Classes #

You have already familiarized yourself with a lot of classes of objects that you can work with in Python: with numbers that can be added and multiplied, for example; with strings that can be converted to capital letters; with lists that you can sort; with dictionaries that can be searched. As you continue studying programming further, you'll discover additional classes of objects: files that can be read; web pages that can be sent to a browser; buttons that can be pressed, and so on. There are countless classes.

And just as you can define a function using def, you can also create a new class himself/herself. This is mainly used when you need a lot of objects in the program that have common behavior. Whole numbers, objects of the int class, have different values but they can all be added. Each list, object of the list class, can have different content, but all lists can be sorted.

The task for this section is to create a class called kitten, which can have different names, but all of them can meow and eat.

Start with meowing:

class Kitten:
    def purr(self):
        print("Meow!")

Just like functions are defined using def, classes have the keyword class, after which you write the name of the class, a colon, and then the indented body of the class. Similar to how def creates a function, the class statement creates a new class and assigns it to a variable with the given name (here Kitten).

Classes are traditionally named with a capital letter to avoid confusion with 'normal' values.

In a class body, you can define methods that look just like functions - except they have a first argument self. But we'll explain that later - first, try purring:

# Creating a specific object
mourek = Kitten()

# Method call
mourek.purr()

When you define a class (using the class block), it doesn't mean that there is a kitten in your program yet. A class is like a recipe or a manual: when you buy a cookbook, you will theoretically know how to bake a cake, what such a cake will look like, and that it can be eaten. But it still doesn't mean that you have the actual cake!

You create a specific object by calling a class: you use the class as a function, Kitten(), and the result is a new object of your class that you can now use.

Meow!

Attributes #

In objects created from 'custom' classes, you can set attributes - information that is stored with the given object. Attributes are marked by writing a period between the value and the name of its attribute.

mourek = Kitten()
mourek.name = 'Mourek'
print(mourek.name)

The string 'Mourek' now "belongs" to a specific kitten. When you create another kitten, you can name it differently - set its attribute name to a different string.

micka = Kitten()
micka.name = 'Micka'

print(micka.name)
print(mourek.name)

Parameter self. #

Now let's briefly return to methods. Specifically, to the self parameter.

Each method has access to a specific object it works on, precisely through the self argument. Now that you have named the kittens, you can use self in the purr method to access the name of the specific kitten.

class Kitten:
    def purr(self):
        print("{}: Meow!".format(self.name))

mourek = Kitten()
mourek.name = 'Mourek'

micka = Kitten()
micka.name = 'Micka'

mourek.purr()
micka.purr()

What happened? The expression mourek.purr represents a method. When you call it (mourek.purr()), the mourek object is passed to the purr function as the first argument, self.

Can such a method take more than one argument? It can - self will be automatically added as the first argument, the rest of the arguments will be taken from the method call. For example:

class Kitten:
    def purr(self):
        print("{}: Meow!".format(self.name))

    def eat(self, food):
        print("{}: Yum yum! I like the {}!".format(self.name, food))" 

mourek = Kitten()
mourek.name = 'Mourek'
mourek.eat('fish')

Summary #

Classes can do much more, but the basics are: all objects of a given class have some common behavior (for example, kittens can meow). And at the same time, each object also has its own information, just for it (for example, a kitten's meowing).

It is worth writing your own class when you have more objects with similar behavior in the program, or when you just need to have a set of functions (or methods) neatly together.

And that's it. You are absolutely great! This was a difficult lesson, so you should be proud of yourself. We are very proud of you for coming this far!

Take a short break - stretch, take a walk, close your eyes - before you start the next chapter. :)