Python Programming

Python implements some of OOP features differently from Java.

Let’s start by printing out a message when the Person object is being created. You will need to define a special __init__() method inside the class to do this.

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class Person:
    def __init__(self):
        print("I created a person! It's alive!")

manager = Person() # invoking the constructor
print(manager)

When you run the program, it should output I created a person! It's alive!, and then <__main__.Person object at 0x7fe70c878c10>.

In Python, the __init__() method acts as the class constructor. The double underscores on both sides is a Python convention that indicates that this is a magic method that does special things. They are called magic or dunder methods (dunder == double underscores). We will see more of these later.

When you create a class instance (Line 5), Python will create a new object (of type Person) in the memory heap, and execute the __init__() method. In the __init__() method, you can customise what happens after the object has been created.

self (in Line 2) refers to the Person object that has just been created. The first parameter of __init__() should always be self. This is so that you can manipulate this self object inside the method. self is similar to the this keyword in Java.

Behind the scenes

What actually happens in the background when you say Person() in Line 5 is that Python will first call Person.__new__() (another magic method that you can but should not mess with), which will allocate some space in memory for the new (empty) object.

Python then calls Person.__init__(self). You can do anything here, but you would typically add attributes to the object (we will do this in the next page; this is actually what self is partly for!)

Finally, Python will return this new object that has been created to the caller.