Classes_and_Instances.md

Author(s): Mohita Liza Bipin, Pranav Shridhar

Editor(s): S Sandeep Pillai

Classes and Instances

• A class defines a set of attributes for an object and an object is an instance of a class.

• Attributes include data members and methods.

• Methods are functions defined within a class and data members are variables which are also defined within a class.

Yes, this seems a little complicated but it really isn't. Here's a simple analogy to help understand better :

A class is very similar to a basic chocolate cake recipe. It has all the ingredients (data) and the instructions (methods) to bake a cake. The object here is the cake.

Many objects can be made from a class and these objects can also be called instances. This process is called instantiation.

Now that the definitions are out of the way we'll be taking a closer look at class and instances.

Classes

Defining a Class

To define a class use the class keyword followed by the class name.

class Potato:

A local namespace containing the attributes is created when the class is defined where all attributes are defined.

Docstrings

Now looking at the class 'Potato' not a lot can be figured out about what exactly this class is for. That is where docstrings come in handy.

Docstrings give anyone reading the code a heads up on what the class is used for.

class example:
    '''This is a docstring'''  # -> Text enclosed by triple quotes. Explains the purpose of the class (when you hover over it in the text editor).

Now let's try applying this to our previous example, Potato.

class Potato:
    '''This class is a template of different types of potatoes'''

To access a docstring use the syntax:

classname.__doc__

print(Potato.__doc__)

So this will print the docstring of Potato and the output will be

This class is a template of different types of potatoes

Attributes

As we saw earlier attributes are both data members and methods so now we're going to add some attributes to Potato

class Potato:
    '''This class is a template of different types of potatoes'''


    def info(self,kind):        # self? what's that!?
       self.kind = kind

'info' is an example of a method and 'kind' is a data member (attribute) describing the type of potato.

'self' is a parameter as whenever a method is called the instance as a whole is passed automatically as the first parameter.

Q: WAIT WAIT WAIT WAIT, WHAT IS THIS SELF KEYWORD???

If you're like me, having learned C++ and coming over to python, you might be utterly confused with the reason python uses the self keyword. In fact, self is not a keyword, you can use any word you like!

So in short, the self object is used to denote an instance variable and its unique to that particular object of the class, not EVERY object of the class.

Consider the code below:

class A():
    x=3

class B():
    def __init__(self):
        self.x=3
        

Here is an important distinction:

A.x is a class variable, and will be shared across all instances of A unless specifically overridden within an instance. B.x is an instance variable, and each instance of B has its own version of it.

The init method is roughly what represents a constructor in Python (like in C/C++). When you call A() Python creates an object for you, and passes it ('self') as the first parameter to the init method.

Instances

Creating an instance

An instance can be defined as follows

instance_name = class_name()

A new instance called 'instance_name' is created here.

Accessing Attributes Through an instance

The next step is to access an object's attributes which can be done using the dot operator '.' you might recognize it from earlier in the docstrings section.

instance_name.data
instance_name.method()

Returning to Potato, we are now going to create an object tato which is a uni-tato (a cross of a unicorn and a potato).

class Potato:
    "This class is a template of different types of potatoes"


    def info(self,kind):
       self.kind = kind


tato = Potato()
tato.info('uni-tato')
print('Type of potato is:',tato.kind)

Output:

Type of potato is: uni-tato

---

Wanna know more??