A collection of design patterns/idioms in Python


A collection of design patterns and idioms in Python.

When an implementation is added or modified, be sure to update this file and rerun (eg. ./ to keep the output comments at the bottom up to date.

Current Patterns:

Creational Patterns:

abstract_factoryuse a generic function with specific factories
borga singleton with shared-state among instances
builderinstead of using multiple constructors, builder object receives parameters and returns constructed objects
factory_methoddelegate a specialized function/method to create instances
lazy_evaluationlazily-evaluated property pattern in Python
poolpreinstantiate and maintain a group of instances of the same type
prototypeuse a factory and clones of a prototype for new instances (if instantiation is expensive)

Structural Patterns:

3-tierdata<->business logic<->presentation separation (strict relationships)
adapteradapt one interface to another using a white-list
bridgea client-provider middleman to soften interface changes
compositeencapsulate and provide access to a number of different objects
decoratorwrap functionality with other functionality in order to affect outputs
facadeuse one class as an API to a number of others
flyweighttransparently reuse existing instances of objects with similar/identical state
front_controllersingle handler requests coming to the application
mvcmodel<->view<->controller (non-strict relationships)
proxyan object funnels operations to something else

Behavioral Patterns:

chainapply a chain of successive handlers to try and process the data
cataloggeneral methods will call different specialized methods based on construction parameter
chaining_methodcontinue callback next object method
commandbundle a command and arguments to call later
iteratortraverse a container and access the container's elements
mediatoran object that knows how to connect other objects and act as a proxy
mementogenerate an opaque token that can be used to go back to a previous state
observerprovide a callback for notification of events/changes to data
publish_subscribea source syndicates events/data to 0+ registered listeners
registrykeep track of all subclasses of a given class
specificationbusiness rules can be recombined by chaining the business rules together using boolean logic
statelogic is organized into a discrete number of potential states and the next state that can be transitioned to
strategyselectable operations over the same data
templatean object imposes a structure but takes pluggable components
visitorinvoke a callback for all items of a collection

Fundamental Patterns:

delegation_patternan object handles a request by delegating to a second object (the delegate)


blackboardarchitectural model, assemble different sub-system knowledge to build a solution, AI approach - non gang of four pattern
graph_searchgraphing algorithms - non gang of four pattern
hsmhierarchical state machine - non gang of four pattern