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    Use Python Classes

    Will ButtonWill Button

    Object oriented classes work much like classes in other languages. Learn how to create them and use them, learn the difference between class variables and instance variables, creating class methods, and learn how to create classes that inherit from other classes.



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    I'm going to start just by creating a generic class, and call it ball. It's going to have a property called color, and I'm going to set it to the value of red. In my lower pane here, I'm going to open a Python console.

    I'm going to access that ball class and say from ball, which is the name of my file, import ball. That's going to import my class. I can create a new instance of that by creating a variable name. I'll call it myBall, and say that it's equal to an instance of the class ball.

    Then if I want to see the properties of it -- in this case, the color -- I can use dotted notation, and it returns the value for that attribute. Using this ball class, every instance of the class will have the color red.

    If I create another ball, and then we check the color attribute of it, it's red also. Most of the time, you're probably going to want to set some default properties for your class objects. For instance, we want to know about the radius, color, and weight of each particular ball.

    To do that, a class uses a special init method that gets called when the class is created. We'll define it, and then it requires some properties. First, it gets a copy of itself, and then the properties that we want to define.

    We said we wanted a radius, color, and the weight. What I'm going to do is I'm going to specify that self.radius, or basically this is another way of saying the radius of the instance that's being created is going to be equal to the radius that was passed to the init method whenever the instance was created.

    Same thing for color, and it's true for the weight as well. I'll save that, and then I'm going to switch back down to my console here. Because we're just running the interactive console, and we've already imported the ball class, I need to exit the console, and then go back into it so that we can reimport it.

    That's only a factor because of the way I'm explaining to you how the class works. Most of the time, this is never going to come up, so I wouldn't worry about it too much. We'll create a red ball. That's going to be an instance of the ball class. It's going to have a radius of 10, a color of red, and weight of two pounds.

    Then we'll create a blue ball. It's going to be a separate instance of the class with a radius of 12, color of blue, and a weight of five. If we take a look at the red ball color, it's red, and the red ball weight is two. Then for the blue ball, just to show you how that works, the color is blue, and the weight of it is five.

    Each instance of the class has its own set of properties that can be different from all other instances. These are known as the instance attributes, meaning that they're different for every object. In the first example, when we define the color red, that's known as a class attribute, because it's the same for every object instantiated from that class.

    Objects instantiated from a class can have default methods as well. Let's create another new class here, and we'll call this one football. We'll start by giving it a docstring, and say that it's just a standard regulation NFL ball.

    Then we'll give it our init, which receives itself as the first argument. Then it has an argument for diameter, color, and pressure. We'll set each of these equal to the parameters specified when the class is instantiated. Then we'll give it a couple of default methods.

    We'll give it an inflate method that takes itself as the first argument, and then the PSI change. We'll say self.pressure is equal to self.pressure plus the PSI change for when the inflate method is called. Then our other function will be deflate, again receiving itself and then the PSI change. We'll say self.pressure is equal to self.pressure minus the PSI change.

    I'm going to save that, go back into my Python console, and then from ball, I'll import the football class. Then we'll create a football for the Bears that's just going going to be a standard regulation football. It'll have a diameter of 22 centimeters, a color of brown, and a pressure of 13 pounds.

    We'll create a ball for the Packers as well. Again, another standard regulation football, 22 centimeter diameter. The color's going to be brown, and I now see that I typed the word color up there instead of the word brown, because I can't type and talk at the same time.

    That's OK. It's not going to affect our example. This one will be 13 PSI. Let's take a look at the Bears' color, just for fun, and then Bears' pressure. Now, if the Bears inflate their ball by two PSI, it goes up to 15.

    For the Packers, their pressure is currently at 13. If the Packers deflate their ball by one pound, it drops it down to 12 pounds. Now, watch this. I'm going to close this out of the way so that I get a little more room on the screen here.

    We've got our class football, and classes can inherit other classes. I'm going to create a class called thePatriotsBall. It's going to inherit the call football. It's going to get everything defined in the football call, and inside of it, I can create a function called inflate.

    The inflate function, whenever called, is going to actually reduce the pressure of the ball by the amount specified. Let's see how that works. I'll scroll this up so you can see it here. Go back into our Python console, and then from ball, we'll import thePatriotsBall.

    Remember, the ball that I specified there with from ball is the name of the Python file I'm importing from. Create a new instance variable called Patriots, which is going to be equal to PatriotsBall. It inherited from ball, so it as the same default options.

    We'll give it a standard NFL diameter of 22 centimeters, a color of brown, and then pressure of NFL regulation 13 pounds. Let's check the pressure on the Patriots' ball. It's 13. Now, the referees ask the Patriots to inflate their ball by two pounds. Now, when we check the pressure, the pressure is actually down to 11.

    I did all of that to show you that functions defined in a class can be overridden because they aren't protected in any way. Tom Brady approves of this behavior, but for you, I would recommend verifying the attributes and methods of any class you're inheriting, before extending it, and possibly overriding a default behavior that you may later need.

    To do that, remember that you can use the dir command, and it will show you the methods that get defined whenever that's created. Then if you do dir on an instance of that class, you also get the attributes belonging to that instance as well. In this case, not only the inflate and deflate methods, but the color, diameter, and pressure as well.