Design Patterns

Design Patterns.md

Design Patterns

Definition

Design Patterns are solutions to to commonly occurring problems.

We need design patterns to ensure our work is consistent, reliable and understandable.

Examples

• Building Architecture
• Electrical and plumbing
• Automobile design
• Mobile phone interfaces

Classification

• Creational - Used to design object creation

  Examples: Singleton, Factory, Builder

• Structural - Used to design relationship between objects

  Examples: Adapter, Facade, Composite

• Behavioural - Used to design object interaction and responsibility

  Examples: Command, Observer, Stratergy

Object Oriented Design Principles (SOLID)

1. Single Responsibility: Means a Class should have only one responsibility i.e, Cooking or Washing but not both
2. Open-Closed: Class should be open for extension usually by inheritance but closed for modification
3. Liskov Substitution: Subclasses should stand in for parents without breaking anything
4. Interface Segregation: Many specific interfaces are better than one do it all interface (in python we use Abstract Base Class combined with multiple inheritance to achieve this)
5. Dependency Inversion: We should program towards Abstraction not concrete Implementation cause Implementation can vary. In python ABC(Abstract Base Classes) can be used just for this.

Note: About Interfaces in Python

Unlike other languages Python do not have standard way to define interfaces. In Python we can implement Interfaces using ABC(Abstract Base Classes)

Commonly used Design Patterns

• Behavioural Patterns

  1. Strategy Pattern

     • Also known as Policy pattern

  2. Observer Pattern

     • Also known as Dependents pattern, Publish-Subscribe pattern
     • Description: When the state of one object changes it's dependents are notified

  3. Command Pattern

     • Also known as Action pattern, Transaction pattern

• Creational Patterns

  1. Singleton Pattern

     • Also known as Anti pattern because of it's downsides

  2. Builder Pattern

  3. Abstract Factory Pattern

     • Example Code

• Structural Patterns

  1. Facade Pattern

     • Unified interface to a set of interfaces

  2. Adapter Pattern

     • Bridging the incompatible APIs