This notebook is special because it contains a complete game... at least, it will when you've finished it. This will use all of the Python you've learned in lessons 1 - 10.


But what does this game do? This is a game where you, as a player, think of an animal, and the computer will ask questions to try and guess what you are thinking. It will look something like:

Initial Data

Before we can go anywhere, we need to set up the initial questions for our animals. We will use a dictionary that forms a tree of questions. What do I mean by that?

Maybe it would be good for me to draw a picture:

Initial Data Structure

Did I call this a tree? Looks more like an upside-down tree, huh? Whatever we call it, we can start at the top, and work our way down the arrows to the bottom.

Let's suppose we are thinking of Bessy, our favorite cow on Grandpa's farm. We start at the top and answer the question in the square, "Does our animal have 4 legs?" Bessy does have 4 legs, so we follow the arrow labeled Yes and get to another square, and ask the question, "Is our animal large and gray?"

Bessie seems large, but she is black and white, so we follow the arrow that says, "No". This brings us to an oval shape with the name of an animal, "Cow". This is the answer, so we end the game.

Let's play this game again, but this time, we are thinking of an octopus. At the top, we answer the question of "4 Legs?" by following the "No" arrow down to the oval that reads "Fish".

But we weren't thinking of a fish. This means, we need a new branch that has a question to distinguish a fish and an octopus.

Initial Data Structure

This new branch needs to replace the Fish oval, so that our new tree... er, upside-down tree now looks more like a pyramid:

Initial Data Structure

Does this make sense? Let's try to make our initial tree using a dictionary called animals.

Each "square" (in our diagrams above) will have three parts:

  • question will hold the question we can ask.
  • yes will either point to an animal or another branch
  • no will either point to an animal or another branch

If the player says no to our first question, we end up with a fish. If the fish isn't correct, we will replace it with a new branch. That branch will have a question, the new animal, and the fish.

You can see this "sub branch" with "yes", as it contains another dictionary. Now that you know how we will keep track of our questions, let's get on with the game.

Play the Game!

We will play our game by calling the playGame function.

This function has two jobs:

  • First, it will begin with some instructions. Feel free to help your player know how to use our program.

  • Second, we ask the player if they want to play another round in our game.

The playGame function calls two other functions that we'll need to make, askYesNo and walkTree. The askYesNo function returns True if our player types in "Yes", and as long as they keep typing in "Yes" to this question, we'll keep calling walkTree.

Play a Round by Walking the Tree

Playing a round means we start at the top of our dictionary, and walk down the tree to the bottom when we get to an animal.

Since the tree is really just a lot of little branches, we could start at any branch, ask its question, and then move down from there. And if we pass in animals as the branch (that is, the top of the tree), we will play an entire game round. So, this function takes a branch as a parameter.

Given a branch, we use the askYesNo function with the question pinned to that branch. That gives us the direction (the lower branch) we need to follow. We store this new branch in the newBranch variable.

If this branch ends on an animal, we call endGame, otherwise, we call the walkTree function again, with the lower branch. Makes sense, right?

The walkTree needs some new functions to help it out. We need:

  • lowerBranch to look at the lower branch
  • foundAnimal to see if we have found an animal
  • endGame to end the round

We'll do these in order.

Looking at a Lower Branch

Moving from branch to branch is called walking the tree, so moving from one branch to another must be a step, right?

If the player answers yes to a question, we need to follow the "yes" branch. Which direction we follow is given by the direction parameter. If direction is True, we return the branch on the "yes", and direction is False, we return the branch on the "no".

To test this function, we can use the top of our animals as a branch, and see what we end up when we give it a True or False for the direction.

Of course, when we start, the False direction gives us a subbranch. Let's create a variable to this, and then test that to see if it ends with either "cow" or "elephant". What this means is that if the lowerBranch gives us something wrong, then our tests will not be right, and we'll be able to fix it.

Found an Animal?

This little function will make it more clear that we have reached the end of the tree and we have bagged ourselves an animal.

This function returns True if we have an animal (that is a string that has the name of the animal) and False if we have another branch with a question.

We know we have an animal if the branch that we pass in is a string, and we are on a branch if we have a dictionary. We use the isinstance function that we've seen before to check what we have. To check if we have a dictionary, we give it the word, dict.

To test this function, we really need just need to pass in both types of data. Our animals variable is a good example of a dictionary we can use.

Ending a Game Round

When we call the endGame function, we've walk down to the bottom and found ourselves with an animal. This animal may be what our player is thinking, in which case, we've won. Or, the player has a new animal for us to learn about.

This function calls two other functions:

  • showAnimal will show an animal properly. I'll tell you what I mean in just a minute.
  • storeNewAnimal will replace the position of an animal with a new subbranch.
  • whichSide will convert a True direction into a "yes" so that we know where to store our new animal.

Let's talk about the whichSide function first.

Which Side is it On?

A helper function changes True to the string "yes" and False to "no" string, so that we can we know where to store our new animal.

Let's test our little whichSide function:

Showing an Animal

What is the difference between a cow and an octopus? One starts with an "A" and the other starts with an "N" ... get it? "a cow" and "aN octopus"? I guess that is a really bad joke, but if you are going to print a sentence in English with the animal word, we need to know if we should put a string "a" or "an" in front of it.

We make a showAnimal function that decides which to print.

Let's make sure that our function works the way we expect:

Storing a New Animal

The player was thinking of something that our program doesn't know about. No problem. We just need to know the name of this animal, and a question that the program can ask to distinguish it.

In order to store it back in our animals tree... er, pyramid, we need to know three things:

  • higherBranch is the point at the end of the branch where we will store
  • side is either "yes" or "no" and gives us the spot in the higher branch to store our new branch
  • oldAnimal is the text string with the name of the animal we thought it might be. We need to move this animal down into our new branch.

After complaining that we lost, we have to ask our player the name of the animal he or she was thinking of. To get let our player type something that and then store it, we use the raw_input function. Whatever is typed, comes back, and we convert it to lower case, so "Porcupine" is assigned to newAnimal as "porcupine".

Next we ned to get a question that we will store in our new branch to decided between the old animal and the new one we are just learning about.

To store our new branch, we just set either the "yes" or "no" in the higherBranch to our new dictionary that has the question and both animals.

You've seen the showAnimal function above, but we need to write a helper function called, turnIntoAQuestion.

Turning a Statement into a Question

When we ask the player for a new question, we need to make sure that what is given to us, doesn't end with a question mark, we need to add it.

To do this, we use the endswith function (which is similar to startswith, but checks the end of the text string.

We can easily test this, right? We just have to make sure that every string of text letters we give it, gives us back a string of text that ends with a question mark.

Yes and No Questions

Almost all of the questions we ask the player of our game are yes and no questions. However, we want to make sure that if the player types "yes" or "y", or even "Yes" (notice the capital) to all mean yes.

In this isYes function, we take the player's answer and first convert it to lower-case. This turns "Yes" into "yes" and "N" into "n". We do this to make the comparison easier.

Since "Yes" and "y" will be the same to our program, we really just need to look at the first letter. To do this, we use the startswith method. A method is like a function, but it only works by being attached to something.

In this function, we can a string of text characters, and attach this lower method to to, and then attach a startswith function to that. We end up with True if our player types "Yes" or "y".

We can make sure this function works, by making a few calls to assert:

We need a function that lets us give it a question that it will show to the player, and then allow our player to type in something. Showing things are easy, we just use print, but how do we get something?

Python has a function, raw_input that when called, Python stops, lets the player type something, and whatever the player types is given back to us. We will take that, and give it to our isYes function.

This means that the askYesNo function will take a string as a question, and return either True for yes and False for No.

Finishing Touches

We have now written all of the functions, including our big function, playGame. In fact, all we need to do is just call it:

If you Play that cell, we'll get an error, because in the Notebook, the raw_input function doesn't work. What we need to do is make a Python Program. Here's what you do:

  • Click the File menu above.
  • Select Download As...
  • Select Python.

This will put a file in your Downloads folder that you can double-click and play! If double-clicking on the file doesn't work, you can open up Terminal and run it from there: