Design Patterns in Python

Principles

• you don't need to use it always!
• disconnect what's equal from what changes
• develop for an interface instead of an implementation
• open to extension, closed to modification
• they help you think out of the box!
• Hollywood principle: superclasses must call child classes when need, and not the other way around

Why to use Design Patterns?

• good programming principles: like coupling and cohesion
• code faster: you know what/how to do before implementation
• code reuse: can be used in multiple projects
• code standardization
• encourage more legible and maintainable code
• provide a common language and jargon for programmers

When to apply?

Pattern	Use it when you need...
Singleton	to share attributes across the whole program
Strategy	different implementations for the same interface
Template	a sequence of steps with different implementations
Adapter	to convert an interface into another
Observer	to notify subscribers in case of events in the publisher
Decorator	to add responsibilities to an object dynamically
State	to implement a finite state machine
Composite	a tree of similar objects
Command	to encapsulate all the info to perform an action (or schedule/undo them)