Creational

Builder

• Separate component when object construction gets too complicated
• Can create mutually cooperating sub-builders
• Often has a fluent interface

Factories

• Factory method more expressive than initializer
• Factory can be an outside class or inner class

Prototype

• Creation of object from an existing object
• Requires explicit deep copy

Singleton

• When you need to ensure just a single instance exisits
• Easy to make with a decorator or metaclass
• Consider using dependency injection

Structural

Adapter

• Converts the interface you get to the interface you need

Bridge

• Decouple abstraction from implementation

Composite

• Allows clients to treat individual objects and compositions of objects uniformly

Decorator

• Attach additional responsibilities to object
• Python has functional decorator

Facade

• Provide a single unified interface over a set of interface
• Friendly and easy-to-use, but can provide access to low-level features

Flyweight

• Efficiently support very large numbers of similar objects

Proxy

• Provide a surrogate object that forwards calls to the real object while performing additional functions
• E.g., access control, communication, logging, etc.

Behavioural

Chain of Responsibility

• Allow components to process information/events in a chain
• Each element in the chain refers to next element; or
• Make a list and go through it

Command

• Encapsulate a request into a separate object
• Good for audit, replay, undo/redo
• Part of CQS/CQRS

Interpreter

• Transform textual input into object-oriented structures
• Used by interpreters, compilers, static analysis tools, etc.
• Compiler Theory is a separate branch of Computer Science

Iterator

• Provide an interface for accessing elements of an aggregate object
• __iter__/__next are stateful, but yield is much more convenient

Mediator

• Provides mediation services between two objects
• E.g., message passing, chat room

Memento

• Yield tokens representing system states
• Tokens do not allow direct manipulation, but can be used in appropriate APIs

Observer

• Allows notifications of changes/happenings in a component

State

• We model system by having one of a possible states and transitions between these states
• Such a system is called a state machine
• Special frameworks exists to orchestrate state machines

Strategy & Template Method

• Both define a skeleton algorithm with details filled in by implementor
• Strategy uses ordinary composition, template method uses inheritance

Visitor

• Allows non-intrusive addition of functionality to hierarchies