A Complete Guide To Python Classes

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In the last lesson of this course, we will explore Python classes, which provide a means of combining data and functionality.

What Are Classes?

Classes are types of objects in Python. Classes allow you to create multiple occurences of the same class, which are called instances. Classes also can include methods similar to the methods that we have used already in this course.

Although you may not have known it at the time, we have used many classes already in this course, including int - the integer class, and str - the string class. Creating customizable classes is very useful and will be explained throughout the rest of this lesson.

To create a new class, we use the keyword class. Class names are somewhat unique in Python because they are one of the only variable names that should not follow camelCase. Instead, class names should have their first letter capitalized.

Once we’ve named the class, we place a colon before an indented block that specifies the class’s attributes and methods.

In a very basic example, here is how we would create a class called Person which has an attribute called age that is given a default value of 0:


class Person:

	age = 0

Once a class is created, we can create instances of this class, which have their own variable values but inherit the methods from the parent class. This means that any methods from the parent class can be used by instances of that class.

The instance is created using a function called __init__. Note that since the __init__ function is missing from the Person class that we created in the example above, there is actually no way for us to create a new instance of Person.

To fix this, let’s add the __init__ function to the Person class. By convention, __init__ functions should always go at the start of class declarations, like this:


class Person:

	def __init__ (self, age):

		self.age = age

You will likely be puzzled by the self variable contained in the block of code above. What is the self parameter? It is a reference to the current instance of the class and is used to access variables that belong to that instance of the class.

Note that, like many variables we have seen throughout this course, the self variable can be named whatever you like. The only restriction is that it must be the first argument that is passed into the __init__ function. Here’s an example of the same class that uses x instead of self:


class Person:

	def __init__ (x, age):

		x.age = age

While self can be changed to whatever you like, the self keyword is generally considered a best practice and you should have a good reason before you use something other than self.

Another concept you should take away from the Person class is that while we simply wrote age = 0 in our first draft of the class, we changed this to self.age = age in a later version.

This is because age was actually passed into the __init__ function as an argument, which means we must call age from self using the dot operator.

The only time you can write a normal variable declaration like age = 0 within a class is when that variable is not passed in as part of the class’s __init__ function.

Let’s continue building out our Person class. Specifically, let’s try and accomplish the following:

  • Track how many parking tickets the person has in a static variable called parkingTickets with an initial value of 0
  • Create a method called giveParkingTicket() that increases the value of the parkingTickets variable by 1

We’ll use the self keyword moving forward. To recap, here’s where our code left off:


class Person:

	def __init__ (self, age):

		self.age = age

First, let’s create the parkingTickets variable and assign it an initial value of 0 like this:


class Person:

	def __init__ (self, age):

		self.age = age

	

	parkingTickets = 0

Next, we’ll need to create the giveParkingTicket() function. Like we did in our last lesson, let’s start by creating a blank version of the function with a docstring:


class Person:

	def __init__ (self, age):

		self.age = age

	

	parkingTickets = 0

	

	def giveParkingTicket(self):


    		"""This function increases the value of parkingTickets by 1"""	

Since giveParkingTicket() does not need to return anything (its simply modifying the value of an outside variable), we know that no return statement is necessary. Instead, we can simply increment the parkingTickets variable using the += 1 increment operators:


class Person:

	def __init__ (self, age):

		self.age = age

	

	parkingTickets = 0

	

	def giveParkingTicket(self):


    		"""This function increases the value of parkingTickets by 1"""	


    		parkingTickets += 1

And we’re done! We now have a working class with a dynamic variable age, a static variable parkingTickets, and a function giveParkingTickets(). To clarify, by ‘dynamic variables’ I mean those that are passed into the class’s brackets upon creation. Static variables are not passed into the brackets upon creation.

The next step is understanding how to create instances of this class. We can create instances of any class by using the class’s name and assigning it to a variable, like this:


me = Person(24)

Once your first Person instance is created, you can access its attributes (both static and dynamic) using the dot operator:


print(me.age)

#Returns 24

print(me.parkingTickets)

#Returns 0

You can also use the dot operator to call methods:


me.giveParkingTicket()

print(me.parkingTickets)

#Returns 1

Moving On

Classes are the hardest topic discussed in this course. Understanding them is critical. Because of that, the practice problems lesson that we’ll navigate to next is the longest sequence of practice problems in this course. Grab a cup of your favorite coffee and get to work!