18. PYTHON CLASSES AND OBJECTS

Python Classes and Objects:

Python is an object oriented programming language. Unlike procedure oriented programming, where the main emphasis is on functions, object oriented programming stress on objects.

Object is simply a collection of data (variables) and methods (functions) that act on those data. And, class is a blueprint for the object.

We can think of class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows etc. Based on these descriptions we build the house. House is the object.

As, many houses can be made from a description, we can create many objects from a class. An object is also called an instance of a class and the process of creating this object is called instantiation.

Defining a Class in Python

Like function definitions begin with the keyword def, in Python, we define a class using the keyword class.

The first string is called docstring and has a brief description about the class. Although not mandatory, this is recommended.

Class Definition:

class MyNewClass:
    '''This is a docstring. I have created a new class'''
    pass

A class creates a new local namespace where all its attributes are defined. Attributes may be data or functions.

There are also special attributes in it that begins with double underscores (__). For example, __doc__ gives us the docstring of that class.

As soon as we define a class, a new class object is created with the same name. This class object allows us to access the different attributes as well as to instantiate new objects of that class.

For Example:

class MyClass:

"This is my second class"

a = 10

def func(self):

print('Hello')

print(MyClass.a)   # Output: 10

print(MyClass.func)  # Output: <function MyClass.func at 0x0000000003079BF8>

print(MyClass.__doc__)   # Output: 'This is my second class'

OUTPUT:

10

<function MyClass.func at 0x0000000003079BF8>

'This is my second class'

Creating an Object in Python:

We saw that the class object could be used to access different attributes.

It can also be used to create new object instances (instantiation) of that class. The procedure to create an object is similar to a function call.

>>> ob = MyClass()

This will create a new instance object named ob. We can access attributes of objects using the object name as prefix.

Attributes may be data or method. Method of an object are corresponding functions of that class. Any function object that is a class attribute defines a method for objects of that class.

This means to say, since MyClass.func is a function object (attribute of class), ob.func will be a method object.

For Example:

class MyClass:

"This is my second class"

a = 10

def func(self):

print('Hello')

# create a new MyClass

ob = MyClass()

# Output: <function MyClass.func at 0x000000000335B0D0>

print(MyClass.func)

# Output: <bound method MyClass.func of <__main__.MyClass object at 0x000000000332DEF0>>

print(ob.func)

# Calling function func()

# Output: Hello

print(ob.func())

OUTPUT:

<function MyClass.func at 0x000000000335B0D0>

<bound method MyClass.func of <__main__.MyClass object at 0x000000000332DEF0>>

Hello

Constructors in Python:

Class functions that begins with double underscore (__) are called special functions as they have special meaning.

Of one particular interest is the __init__() function. This special function gets called whenever a new object of that class is instantiated.

This type of function is also called constructors in Object Oriented Programming (OOP). We normally use it to initialize all the variables.

For Example:

class ComplexNumber:

    def __init__(self,r = 0,i = 0):

        self.real = r

        self.imag = i

    def getData(self):

        print("{0}+{1}j".format(self.real,self.imag))

# Create a new ComplexNumber object

c1 = ComplexNumber(2,3)

# Call getData() function

# Output: 2+3j

c1.getData()

# Create another ComplexNumber object

# and create a new attribute 'attr'

c2 = ComplexNumber(5)

c2.attr = 10

# Output: (5, 0, 10)

print((c2.real, c2.imag, c2.attr))

# but c1 object doesn't have attribute 'attr'

# AttributeError: 'ComplexNumber' object has no attribute 'attr'

print(c1.attr)

Deleting Attributes and Objects:

Any attribute of an object can be deleted anytime, using the del statement.

For Example:

>>> c1 = ComplexNumber(2,3)
>>> del c1.imag
>>> c1.getData()
Traceback (most recent call last):
...
AttributeError: 'ComplexNumber' object has no attribute 'imag'

>>> del ComplexNumber.getData
>>> c1.getData()
Traceback (most recent call last):
...
AttributeError: 'ComplexNumber' object has no attribute 'getData'

We can even delete the object itself, using the del statement.

>>> c1 = ComplexNumber(1,3)
>>> del c1
>>> c1
Traceback (most recent call last):
...
NameError: name 'c1' is not defined

Actually, it is more complicated than that. When we do c1 = ComplexNumber(1,3), a new instance object is created in memory and the name c1 binds with it.

On the command del c1, this binding is removed and the name c1 is deleted from the corresponding namespace. The object however continues to exist in memory and if no other name is bound to it, it is later automatically destroyed.

This automatic destruction of unreferenced objects in Python is also called garbage collection.