Ø
Class: A user-defined
prototype for an object that defines a set of attributes that characterize any
object of the class. The attributes are data members (class variables and
instance variables) and methods, accessed via dot notation.
Ø
Class variable: A variable that is
shared by all instances of a class. Class variables are defined within a class
but outside any of the class's methods. Class variables aren't used as
frequently as instance variables are.
Ø
Data member: A class variable or
instance variable that holds data associated with a class and its objects.
Ø
Function overloading: The assignment of more
than one behavior to a particular function. The operation performed varies by
the types of objects (arguments) involved.
Ø
Instance variable: A variable that is
defined inside a method and belongs only to the current instance of a class.
Ø
Inheritance : The transfer of the
characteristics of a class to other classes that are derived from it.
Ø
Instance: An individual object
of a certain class. An object obj that belongs to a class Circle, for example,
is an instance of the class Circle.
Ø
Instantiation : The creation of an
instance of a class.
Ø
Method : A special kind of
function that is defined in a class definition.
Ø
Object : A unique instance of a
data structure that's defined by its class. An object comprises both data
members (class variables and instance variables) and methods.
Ø
Operator overloading: The assignment of more
than one function to a particular operator.
One thing that you will get to know about programming, is that
programmers like to be lazy. If something has been done before, why should you
do it again?
That is what functions
cover in Python. You've already had your code do something special. Now you
want to do it again. You put that special code into a function, and re-use it
for all it is worth. You can refer to a function anywhere in your code, and the
computer will always know what you are talking about. Handy, eh?
Of
course, functions have their limitations. Functions don't store any information
like variables do - every time a function is run, it starts afresh. However,
certain functions and variables are related to each other very closely, and
need to interact with each other a lot. For example, imagine you have a golf
club. It has information about it (i.e. variables) like the length of the
shaft, the material of the grip, and the material of the head. It also has
functions associated with it, like the function of swinging your golf club, or
the function of breaking it in pure frustration. For those functions, you need
to know the variables of the shaft length, head material, etc.
That
can easily be worked around with normal functions. Parameters affect the effect
of a function. But what if a function needs to affect variables? What happens
if each time you use your golf club, the shaft gets weaker, the grip on the
handle wears away a little, you get that little more frustrated, and a new
scratch is formed on the head of the club? A function cannot do that. A
function only makes one output, not four or five, or five hundred. What is
needed is a way to group functions and variables that are closely related into
one place so that they can interact with each other.
Chances
are that you also have more than one golf club. Without classes, you need to
write a whole heap of code for each different golf club. This is a pain, seeing
that all clubs share common features, it is just that some have changed
properties - like what the shaft is made of, and it's weight. The ideal
situation would be to have a design of your basic golf club. Each time you
create a new club, simply specify its attributes - the length of its shaft, its
weight, etc.
Or
what if you want a golf club, which has added extra features? Maybe you decide
to attach a clock to your golf club (why, I don't know - it was your idea).
Does this mean that we have to create this golf club from scratch? We would
have to write code first for our basic golf club, plus all of that again, and
the code for the clock, for our new design. Wouldn't it be better if we were to
just take our existing golf club, and then tack the code for the clock to it?
These
problems that a thing called object-oriented-programming solves. It puts
functions and variables together in a way that they can see each other and work
together, be replicated, and altered as needed, and not when unneeded. And we
use a thing called a 'class' to do this.
Creating a Class:
What
is a class? Think of a class as a blueprint. It isn't something in itself, it
simply describes how to make something. You can create lots of objects from
that blueprint - known technically as an instance.
So how do you make
these so-called 'classes'? Very easily, with the class operator:
Code Example 1 - defining a class
# Defining a class
class class_name:
[statement 1]
[statement 2]
[statement 3]
[etc.]
Makes little sense?
Thats okay, here is an example that creates the definition of a Shape:
Code Example 2 - Example of a Class
#An example of a class
class Shape:
def __init__(self,x,y):
self.x = x
self.y = y
description = "This shape has not been described yet"
author = "Nobody has claimed to make this shape yet"
def area(self):
return self.x * self.y
def perimeter(self):
return 2 * self.x + 2 * self.y
def describe(self,text):
self.description = text
def authorName(self,text):
self.author = text
def scaleSize(self,scale):
self.x = self.x * scale
self.y = self.y * scale
What
you have created is a description of a shape (That is, the variables) and what
operations you can do with the shape (That is, the fuctions). This is very
important - you have not made an actual shape, simply the description of what a
shape is. The shape has a width (x), a height (y), and an area and perimeter
(area(self) and perimeter(self)). No code is run when you define a class - you
are simply making functions and variables.
The
function called __init__ is run when we create an instance of Shape - that is,
when we create an actual shape, as opposed to the 'blueprint' we have here,
__init__ is run. You will understand how this works later.
self
is how we refer to things in the class from within itself. self is the first
parameter in any function defined inside a class. Any function or variable
created on the first level of indentation (that is, lines of code that start
one TAB to the right of where we put class Shape is automatically put into
self. To access these functions and variables elsewhere inside the class, their
name must be preceeded with self and a full-stop (e.g. self.variable_name).
Using a Class:
Its
all well and good that we can make a class, but how do we use one? Here is an
example, of what we call creating an instance of a class. Assume that the code
example 2 has already been run:
Code Example 3 - Creating a class
rectangle = Shape(100, 45)
What has been done? It
takes a little explaining...
The __init__ function
really comes into play at this time. We create an instance of a class by first
giving its name (in this case, Shape) and then, in brackets, the values to pass
to the __init__ function. The init function runs (using the parameters you gave
it in brackets) and then spits out an instance of that class, which in this
case is assigned to the name rectangle.
Think of our class instance, rectangle, as a self-contained
collection of variables and functions. In the same way that we used self to
access functions and variables of the class instance from within itself, we use
the name that we assigned to it now (rectangle) to access functions and
variables of the class instance from outside of itself. Following on from the
code we ran above, we would do this:
Code Example 4 - accessing attributes from outside an instance
#finding the area of your rectangle:
print rectangle.area()
#finding the perimeter of your rectangle:
print rectangle.perimeter()
#describing the rectangle
rectangle.describe("A wide rectangle, more than
twice\
as wide as it is tall")
#making the rectangle 50% smaller
rectangle.scaleSize(0.5)
#re-printing the new area of the rectangle
print rectangle.area()
As
you see, where self would be used from within the class instance, its assigned
name is used when outside the class. We do this to view and change the
variables inside the class, and to access the functions that are there.
We
aren't limited to a single instance of a class - we could have as many
instances as we like. I could do this:
Code Example 5 - More than one instance
long_rectangle = Shape(120,10)
fat_rectangle = Shape(130,120)
Both
long_rectangle and fat_rectangle have their own functions and variables
contained inside them - they are totally independent of each other. There is no
limit to the number of instances I could create.
