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In this module, we're going to talk about objects and classes.

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Python has many different kinds of data types: integers,

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floats, strings, lists, dictionaries, booleans.

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In Python, each is an object.

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Every object has the following: a type,

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internal representation, a set of functions called methods to interact with the data.

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An object is an instance of a particular type.

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For example, we have two types,

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type one and type two.

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We can have several objects of type one as shown in yellow.

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Each object is an instance of type one.

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We also have several objects of type two shown in green.

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Each object is an instance of type two.

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Let's do several less abstract examples.

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Every time we create an integer,

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we are creating an instance of type integer,

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or we are creating an integer object.

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In this case, we are creating five instances of type integer or five integer objects.

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Similarly, every time we create a list,

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we are creating an instance of type list,

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or we are creating a list object.

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In this case, we are creating five instances of type list or five list objects.

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We could find out the type of an object by using the type command.

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In this case, we have an object of type list,

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we have an object of type integer,

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we have an object of type string.

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Finally, we have an object of type dictionary.

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A class or type's methods are functions

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that every instance of that class or type provides.

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It's how you interact with the object.

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We have been using methods all this time,

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for example, on lists.

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Sorting is an example of a method that interacts with the data in the object.

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Consider the list ratings,

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the data is a series of numbers contained within the list.

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The method sort will change the data within the object.

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We call the method by adding a period at the end of the object's name,

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and the method's name we would like to call with parentheses.

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We have the rating's list represented in orange.

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The data contained in the list is a sequence of numbers.

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We call the sort method,

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this changes the data contained in the object.

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You can say it changes the state of the object.

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We can call the reverse method on the list,

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changing the list again.

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We call the method, reversing the order of the sequence within the object.

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In many cases, you don't have to know the inner workings of the class and its methods,

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you just have to know how to use them.

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Next, we will cover how to construct your own classes.

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You can create your own type or class in Python.

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In this section, you'll create a class.

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The class has data attributes.

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The class has methods.

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We then create instances or instances of that class or objects.

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The class data attributes define the class.

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Let's create two classes.

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The first class will be a circle,

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the second will be a rectangle.

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Let's think about what constitutes a circle.

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Examining this image, all we need is a radius to define a circle,

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and let's add color to make it easier to

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distinguish between different instances of the class later.

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Therefore, our class data attributes are radius and color.

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Similarly, examining the image in order to define a rectangle,

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we need the height and width.

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We will also add color to distinguish between instances later.

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Therefore, the data attributes are color, height, and width.

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To create the class circle,

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you will need to include the class definition.

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This tells Python you're creating your own class,

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the name of the class.

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For this course in parentheses,

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you will always place the term object,

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this is the parent of the class.

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For the class rectangle,

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we changed the name of the class,

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but the rest is kept the same.

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Classes are outlines we have to set the attributes to create objects.

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We can create an object that is an instance of type circle.

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The color data attribute is red,

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and the data attribute radius is four.

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We could also create a second object that is an instance of type circle.

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In this case, the color data attribute is green,

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and the data attribute radius is two.

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We can also create an object that is an instance of type rectangle.

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The color data attribute is blue,

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and the data attribute of height and width is two.

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The second object is also an instance of type rectangle.

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In this case, the color data attribute is yellow,

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and the height is one,

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and the width is three.

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We now have different objects of class circle or type circle.

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We also have different objects of class rectangle or type rectangle.

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Let us continue building the circle class in Python.

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We define our class.

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We then initialize each instance of the class with data attributes,

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radius, and color using the class constructor.

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The function init is a constructor.

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It's a special function that tells Python you are making a new class.

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There are other special functions in Python to make more complex classes.

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The radius and color parameters are used to

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initialize the radius and color data attributes of the class instance.

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The self parameter refers to the newly created instance of the class.

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The parameters, radius, and color can be used in the constructors body to

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access the values passed to the class constructor when the class is constructed.

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We could set the value of

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the radius and color data attributes to the values passed to the constructor method.

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Similarly, we can define the class rectangle in Python.

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The name of the class is different.

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This time, the class data attributes are color, height, and width.

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After we've created the class,

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in order to create an object of class circle,

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we introduce a variable.

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This will be the name of the object.

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We create the object by using the object constructor.

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The object constructor consists of the name of the class as well as the parameters.

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These are the data attributes.

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When we create a circle object,

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we call the code like a function.

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The arguments passed to the circle constructor are used to

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initialize the data attributes of the newly created circle instance.

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It is helpful to think of self as a box that

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contains all the data attributes of the object.

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Typing the object's name followed by a dot and the data attribute

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name gives us the data attribute value, for example, radius.

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In this case, the radius is 10.

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We can do the same for color.

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We can see the relationship between the self parameter and the object.

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In Python, we can also set or change the data attribute directly.

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Typing the object's name followed by a dot and the data attribute name,

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and set it equal to the corresponding value.

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We can verify that the color data attribute has changed.

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Usually, in order to change the data in an object,

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we define methods in the class.

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Let's discuss methods.

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We have seen how data attributes consist of the data defining the objects.

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Methods are functions that interact and change the data attributes,

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changing or using the data attributes of the object.

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Let's say we would like to change the size of a circle.

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This involves changing the radius attribute.

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We add a method, add radius to the class circle.

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The method has a function that requires the self as well as other parameters.

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In this case, we are going to add a value to the radius,

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We denote that value as r. We are going to add r to the data attribute radius.

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Let's see how this part of the code works when we create

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an object and call the add_radius method.

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As before, we create an object with the object constructor.

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We pass two arguments to the constructor.

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The radius is set to two and the color is set to red.

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In the constructor's body,

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the data attributes are set.

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We can use the box analogy to see the current state of the object.

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We call the method by adding a dot followed by the method, name, and parentheses.

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In this case, the argument of the function is the amount we would like to add.

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We do not need to worry about the self parameter when calling the method.

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Just like with the constructor,

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Python will take care of that for us.

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In many cases, there may not be

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any parameters other than self specified in the method's definition.

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So we don't pass any arguments when calling the function.

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Internally, the method is called with a value of eight,

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and the proper self object.

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The method assigns a new value to self radius.

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This changes the object,

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in particular, the radius data attribute.

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When we call the add_radius method,

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this changes the object by changing the value of the radius data attribute.

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We can add default values to the parameters of a class as constructor.

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In the labs, we also create the method called drawCircle.

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See the lab for the implementation of drawCircle.

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In the labs, we can create a new object of type circle using the constructor.

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The color will be red and the radius will be three.

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We can access the data attribute radius.

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We can access the attribute color.

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Finally, we can use the method drawCircle to draw the circle.

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Similarly, we can create a new object of type circle.

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We can access the data attribute of radius.

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We can access the data attribute color.

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We can use the method drawCircle to draw the circle.

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In summary, we have created an object of

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class circle called RedCircle with a radius attribute of three,

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and a color attribute of red.

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We also created an object of class circle called BlueCircle,

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with a radius attribute of 10 and a color attribute of blue.

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In the lab, we have a similar class for rectangle.

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We can create a new object of type rectangle using the constructor.

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We can access a data attribute of height.

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We can also access the data attribute of width.

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We could do the same for the data attribute of color.

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We can use the method drawRectangle to draw the rectangle.

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So we have a class,

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an object that is a realization or instantiation of that class.

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For example, we can create two objects of class Circle,

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or two objects of class Rectangle.

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The dir function is useful for obtaining the list

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of data attributes and methods associated with a class.

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The object you're interested in is passed as an argument.

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The return value is a list of the objects data attributes.

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The attribute surrounded by underscores are for internal use,

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and you shouldn't have to worry about them.

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The regular looking attributes are the ones you should concern yourself with.

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These are the objects,

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methods, and data attributes.

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There is a lot more you can do with objects in Python.

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Check Python.org for more info.

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(Music)