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Object data is data that gets carried around with the object.

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And so when you have several objects, even several objects that are based on the

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same class, they can have their own set of data.

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Let's take a look at how this is done.

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And we'll make a working copy of classes.py.

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We'll call this, casses-working.py.

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I'll go ahead and open our working copy.

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We see that we have our Duck object and the donald instance of the object.

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And all of the attributes in this object are code. They are methods.

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So let's add some data to the mix.

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We'll do this by first creating a constructor by naming a method with two

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underscores and the word init and two more underscores.

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And we'll pass it a value, call it color and we'll give it a default value of white.

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It's always a good idea to give your variables default value, unless they are

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absolutely going to be required.

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And we'll save that in an object variable.

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Object Variable is carried with the object itself.

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Remember that self is a reference to the object.

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So I say self._color = color.

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Now, I named it with an underscore for a couple of reasons.

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One reason is that I want to tell myself this is an attribute that I'm using

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locally, that I'm not going to be using directly.

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In other words, I'm not going to be accessing this variable from outside of the object.

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All of the access is going to be done from methods within the object.

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And for the most part, unless there's a really overriding reason to access

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data outside of the object without a method, then it's a really good idea to

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only do it with a method.

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Let me show you what I mean.

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It I wanted to, I could assign a value to color here. I can say donald._color = 'blue'.

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And down here, we'll just get rid of these and we'll print donald._color.

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And so, I'll save this and we'll run it and we'll see that it says blue.

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And if I print this out twice, we'll see that the first is says white, and then it says blue.

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And that's because I've gone and I've changed it here.

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And so this is what you call a side effect, and these can be very, very hard to keep track of.

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If instead, I just say I'm not going to do it that way at all.

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I'm going to do this all through Accessor Methods.

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I've a separate method for getting the color and I call this get_color and this

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will return self._color.

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And then I have another method for setting the color and I'll call this one,

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set_color, and I'll say self._color = color.

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Now, it's all controlled and I can keep track of it much easier.

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And so if I want to print donald's color, I can say print donald.get_color().

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Save that and we'll run it.

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And if I want to change his color, then I have a method that.

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donald.set_color(), and we'll say that their color is blue.

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And then we'll go ahead and print it again.

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Let's go ahead and save this and run it and see what it does.

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You might be saying to yourself, okay so what's the difference? We just made that harder.

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But did we actually make it better, and the answer is, yeah we made it better.

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The reason that it's better is that now we have code itself has control over

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what happens when that gets set.

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Let's say when I set their color that I need to save it in a database, or that

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I need to do some other things. Maybe it's not just a color, maybe it's some

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other controlling configuration thing. Maybe ducks that have blue feathers have

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to also have blue feet.

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So, when the color gets set, then I know that I've got a method that's going to

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also set the color of the feet.

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That's what going to also save it in the database or set some attribute some

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place else, based on what this change is.

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If I'm allowing it to get set just arbitrarily from outside of the object then I

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don't have that control.

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Then we might end up with the duck that's got blue feathers and green feet and

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that would never work.

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Bu enforcing this rule, and you'll notice when you read about object oriented

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code and when you learn about object oriented techniques, using these accessor

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methods is a standard and the reason for that is that it avoids side effects

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and avoids the kinds of problems that can happen when you lose control over how

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the data is being used.

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Remember one of the major advantages of object oriented programming is the encapsulation.

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And once you have that encapsulation, then that gives you control, that allows

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you to know exactly how your data is being used.

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Now the other thing that we'll notice about this is that this does not scale well.

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If I want it to be able to set say some other flags, some other values, some

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other attributes, that can quickly get out of hand.

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If they are more than two or three of these, it would become a real problem to keep track of.

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So what we tend to do in Python is to use a dictionary and to use what's

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called keyword arguments.

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If instead of just setting the color like this, I allow keyword arguments,

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kwargs like that and then based on those keyword arguments, I can set the color like this.

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That one even allows me to have a default value of white and then when I want to

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set it at the beginning, and let's just take these out here and I can say color

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equals blue, like that.

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Save that and run it and we get blue.

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And if I don't pass the argument at all, save that and run it, then we get white.

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So this scales. If I wanted to say set feet = 2, I can do that.

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Save it and run it.

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Of course, I'm not setting the color, but I'm setting another attribute.

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And if I wanted, I can have a getter for feet.

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The other side of this that doesn't scale well though is that I'm saving the

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color in a variable like this.

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Again, that could get on unwieldy and it could become difficult to keep track of.

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So, I can use a dictionary for that as well.

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In fact, I could use a dictionary for all of them and just call these variables

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and assign it to kwargs, like that.

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And for my set_color and get_color, if I wanted to have separate accessors for

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those, I could still do that. I could say variable sub color and assign that to the color.

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And I could say variables.get('color') and give it a default value and I like to

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use None in a circumstance like this and I can save that.

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And when I run it, you'll see that we get None down here.

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That just tells me that donald's color has not been set.

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But instead of this, we can make one of these that scales as well.

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Now, we're really getting into some power here.

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We can say set_variable and allow it the name. I'll call that k and then v for

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value, self.variables(), so k equals v, and get_variable(self, k).

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Return self.variables.get(k, None).

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Now instead of get_color, I can say get_variable("color").

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Save that and run it. But I've set feet. I haven't set color.

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So I can just put feet in here and then I get the number of feet.

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So, this allows us scalability.

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This allows us, if we wanted to, even after donald has been created, we could

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set his color, set_variable("color") to blue, and then down here, I can get his color as well.

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And now, we have this flexibility.

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We have a very small amount of code that can do a lot of different things.

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So, this is a technique that you'll see commonly used in Python where you are

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storing your object data in dictionary objects.

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It allows you a lot of flexibility.

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It allows you to use a lot of different data, a lot of flags, a lot of

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attributes, and to do different things with them, to save them to the databases,

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to use them as configuration options, and to vary easily be able to set them and

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get them and control them.

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So, this is one method.

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Obviously, it's not the only method and we'll see a lot of examples through out

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the rest of this course and different ways to handle object data, but this is in

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a nut shell how you use object data with objects and classes in Python.