Python (5): OOP Concepts

OOP Concepts

P: Polymorphism
I: Inheritance
E: Encapsulation

A: Abstraction

Apple PIE

How and where OOP concepts are used?
    

Because using classes well requires some up-front planning, they tend to be of more interest to people who work in strategic mode (doing long-term product development) than to people who work in tactical mode (where time is in very short supply).

The concept of “self”

class C2: pass # Make superclass objects
class C3: pass

class C1(C2, C3): # Make and link class C1
    def setname(self, who): # Assign name: C1.setname
        self.name = who # Self is either I1 or I2

I1 = C1() # Make two instances
I2 = C1()
I1.setname('bob') # Sets I1.name to 'bob'
I2.setname('sue') # Sets I2.name to 'sue’
print(I1.name) # Prints 'bob'

There’s nothing syntactically unique about def in this context. Operationally, though, when a def appears inside a class like this, it is usually known as a method, and it automatically receives a special first argument—called self by convention—that provides a handle back to the instance to be processed. Any values you pass to the method yourself go to arguments after self (here, to who).

If you’ve ever used C++ or Java, you’ll recognize that Python’s self is the same as the “this” pointer, but self is always explicit in both headers and bodies of Python methods to make attribute accesses more obvious: a name has fewer possible meanings.

Concept of Inheritance of Variables and Methods
class Employee: # General superclass
    def __init__(self, who = "NA", sal = 0): # Set name when constructed
        self.name = who
        self.salary = sal

    def getDetails(self):
        rtn = (self.name, self.salary)
        print("rtn: ", rtn)
        return rtn

class Engineer(Employee): # Specialized subclass
   pass

print("Defaults:")
e = Engineer()
e.getDetails()

print()
print("Custom:")
e = Engineer(who = 'Ashish', sal = 100)
e.getDetails() 

$ python inheritance.py

Defaults:
rtn:  ('NA', 0)

Custom:
rtn:  ('Ashish', 100)

Error Alert

class Employee: # General superclass
    def __init__(self, who, sal): # Set name when constructed
        self.name = who
        self.salary = sal

class Engineer(Employee): # Specialized subclass
    pass
 
e = Engineer()

$ python inheritance.py
Traceback (most recent call last):
File "inheritance.py", line 14, in <module>
    e = Engineer()
TypeError: __init__() missing 2 required positional arguments: 'who' and 'sal' 

Polymorphism

class Employee:
    def __init__(self, who = "NA", sal = 0): 
        self.name = who
        self.salary = sal

    def getDetails(self):
        rtn = (self.name, self.salary)
        print("rtn: ", rtn)
        return rtn

class Engineer(Employee): 
    def reviseSalary(self):
        self.salary =+ 100

    def reviseSalary(self, inc = 200):
        self.salary =+ inc

e = Engineer(who = 'Ashish', sal = 100)
e.getDetails()
e.reviseSalary()
e.getDetails()
e.reviseSalary(inc = 500)
e.getDetails()

$ python polymorphism.py
rtn:  ('Ashish', 100)
rtn:  ('Ashish', 200)
rtn:  ('Ashish', 500)

Abstraction

When we call "print()" for an object it prints some object details like these:

<__main__.Employee object at 0x000001F549F7A1D0>

However, when we define the Python's in-built __str__() for an object, we are essentially telling what should happen when print() is called for the object and end-user would not have to know how to get user-friendly details of the object. This is abstraction (abstraction of print()'s output in __str__() redefining it).

class Employee:
    def __init__(self, who = "NA", sal = 0): 
        self.name = who
        self.salary = sal

    def getDetails(self):
        rtn = (self.name, self.salary)
        print("rtn: ", rtn)
        return rtn

class Engineer(Employee): 
    def reviseSalary(self):
        self.salary =+ 100

    def reviseSalary(self, inc = 200):
        self.salary =+ inc

    def __str__(self): # Higher priority than __repr__ when using with print()
        return '[From __str__: %s, %s]' % (self.name, self.salary)

    #def __repr__(self):
    #    return '[From __repr__: %s, %s]' % (self.name, self.salary)

e = Engineer(who = 'Ashish', sal = 100)
e.getDetails()

print("e:", e)
print("e.__repr__():", e.__repr__())

f = Employee()
print("f = Employee():", f) 

$ python abstraction.py
rtn:  ('Ashish', 100)
e: [From __str__: Ashish, 100]
e.__repr__(): <__main__.Engineer object at 0x000001F549C2F8D0>
f = Employee(): <__main__.Employee object at 0x000001F549F7A1D0>

Tags: Python,Technology, Title: Python (5): OOP Concepts