Objects in Python

• We will understand the following
  1. How to create classes and instantiate objects in Python
  2. How to add attributes and operations/methods/behaviors to Python objects

Creating Python classes

• Lets create a new file called as helloclass.py with the following

class HelloClass:
    pass

• This is our first object-oriented program.
• To name your classes Lets try to understand naming conventions from PEP-8. ReferHere. Class names should be named using CapWords
• pass keyword indicates that no further action needs to be taken
• Now lets create objects

object_1 = HelloClass()
object_2 = HelloClass()
print(object_1)
print(object_2)

• This code is creating objects (instantiates) two objects from HelloClass named object_1 & object_2
• When printed these two objects tell us what memory address they live in

Adding Attributes

• Lets create one more file called point.py and add a Point class

class Point:
    pass

p1 = Point()
p2 = Point()

p1.x = 6
p2.x = 5
p1.y = 7
p2.y = 6
print(p1.x, p1.y)
print(p2.x, p2.y)

• Lets execute this program python point.py
• All we need to do to assign a value to an attribute on an object is <object>.<attribute> = <value>

Add Operations or behavior

• Now lets create a new implementation of our Point class

class Point:
    def reset(self):
        self.x = 0
        self.y = 0

p1 = Point()
p2 = Point()

p1.x = 6
p2.x = 5
p1.y = 7
p2.y = 6

print(p1.x, p1.y)
print(p2.x, p2.y)
p1.reset()
p2.reset()
print(p1.x, p1.y)
print(p2.x, p2.y)

• Now lets execute this code
• Operations or behaviors of class will be defined using def keyword

def <operation_name>(<parameters>):
    <code>

• Now to understand about self lets look at this code

class Point:
    def reset(self):
        print("resetting")
        self.x = 0
        self.y = 0

    def square(this):
        print("performing square")
        this.x = this.x**2
        this.y = this.y**2

    def print(current):
        print(current.x, current.y)

p1 = Point()
p2 = Point()

p1.x = 6
p2.x = 5
p1.y = 7
p2.y = 6

• All operations of the class will have the first parameter with any name, but this argument is conventionally named as self. I’ve never seen a python programmer use any other name
• Lets follow the convention and change the code to self in operations

class Point:
    def reset(self):
        print("resetting")
        self.x = 0
        self.y = 0

    def square(self):
        print("performing square")
        self.x = self.x**2
        self.y = self.y**2

    def print(self):
        print(self.x, self.y)

p1 = Point()
p2 = Point()

p1.x = 6
p2.x = 5
p1.y = 7
p2.y = 6

p1.print()
p2.print()
p1.square()
p2.square()
p1.print()
p2.print()
p1.reset()
p2.reset()
p1.print()
p2.print()

• When we call operations we use <object>.<operation_name>(), then python internally passes the object to the self argument or self argument will have current objects reference
• We can call the operations in a different way <class>.<operation_name>(<object>)

p3 = Point()
p3.x = 2
p3.y = 2
Point.print(p3)

• What will happen if we don’t use self

class Point:
    def reset():
       pass

p1 = Point()
p1.reset()

• The error message is simply trying to tell you forgot self argument, whenever you call an operation from object, object itself will be passed as an argument to your method.
• Lets add more arguments to operations

class Point:
    def reset(self):
        print("resetting")
        self.x = 0
        self.y = 0

    def square(self):
        print("performing square")
        self.x = self.x**2
        self.y = self.y**2

    def print(self):
        print(self.x, self.y)

    def move(self, x, y):
        self.x = x
        self.y = y
    
    def add(self, x, y):
        self.x = self.x + x
        self.y = self.y + y

p = Point()
p.x = 4
p.y = 5
p.print()
p.move(2,3)
p.print()
p.add(1,1)
p.print()

• How to import classes in python interactive shell (REPL)

python -i point.py