Python Programming

Python implements some of OOP features differently from C++.

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 pointer in C++.

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.