ANSWER :

Type-based dispatch:

A programming pattern that checks the type of an operand and invokes different functions for different types.

This can be illustrated using a Program. Consider a problem of creating a class called Time, adding two Time objects, adding a number to Time object etc. that we had considered in previous section. Here is a complete program with more of OOP concepts.

Fig 6.1 Demonstration of Type-based Dispatched & Overloading

Fig 6.2 Output of Demonstration of Type-based Dispatched & Overloading

Working of above program is explained hereunder -

The class Time has __init__() method for initialization of instance attributes hour, min and sec. The default values of all these are being zero

The method time_to_int() is used convert a Time object (hours, min and sec) into single integer representing time in number of seconds.

The method int_to_time() is written to convert the argument seconds into time object in the form of hours, min and sec. The built-in method

divmod()

gives the quotient as well as remainder after dividing first argument by second argument given to it.

Special method __eq__() is for overloading equality (==) operator. We can say one Time object is equal to the other Time object if underlying hours, minutes and seconds are equal respectively. Thus, we are comparing these instance attributes individually and returning either True of False.

When we try to perform addition, there are 3 cases -

i. Adding two time objects like T3=T1+T2.

ii. Adding integer to Time object like T4=T1+75

iii. Adding Time object to an integer like T5=130+T1

Each of these cases requires different logic. When first two cases are considered, the first argument will be T1 and hence self will be created and passed to __add__() method. Inside this method, we will check the type of second argument using isinstance() method. If the second argument is Time object, then we call addTime() method. In this method, we will first convert both Time objects to integer (seconds) and then the resulting sum into Time object again. So, we make use time_to_int() and int_to_time() here. When the 2^nd argument is an integer,

it is obvious that it is number of seconds. Hence, we need to call increment() method.

Thus, based on the type of argument received in a method, we take appropriate action. This is known as

type-based dispatch.

In the 3^rd case like T5=130+T1, Python tries to convert first argument 130 into self, which is not possible. Hence, there will be an error. This indicates that for Python, T1+5 is not same as 5+T1 (Commutative law doesn't hold good!!). To avoid the possible error, we need to implement

right-side addition

method __radd__(). Inside this method, we can call overloaded method __add__().

The beauty of Python lies in surprising the programmer with more facilities!! As we have implemented __add__() method (that is, overloading of + operator), the builtin sum() will is capable of adding multiple objects given in a sequence. This is due to

Polymorphism

in Python

.

Consider a list containing Time objects, and then call

sum() on that list as -

T6=sum([T1,T2,T3,T4])

The sum() internally calls __add__() method multiple times and hence gives the appropriate result. Note down the square-brackets used to combine Time objects as a list and thenc passing it to sum().

Thus, the program given here depicts many features of OOP concepts.

ANSWER :

7.1 Class:

Refer 1^st Question & Answer or Click Here

7.2 Instantiate of a Class:

Refer 4^th Question & Answer or Click Here

IN-Short:

Class:

A programmer-defined type. A class definition creates a new class object.

Class object:

An object that contains information about a programmer-defined type. The class object can be used to create instances of the type.

Instance:

An object that belongs to a class.

Instantiate:

To create a new object.

ANSWER :

Class Variables:

Declared inside the class definition (but outside any of the instance methods). They are not tied to any particular object of the class, hence shared across all the objects of the class. Modifying a class variable affects all objects instance at the same time.

Instance Variable:

Declared inside the constructor method of class (the __init__ method). They are tied to the particular object instance of the class, hence the contents of an instance variable are completely independent from one object instance to the other.

ANSWER :

Fig 9.1 Demonstrating a Program to calculate Age & How many Days Left for Upcoming Birthday

Fig 9.2 Output of Demonstrating a Program to calculate Age & How many Days Left for Upcoming Birthday

ANSWER :