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The concept of object-oriented programming originate from the natural world, which is natural to use ''Class'' and ''Instance''. ''Class'' is an abstract concept referring to a general category of objects. For example, we can define a ''Class'' to be students, so instances of the defined ''Class'' are specific students such as Bart, Lisa, and Simpson. 

The level of abstraction of object-oriented program is higher than functions, because a ''Class'' includes both data and operations of the data.

Data Encapsulation, inheritance, and Polymorphism are three fundamental aspects of object-oriented programming. 

=Class and Instance=

A ''Class'' refers to an abstract template of codes, while a ''Instance'' is defined as specific objects based a ''Class''. Every ''Instance'' shares the same function inherited from the ''Class'', but data for each ''Instance'' might be different.

<syntaxhighlight lang=python>
class Student(object):
    pass
</syntaxhighlight >

The code above defines a Class. Right after the keyword '''class''' is the name of the class '''Student''', which usually starts with an upper case letter. Then '''(object)''' indicates which class is the Student inherited from. If you don't have a specific class to inherit, use ''object'' since it is the root class where all class inherits. 

After defining the class, we can create a instance for ''Student''.

<syntaxhighlight lang=python>
>>> class Student(object):
...     pass
...
>>> bart = Student() # bart is a instance of Student()
>>> bart
<__main__.Student object at 0x101be77f0>
>>> Student # Student is a class
<class '__main__.Student'>
</syntaxhighlight >

You can bind variables to the instance as a property. For example, bind a ''name'' property to instance bart:

<syntaxhighlight lang=python>
>>> bart.name = 'duke'
>>> bart.name
'duke'
</syntaxhighlight >

Meanwhile, class also serves as a template to write public properties into the class, which is done by a special function '''__init__''':

<syntaxhighlight lang=python>
class Student(object):
    def __init__(self, name, score):
        self.name = name
        self.score = score
</syntaxhighlight >

The first argument of the '''__init__''' function is always '''self''', which represents the created instance. Within the '''__init__''' function, you can bind different properties on '''self''' because '''self''' is the instance.

With the '''__init__''' function, you have to have corresponding arguments when creating an instance, but '''self''' is not required because Python automatically pass the '''self''' argument into the function. The following is a example

<syntaxhighlight lang=python>
>>> class Student(object):
...    def __init__(self, name, score):
...        self.name = name
...        self.score = score
...
>>> bart = Student('duke', 99)
>>> bart.name
'duke'
>>> bart.score
99
>>> bart_test = Student('NCState')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-97f4e2f67951> in <module>()
----> 1 bart_test = Student('NCState')

TypeError: __init__() takes exactly 3 arguments (2 given)
</syntaxhighlight >

==Data Encapsulation==

Since the variables are accessible within the function, we can write a function to encapsulate them so that we don't have to write another function to read them after creating an instance. The functions ''print_score'' is called the function of the class Student. In the following example, the code '''bart.print_score()''' executes the function of the class within the instance '''bart'''.

<syntaxhighlight lang=python>
>>> class Student(object):
...     def __init__(self, name, score):
...         self.name = name
...         self.score = score
...     def print_score(self):
...         print("%s: %s" % (self.name, self.score))
...
>>> bart = Student('zhang', 99)
>>> bart.print_score()
zhang: 99
</syntaxhighlight >

=Inheritance and Polymorphism=

=Properties of Instance=
==__str__==


==__iter__ and __next__==


==__getitem__==


==__getattr__==


==__call__==

=Multiple Inheritance=

=Enumeration=