# OOPS 2 - Dunders, Inheritance Continued

---
title: Agenda
description:
duration: 300
card_type: cue_card
---

### Agenda

* Dunder / Magic methods
* Inheritance
* Private Properties
* Multiple Inheritance
* Method Resolution Order

---
title: Quiz-1
description: Quiz-1
duration: 60
card_type: quiz_card
---

# Question

How many objects would be created by runing the following code?

Code

class A:
    print("Am I a class?")

A()

obj1 = A()

if A():
    print(A())

# Choices

- [x] 4
- [ ] 2
- [ ] 3
- [ ] 0

---
title: Dunder / Magic methods
description:
duration: 1200
card_type: cue_card
---

## Dunder / Magic methods

* These are special methods that have double underscores at the beginning and end of their names (e.g., __init__).
* These methods provide a way for a class to define its behavior.
* By modifying these methods, you can customize the default behavior of your objects.

Code

class Car:
    def __init__(self, name, mileage):
        self.name = name
        self.mileage = mileage

Code

c1 = Car("Nexon", 12)
c2 = Car("Altroz", 15)

Code

print(c1)
# Name -> Mileage

> Output

<__main__.Car object at 0x7bd1cb5cc880>

Whenever call the `print()` function over some object then there should always be a pre-defined message linked to that object that gets printed.

Hence, printing is a behaviour in Python.

With dunder/magic methods, we can modify this behaviour.

Code

class Car:
    def __init__(self, name, mileage):
        self.name = name
        self.mileage = mileage

    # Whenever an object is printed, __str__() function is called.
    def __str__(self):
        return f"{self.name} -> {self.mileage}"

    def __add__(self, other):
        return self.mileage + other.mileage

    def __lt__(self, other):
        return self.mileage < other.mileage

    def __call__(self):
        print("I WAS CALLED")

Code

c1 = Car("Nexon", 12)
c2 = Car("Altroz", 15)

Code

print(c1)

> Output

Nexon -> 12

Code

c1 + c2
# Car.__add__(c1, c2)

> Output

27

---
title: Quiz-2
description: Quiz-2
duration: 60
card_type: quiz_card
---

# Question

What will be the output of the following code?

Code

class A:
  def __init__(self, a):
    self.a = a
  def __add__(self, b):
    if isinstance(b, int):
      return A(self.a + b)
    return A(self.a + b.a)
  def __str__(self):
    return f"{self.a}"


one = A(1)
two = A(2)

print(one + two + 100)

# Choices

- [x] 103
- [ ] 3
- [ ] Error
- [ ] None of the above

---
title: Inheritance
description:
duration: 1200
card_type: cue_card
---

## Inheritance

<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/015/907/original/Screenshot_2022-10-07_at_10.04.40_AM.png?1665117262">

\
Code

class Parent:
  def __init__(self):
    print("parent class")

class Child(Parent): # inheriting parent class
  def __init__(self):
    print("child class")

Code

class SchoolMember:
    def __init__(self, name):
        self.name = name

class Student(SchoolMember): 
    def __init__(self, name, grade):
        self.grade = grade

class Staff(SchoolMember):
    def __init__(self, name, salary):
        self.salary = salary

class Teacher(Staff):
    def __init__(self, name, salary, subject):
        self.subject = subject

<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/015/908/original/Screenshot_2022-10-07_at_10.05.56_AM.png?1665117338">

\
Code

s1 = Student("Krishna", "A")

type(s1)

> Output

__main__.Student

Code

s1.grade

> Output

'A'

Code

s1.name # class inherited but still can't use the properties

> Output

---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-17-58cbae705711> in <cell line: 1>()
----> 1 s1.name # class inherited still cannot use the properties

AttributeError: 'Student' object has no attribute 'name'

#### The problem is that we never called the `__init__()` function of the parent class inside the child class.

#### We can use the `super()` method to directly call the methods of parent class.

Code

class SchoolMember:
    def __init__(self, name):
        self.name = name

class Student(SchoolMember):
    def __init__(self, name, grade):
        super().__init__(name)
        # super() automatically corresponds to the direct parent class
        self.grade = grade

class Staff(SchoolMember):
    def __init__(self, name, salary):
        super().__init__(name)
        self.salary = salary

class Teacher(Staff):
    def __init__(self, name, salary, subject):
        super().__init__(name, salary)
        self.subject = subject

Code

s1 = Student("Sharath", "A")
print(s1.name)

> Output

Sharath

Code

s1.name = "Random"
print(s1.name)

> Output

Random

Code

t1 = Teacher("Bipin", 10, "Python")

t1.name, t1.salary, t1.subject

> Output

('Bipin', 10, 'Python')

---
title: Quiz-3
description: Quiz-3
duration: 60
card_type: quiz_card
---

# Question

What would be the output of the following?

Code

class A:
    def do_something(self):
        print("I do something!", end=" ")

class B(A):
    def do_something(self):
        super().do_something()
        print("I also do something!", end=" ")

b = B()
b.do_something()

# Choices

- [ ] I also do something!
- [ ] I also do something! I do something!
- [x] I do something! I also do something!

---
title: Need of private properties
description:
duration: 600
card_type: cue_card
---

## Need of private properties -

Code

class BankAccount:
    def __init__(self, balance):
        self.balance = balance

    def withdrawl(self, amount):
        self.balance -= amount

    def deposit(self, amount):
        self.balance += amount

Code

b1 = BankAccount(10000)

Code

b1.deposit(5000)

b1.balance

> Output

15000

Code

b1.withdrawl(15000)

b1.balance

> Output

0

Code

b1.balance = 23456789854323456789987654345678909876543

b1.balance

> Output

23456789854323456789987654345678909876543

Just like that, any property within the instance can be modified to anything.

---
title: Break & Doubt Resolution
description:
duration: 600
card_type: cue_card
---

### Break & Doubt Resolution

`Instructor Note:`
* Kindly take this time (up to 5-10 mins) to give a short break to the learners.
* Meanwhile, you can ask the them to share their doubts (if any) regarding the topics covered so far.

---
title: Private Properties
description:
duration: 1200
card_type: cue_card
---

## Private Properties

- will be accessible within the class.
- won't be accessible outside the class definition.

Code

class BankAccount:
    def __init__(self, balance):
        # initializing private property
        self.__balance = balance

    # Setter
    def withdraw(self, amount):
        self.__balance -= amount

    def deposit(self, amount):
        self.__balance += amount

    # Getter
    def check_balance(self):
        return self.__balance

Code

b1 = BankAccount(10000)

# cannot be accessed like this
b1.__balance

> Output

---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-34-88d3b8138d4e> in <cell line: 2>()
1 # cannot be accessed like this
----> 2 b1.__balance

AttributeError: 'BankAccount' object has no attribute '__balance'

Code

b1.deposit(10000)

b1.withdraw(5000)

b1.check_balance()

> Output

25000

#### Nothing can be completely *private* in Python.

When we create variables like `__private_var`, Python internally creates it with name of `_ClassName__private_var`.

Hence, we can still access it with the above name.

<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/015/909/original/Screenshot_2022-10-07_at_10.18.54_AM.png?1665118116">

\
Code

b1._BankAccount__balance = 159854568765456789876545678765678
# Name Mangling

b1.check_balance()

> Output

159854568765456789876545678765678

Code

# Checking the properties and methods within any object -
dir(b1)

> Output

['_BankAccount__balance',
'__class__',
'__delattr__',
'__dict__',
'__dir__',
'__doc__',
'__eq__',
'__format__',
'__ge__',
'__getattribute__',
'__gt__',
'__hash__',
'__init__',
'__init_subclass__',
'__le__',
'__lt__',
'__module__',
'__ne__',
'__new__',
'__reduce__',
'__reduce_ex__',
'__repr__',
'__setattr__',
'__sizeof__',
'__str__',
'__subclasshook__',
'__weakref__',
'check_balance',
'deposit',
'withdraw']

Code

b1.__balance = "random balance"

dir(b1)

> Output

['_BankAccount__balance',
'__balance',
'__class__',
'__delattr__',
'__dict__',
'__dir__',
'__doc__',
'__eq__',
'__format__',
'__ge__',
'__getattribute__',
'__gt__',
'__hash__',
'__init__',
'__init_subclass__',
'__le__',
'__lt__',
'__module__',
'__ne__',
'__new__',
'__reduce__',
'__reduce_ex__',
'__repr__',
'__setattr__',
'__sizeof__',
'__str__',
'__subclasshook__',
'__weakref__',
'check_balance',
'deposit',
'withdraw']

---
title: Multiple Inheritance
description:
duration: 900
card_type: cue_card
---

## Multiple Inheritance

- Inheriting more than one class.

Code

class A:
    def __init__(self, a):
        self.a = a

class B:
    def __init__(self, b):
        self.b = b

# C inherits from both A and B
class C(A, B):
    def __init__(self, a, b, c):
        # Assumption 1
        # super().__init__(a, b)

        # Assumption 2
        # super().__init__(a)
        # super().__init__(b)

        # Assumption 3
        A.__init__(self, a)
        B.__init__(self, b)
        self.c = c

Code

c1 = C(1,2,3)

c1.c

> Output

3

---
title: Method Resolution Order
description:
duration: 1200
card_type: cue_card
---

## Method Resolution Order

<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/015/910/original/Screenshot_2022-10-07_at_10.31.02_AM.png?1665118892">

\
Code

class A:
    x = 10

class B(A):
    pass

class C(B):
    pass

class D(A):
    x = 5

class E(C, D):
    pass

# E -> C -> B -> D -> A

Code

e1 = E()
e1.x

> Output

5

Code

# It'll give the method resolution order
E.__mro__

> Output

(__main__.E, __main__.C, __main__.B, __main__.D, __main__.A, object)

---
title: Quiz-4
description: Quiz-4
duration: 60
card_type: quiz_card
---

# Question

Select the correct Method Resolution Order for class `E`.

Code

class A:
  pass
class B:
  pass
class C(A, B):
  pass
class D:
  pass
class E(C, D):
  pass

# Choices

- [x] E -> C -> A -> B -> D
- [ ] E -> D -> C -> B -> A
- [ ] E -> A -> B -> D -> C
- [ ] None of the above

---
title: Practice Coding Question(s)
description:
duration: 600
card_type: cue_card
---

### Practice Coding Question(s)

You can pick the following question and solve it during the lecture itself.

This will help the learners to get familiar with the problem solving process and motivate them to solve the assignments.

<span style="background-color: pink;">Make sure to start the doubt session before you start solving the question.</span>

Q. https://www.scaler.com/hire/test/problem/96310/