Now that our classes contain attributes, the next thing we can add are the methods. For example, the Student class contains two methods: birthday() and grade().
Let’s start by adding the birthday() method. A method is just a function inside of a class, so we’ll need use the def keyword, a name, and then parentheses containing the parameters followed by a colon.
Below the colon and indented one level, we’ll add the code that should be executed when that method is called. For a student’s birthday, we’ll just add 1 to the student’s age.
However, since birthday() is a class method, we’ll need to add a special parameter to the list of parameters. Most Python developers refer to this parameter as self, but the name is actually up to you. The self parameter must always be the first parameter defined in any class method. However, when we call the methods as we’ll see later in this module, we don’t have to provide an argument for it. Instead, the Python interpreter automatically includes a version of the current object that the method was called on as the first parameter.
Therefore, since the age is a class attribute instead of a variable inside of the birthday() method, we’ll need to use the self parameter, followed by a period, in front of the variable age. The self parameter simply refers to the current object, and helps us know that we are referring to the age attribute in the class. We’ll see why that is important in the next method.
The grade() method is a bit more complex, since it accepts several parameters. We can refer to the UML diagram to find the names and types of each parameter. Also, we’ll need to remember to add the self parameter at the beginning of the list!
Inside of the grade() method, we’ll need to recalculate the GPA. So, first we’ll need to figure out how many grade points the student has by multiplying the student’s GPA by the number of credits. Remember, the GPA stands for “grade point average”, which is the number of grade points divided by the number of credits, so to find the number of grade points we can just reverse that process.
However, since the GPA is a floating-point number, we may have to convert it to an integer, since grade points should be a whole number. It is tempting to just cast it to an integer, but what if we get a result such as 26.9999 due to floating-point error? Instead, we’ll want to round that value to convert it to an integer, so we can use the round() method to do just that.
After that, we can simply update the number of credits and the grade points with the attributes given as parameters to this method. Notice that we have a parameter named credits as well as a class attribute named credits. Because of that, it is important to use the self parameter when referring to the class attribute, since any reference to credits without it will be assumed to be the parameter instead.
Finally, we can divide the updated number of grade points by the new number of credits to update the student’s GPA and store it back in the class attribute for GPA.
There we go! We’ve now added some methods to our class. See if you can do the same for the Teacher class.