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Classes and Objects

To understand classes, we first need to know about abstract datatype.

Abstract Datatype

Let us consider an implementation of Stack in python language.

We see that the Stack implementation itself gives functions to manipulate data, eg. push()or pop() and the implementation is private.

So basically, an abstract datatype:

  • Stores some data.
  • Provides designated functions to manipulate data.
  • Separates the (private) implementation and the (public) specification

Class

A implementation of is Abstract Datatype a class.

A class is basically a:

  • Template for a user defined datatype.
  • How data is stored
  • How public functions manipulate the data

Object

An object is an instance of a class.

Class as a real-world example

2D Points

A class created in python always needs to have the `__init__()` function. It is a special function and is known as constructor. The only objective of the constructor is to initialise the data members of a class a default value.

Each function of a class accepts a compulsory parameter known as self. It contains the a particular instance of a class to be worked upon. 

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class Point:
  def __init__(self, a = 0, b = 0):
    self.x = a
    self.y = b

Let us define another function called translate that shifts the point \((x,y)\) by \((\Delta x, \Delta y)\).

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def translate(self, deltax, deltay):
  self.x += deltax
  self.y += deltay

Let us define another function that gets us the distance of the point from origin using the formula \(d=\sqrt{x^2+y^2}\)

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def odistance(self):
  d = ((x ** 2) + (y ** 2)) ** (0.5)
  return d

Therefore the whole class Point is as follows:

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class Point:
  def __init__(self, a = 0, b = 0):
    self.x = a
    self.y = b

  def translate(self, deltax, deltay):
    self.x += deltax
    self.y += deltay

  def odistance(self):
    d = ((x ** 2) + (y ** 2)) ** (0.5)
    return d

Special Class functions

Function name Use
__init__() Constructor
__str__() Converts object to string. - It is implicitly called by print as print can only be used with strings.
__add__() Implicitly called by +

Similarly there are many more functions such as __mult__() for multiplication, __le__() for less than operator and so on.