CLASSES Python Workshop

Introduction  Compared with other programming languages, Python’s class mechanism adds classes with a minimum of new syntax and semantics.  It is a mixture of the class mechanisms found in C++ and Modula-3.  Python classes provide all the standard features of Object Oriented Programming

Objects and Classes  An object is the basic Python building block, the 'lego brick' with which every program is built. All the elements that we've met up until now - integers, strings, lists, functions etc. - they're all objects.  A class is simply a user-defined object that lets you keep a bunch of closely related things "together".

Object Oriented Programming  Inheritance  A way to compartmentalize and reuse classes/objects already defined  Encapsulation  Grouping together of things that logically belong to each other  Polymorphism  the ability to create a variable, a function, or an object that has more than one form

Class foo # Class foo # class Foo: def __str__(self): return "I am an instance of Foo!!!" #main program f = Foo() print f

Class Components  Class class-identifier class Foo:  Constructor  special function of the class that is called whenever we create a new instance of the class def __str__(self):  Return return "I am an instance of Foo!!!"

Instantiation and Call Note how we instantiated the class with f = Foo() This we now have a new object f which has characteristics of this class, and we can use f print f

Illustration  The following illustration is simple to follow. It is based on our understanding of complex numbers that have two components  Real part  Imaginary part  Let’s see how we can work this!!

Class Complex class Complex: def __init__(self, realpart, imagpart): self.r = realpart self.i = imagpart x = Complex(3.0, -4.5) print “x real =“, x.r print “x imaginary = “, x.i

Explanation  As you can see, the definition of the class class Complex: def __init__(self, realpart, imagpart): self.r = realpart self.i = imagpart  The instantiation of the object x x = Complex(3.0, -4.5)  The use of the object attributes to print the respective parts print “x real =“, x.r print “x imaginary =“, x.i

Question  Can you see how the constructor is used to determine the parameters passed to the class when an object is instantiated?  def __init__(self, realpart, imagpart):  x = Complex(3.0, -4.5)

Question  Can you see how the attributes are used? self.r = realpart self.i = imagpart  And from the main program print “x real =“, x.r print “x imaginary =“, x.i