Object Oriented Programming in Python: Constructor

So far, we have learned..

• programmer can make new type of data using Python's class
• class is composed of data and methods (functions) which handle the data

Input

class ApplicationInfo:
    def set_info(self, name, company, major):
        self.name = name
        self.company = company
        self.major = major 
    
    def print_neat(self):
        print('----------------------------')
        print('Name : ', self.name)
        print('Company  ', self.company)
        print('Major : ', self.major)
        print('----------------------------')

Input

application = ApplicationInfo()
application.set_info('Liam Gallagher', 'Uber', 'Biology')
application.print_neat()

Output

----------------------------
Name :  Liam Gallagher
Company   Uber
Major :  Biology
----------------------------

This is the code that we used from the previous post. As we all know by now, to make instance we had to call the className() and add "." after the method to access the instance method.

You may also remember the metaphor used previously to describe the relationship between class (waffle machine) and instance (waffle).

Now, let us make extra special waffle - a "chocolate chip waffle". Although, I am no expert in making a waffle. The steps in making a chocolate chip waffle will be..

• Make a waffle dough mixed with chocolate
• Pour the waffle dough inside the machine (create instances)
• Enjoy the waffle (instance returned)

Let us again look at the code that we have written so far.

application = ApplicationInfo()
application.set_info('Liam Gallagher', 'Uber', 'Biology')
application.print_neat()

The code above rather seems like..

• Make a waffle (create instance)
• Put chocalate chips inside the waffle (call instance method)
• Enjoy the waffle (instance returned)

How can we add chocalate chips when making the waffle or precisely when creating the instance?

Constructor - init()

Python class has constructor method which is automatically called after the class is created.

Input

class makeClass:
    def __init__(self):
        print('Class Created !')

myClass = makeClass()

Output

Class Created !

At the moment when the instance is created, we see that the constructor method is automatically called.

Now, let us implement this into our ApplicationInfo class.

Input

class ApplicationInfo:
    def __init__(self, name, company, major):
        self.name = name
        self.company = company
        self.major = major 
    
    def print_neat(self):
        print('----------------------------')
        print('Name : ', self.name)
        print('Company  ', self.company)
        print('Major : ', self.major)
        print('----------------------------')

Input

application = ApplicationInfo()

Output

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-20-41e426f145dc> in <module>
----> 1 application = ApplicationInfo()

TypeError: __init__() missing 3 required positional arguments: 'name', 'company', and 'major'

Since we did not add arguments inside the costructor method, error occurs in the process. Instead, we will pass three arguments as the constructor method is designed.

Input

application = ApplicationInfo("Randy Newman", "Pixar", "Aerospace Engineering")
application.print_neat()

Output

----------------------------
Name :  Randy Newman
Company   Pixar
Major :  Aerospace Engineering
----------------------------