Learning Python- Intermediate course: Day 38, OOP

Today we will learn about OOP in Python. Today we will make a simple class and use instances of the classes.

Almost everything in Python is an Object, with its properties and methods. A class is like an object blueprint for creating objects. To create a class, use the keyword class:

Here is an example of a sample class commented wherever required

class sample: # creating a class named sample

    # ATTRIBUTES
    x=5 # class data has variable  named x with value five.

# OBJECT CREATION(INSTANTIAZATION OF AN OBJECT)
mysample=sample() # creating an instance of the class

#Accessing the parameters
print(mysample.x) # We can access the attribute values using the '.' operator.

Let us see one more sample

class sample:
    number=20
    character='3'

a=sample()
print(a.number+int(a.character))

Class methods

We can put functions inside a class, which are called as 'methods'
Syntax

def functionname (classname,......arguments......):
 ...
 ...
 ...

The below example will make things very clear

class sample:
    name="Tom"
    number=20
    character='3'
    def printname(sample):
        print(sample.name)

a=sample()
a.printname()

Tom

In the method, one of the arguments passed was the class itself. This is necessary syntax in Python. If it is not done in the same manner, the program won't run. Below are code snippets of how a program won't run.

Common mistakes

1) Didnt use the dot operator in the method syntax

class sample:
    name="Tom"
    number=20
    character='3'
    def printname(sample):
        print(name)

a=sample()
a.printname()

Traceback (most recent call last):
  File "main.py", line 9, in <module>
    a.printname()
  File "main.py", line 6, in printname
    print(name)
NameError: name 'name' is not defined

2) Forgot to mention the classname in the arguments

class sample:
    name="Tom"
    number=20
    character='3'
    def printname():
        print(name)

a=sample()
a.printname()

Traceback (most recent call last):
  File "main.py", line 9, in <module>
    a.printname()
TypeError: printname() takes 0 positional arguments but 1 was given

Tired of writing the name of the class every time? Well use the `self` keyword

The self keyword can be used in place of the entire class name

class sample:
    name="Tom"
    number=20
    character='3'
    def printname(self):
        print(sample.name)

a=sample()
a.printname()

Tom

Even replacing the class name by self inside the method works

class sample:
    name="Tom"
    number=20
    character='3'
    def printname(self):
        print(self.name)

a=sample()
a.printname()

Tom

Returning values

We can make the methods return value as we would do in the usual methods

class sample:
    name="Tom"
    number=20
    character='3'
    def printname(self):
        return (self.name)

a=sample()
print(a.printname())

Tom

Let us take one more example related to the methods.

class sample:
    rate=15

    def set_amount(self,amount):
        sample.amount=amount
    def print_tax(self,amount):
        self.set_amount(amount) 
        # Exercise why not self.set_amount(self,amount) ?
        print(self.rate*self.amount/100)


a=sample()
a.print_tax(20)

3.0

Here, set_amount(self) is a setter method, i.e. This methods sets the values of the method object.

Exercise

1) What will happen if we replace self.set_amount(amount) by self.set_amount(self,amount)?
2) Write a program which contains a class student which contains variables Id, name and roll number
The class must have methods showId and showrollnumber()

Private methods

We will now see how to make class methods as private. The private methods can only be accessed from inside the class. No one outside can access it.
In the above example, the set_amount is used only inside the class. So why not mark it private? We can set private methods in Python by adding two underscores to the class name, like this __set_amount Rest all remains the same

class sample:
    rate=15

    def __set_amount(self,amount):
        sample.amount=amount
    def print_tax(self,amount):
        self.__set_amount(amount) 
        print(self.rate*self.amount/100)


a=sample()
a.print_tax(20)

3.0

This private method can not be accessed outside of the class

class sample:
    rate=15

    def __set_amount(self,amount):
        sample.amount=amount
    def print_tax(self,amount):
        self.__set_amount(amount) 
        print(self.rate*self.amount/100)


a=sample()
a.__set_amount(2)
a.print_tax(20)

Traceback (most recent call last):
  File "main.py", line 12, in <module>
    a.__set_amount(2)
AttributeError: 'sample' object has no attribute '__set_amount'

So friends that was all for this part. In the next part we will study the __init__ or the class constructor.

Thank you