Object-oriented programming in Python
A class is a block of code that defines data and behaviour.
An object is instantiated from a class. It contains its own data and behaviour, as defined by the class.
The advantage of encapsulating data and behaviour in this way is that it allows the programmer to effectively model complex real-world behaviour. In practical terms, it can help keep a program organised into manageable parts that do not become impossibly hard to maintain as an application grows in size.
In Python, a class is defined as follows:
class announces a new class definition, just as
def announces a function definition. Note the capitalisation of Thing. In Python, this is a convention that you should take care to stick to.
Classes exist in a class heirarchy, where subclasses that define more specific behaviour are descended from superclasses that define more general behaviour. In Python all classes inherit from the
object class, which is so general it cannot be used at all, except through sublclassing. In other words, it is an abstract class.
An object is instantiated like this:
whatsit = Thing()
In a program, it is the objects that are created and manipulated to get meaningful output and the classes that define what those objects can contain and what they can do.
The behaviour of an object is defined in its methods. Methods are just functions that are available to objects of a class.
print "Hello, I am a thing."
whatsit = Thing()
>>> Hello, I am a thing
In Python, every method of a class must be defined with an object of that class as its first argument. This is a quirk of the implementation of methods in Python. By convention, the argument name used is self.
When an object calls one of its methods, the object itself is inserted as the first argument of the method. In this way, the definition of a method has one more argument than when the method is called.
When an object is instantiated, it is normal to do so with arguments. These arguments are passed to a constructor. A constructor is a method that is called when an object is first created. In Python, this method is called
__init__ (a so-called magic method in Python, of which there are several).
def __init__(self, colour):
self.colour = colour
print "Hello, I am " + self.colour
whatsit = Thing('blue')
>>> Hello, I am blue
The data of an object are generally known as attributes, which are just variables that are defined in the object. In this case, the whatsit object has an attribute of colour, as defined in the Thing class, with value ‘blue’.
Objects can be composed of attributes that are themselves objects. In this way, methods can be called from objects within objects:
When an object calls one of its methods, if the method is not defined in the class from which that object derives, then the superclasses are scrutinised for a method with the appropriate name.
Classes can inherit their behaviour from more than one superclass but, in general, this is not recommended practice. Classes defined with more than one superclass can often lead to confusion.