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. Class methods
10. Multiple Inheritance and how lookup works
11. Method Resolution Order (MRO)
12. Decorators
13. Static methods
14. Class methods

NOTES

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01. Classes

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02. Instances, Instance methods, Instance attributes

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03. Class attributes

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04. The init constructor

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

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06. Encapsulation

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07. Polymorphism

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08. Instance methods

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09. Class methods

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10. Multiple Inheritance and how lookup works

• 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.

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11. Method Resolution Order (MRO)

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

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

• Python, by default, uses a depth-first ordering while following 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

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12. Decorators

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13. Static methods

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14. Class methods

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15.