Object Orientation
Wikipedia provides the following definition for object-oriented programming:
Object-oriented programming (OOP) is a programming paradigm that represents concepts as “objects” that have
data fields (attributes that describe the object) and associated procedures known as methods. Objects, which are
usually instances of classes, are used to interact with one another to design applications and computer programs.
This already provides the main technical terms that are also used in the Python world for
classes and objects and that will be made clearer in the remainder of this section.
Basics of Python Classes
We start by defining a new class (of objects). To this end, use the statement class, which
is applied like a def statement for function definitions. The following code defines a new
Python class named ExampleOne. This class does nothing but “exist.” The pass command
simply does what its name says — it passes and does nothing:
In [ 1 ]: class ExampleOne(object):
pass
However, the existence of the class ExampleOne allows us to generate instances of the
class as new Python objects:
In [ 2 ]: c = ExampleOne()
In addition, since this class inherits from the general object class, it already has some
batteries included. For example, the following provides the string representation of the
newly generated object based on our class:
In [ 3 ]: c.__str__()
Out[3]: ‘<__main__.ExampleOne object at 0x7f8fcc28ef10>’
We can also use type to learn about the type of the object — in this case, an instance of
the class ExampleOne:
In [ 4 ]: type(c)
Out[4]: __main__.ExampleOne
Let us now define a class that has two attributes, say, a and b. To this end, we define a
special method called init that is automatically invoked at every instantiation of the class.
Note that the object itself — i.e., by Python convention, self — is also a parameter of this
function:
In [ 5 ]: class ExampleTwo(object):
def __init__(self, a, b):
self.a = a
self.b = b
Instantiating the new class ExampleTwo now takes two values, one for attribute a and one
for attribute b. Note in the preceding definition that these attributes are referenced
internally (i.e., in the class definition) by self.a and self.b, respectively:
In [ 6 ]: c = ExampleTwo( 1 , ‘text’)
Similarly, we can access the values of the attributes of the object c as follows:
In [ 7 ]: c.a
Out[7]: 1
In [ 8 ]: c.b
Out[8]: ‘text’
