Object Oriented Programming in Python

1. Classes
2. Instances, Instance methods, Instance attributes
3. Class attributes
4. The init constructor
5. Inheritance (Inheriting {attributes,methods,constructors etc..})
6. Encapsulation
7. Polymorphism
8. Instance methods
9. Multiple Inheritance and method/attribute lookup
10. Method Resolution Order (MRO)
11. Decorators
12. Static methods
13. Class methods

NOTES

---

01. Classes

Classes are the building blocks in Object Oriented Programming.

Classes can be seen as blueprints from which you create your Instances.

---

02. Instances, Instance methods, Instance attributes

---

03. Class attributes

Attributes or methods specific to a class are called Class attributes

Example:

class MyClass(object):
    value = 10
    
    def __init__(self):
        pass

Here value is a class attribute. These are used when certain values need to be set outside a function.

---

04. The init constructor

The init() constructor is a magic method which gets called when a class is instantiated.

Any attributes set under the init() constructor will be instantiated at the time of instance creation.

---

05. Inheritance (Inheriting {attributes,methods,constructors etc..})

---

06. Encapsulation

---

07. Polymorphism

---

08. Instance methods

---

09. Multiple Inheritance and method/attribute lookup

• Any class can inherit from other classes.
• Any python class can inherit from multiple classes at the same time.
• The class that inherits another class is called the Base/Child class.
• The class being inherited by the Child class is called the Parent class.
• The child class inherits any methods and attributes defined in the parent classes.
• Python uses a depth-first method resolution order (MRO) to fetch methods.
• When two classes inherits from the same class, from Python2.3 onwards, the MRO omits the first occurrence of the class.
• This new MRO lookup method applies from Python2.3, and is for the new-style classes. NOTE: New style classes inherits from the 'object' parent class.

---

10. Method Resolution Order (MRO)

• Python has a method lookup order, called MRO (Method Resolution Order)

• The MRO path can be printed to stdout using print <class-name>.mro()

• Python, by default, uses a depth-first lookup path for MRO.

• ie.. Imagine you have four classes, A, B, C, D.

  1. You instance is created from D.
  2. D inherits from B and C
  3. B inherits from A.
  4. Both C and A has a method with the same name.
  5. Since python follows a depth-first MRO, the method is called from A

REFERENCE: Check the code examples in:

• 14-multiple-inheritance-1.py
• 15-multiple-inheritance-2.py

In some cases, the inheritance can get quite cumbersome when multiple classes inherit from the same classes, in multiple levels.

NOTE : From Python2.3, the MRO has changed slightly in order to speed up the method lookups.

The MRO lookup now skips/omits the initial occurrences of classes which occurs multiple time in the lookup path.

• Example:
  1. Four classes, A, B, C, D.
  2. D inherits from both B and C
  3. B inherits from A
  4. C inherits from A
  5. Both C and A contains a similar named method.
  6. Your instance in created from class D.
  7. You try a lookup for the method which is named both in A and C.
  8. The usual lookup should be D -> B -> A -> C -> A a. Hence since the method exists in A and C, it should return from A. b. But since the class A is occurring twice and due to the new MRO method, the lookup will be D -> B -> C -> A.
  9. The lookup should return the method from class C, rather than A.

REFERENCE: Check the code example in:

• 16-multiple-inheritance-3.py

---

11. Decorators

---

12. Static methods

---

13. Class methods

---

14. Magic methods

Magic methods