How to pratically learn python from zero: chapter four: Classes

There is a concept called OOP in computer science.It is called object-oriented programming.

Object-oriented programming (OOP) is a programming language model organized around objects rather than "actions" and data rather than logic. Historically, a program has been viewed as a logical procedure that takes input data, processes it, and produces output data.

In Python, accordingly, there is a concept called 'classes', it makes the Python programming become OOP. It mainly groups some functions together and make a class. If you feel OOP is difficult to understand or something, you can interpret class to be a bunch of functions grouped together to fulfill a task etc.

Since class is a list of functions, first we need to learn to write a function in python. It has the following format:

def functionname( parameters ):
   "function_docstring"
   function_suite
   return [expression]

Here is a simple example:

def cal1(x,y):

     z=x+y

     return z

print cal1(5,6)

output:

11

Python naming convention:

1. Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character 

variable names.

2. Package and Module Names:

Modules should have short, all-lowercase names.  Underscores can be used in the module name if it improves readability. 

 Python packages should also have short, all-lowercase names, although the use of underscores is discouraged.

3. Class names should normally use the CapWords convention.
     

Basic format of a class in python is like:

class ClassName:

    <statement-1>

    ......

   <statement-n>

And in a class, there is an instantiation operation which is for a specific initial state. Therefore a class may define a 

special method named __init__  like this: 

def __init__(self):

    self.data=[]

But for some of the classes, we can skip the initial steps, i.e. not include __init__ in our class. 

After the __init__, we can add more functions. 

Here is simple class called Dog:

class Dog:

    def __init__(self, name):
        self.name = name
        self.tricks = []    # creates a new empty list for each dog

    def add_trick(self, trick):
        self.tricks.append(trick)

And we can start a class instance like below:

d=Dog('Puppy')

And use the function in the class:

d.add_tricks('roll over')

And the original class will change the values:

d.tricks

['roll over']