next exercise unit.