Design Patterns

Pattern purpose

Behavioral

Deal with composition of classes or objects

• Chain of responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template method
• Visitor

Structural

Deal with composition of classes or objects

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

Creational

Creation of object

• Abstract factory
• Builder
• Factory method
• Prototype
• Singleton

Behavioral patterns

These patterns are concerned with algorithms and the assigment of responsibilities between objects.

• Behavioral class patterns use inheritance to distribute behavior between classes
• Behavioral object patterns use object composition rather inheritance

Chain of responsibility

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

• Usage when
  • more than 1 object may handle a request and handler is not known.
  • you want to issue a request to one of several objects without specifying the receiver explicitly
  • the set of objects that can handle a request should be specified dynamically
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/chain_of_responsibility/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/chain_of_responsibility/notebook.ipynb

Command

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests and support undoable operations.

• Usage when
  • need to parameterize objects by an action to perform
  • need to specify, queue and execute requests at different times
  • need to support undo operations
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/command/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/command/notebook.ipynb

Interpreter

Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

• Usage when
  • there is a language to interpret and you can represent statements in the language as abstract trees. It works best when the grammar is simple and efficiency is not a critical concern.
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/interpreter/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/interpreter/notebook.ipynb

Iterator

Provide a way to access the elements of an aggregate object sequentially without exposing tits underlying representation.

• Usage when
  • to access an aggregate object’s contents without exposing its internal representation
  • to support multiple traversals of aggregate objects
  • to provide a uniform interface for traversing different aggregate structures
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/iterator/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/iterator/notebook.ipynb

Mediator

Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly and it lets you vary their interactions indepentently.

• Usage when
  • you have set of objects that communicate but interdependencies increase complexity
  • you have an object that communicates with many other objects but it is difficult to reuse that object
  • want to customize behavior that is distributed across several classes without lots of subclassing
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/mediator/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/mediator/notebook.ipynb

Memento

Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.

• Usage when
  • need to implement undo operation
  • want to hide internal implementation details about the state behind memento encapsulation
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/memento/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/memento/notebook.ipynb

Observer

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

• Usage when
  • you don’t want objects tightly coupled
  • a change to one object requires changing others
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/observer/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/observer/notebook.ipynb

State

Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

• Usage when
  • an object’s behavior depends on its state and must change its behavior at run-time depending on that state
  • operations have large, multipart conditional statements that depend on the object’s state
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/state/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/state/notebook.ipynb

Strategy

Define a family of algorithms, encapsulate each one and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

• Usage when
  • need different variants of an algorithm
  • have many related classes differ only in their behavior allowing to configure a class with one of many behaviors
  • an algorithm uses data that clients shouldn’t know about, so use this pattern to hide complex, algorithm-specific data structures
  • you have multiple conditionals for different behaviors, move conditional branches into their own strategy class
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/strategy/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/strategy/notebook.ipynb

Template method

Define a skeleton of an algorithm in an operation, deferring some steps to subclasses. Template method lets subclasses redefine certain steps of an algorithm without changing the algorithm structure.

• Usage when
  • need to implement invariant parts of an algorithm once in abstract class and leave to subclasses to implement varying behavior
  • common behavior among subclasses should be factored and localized in a common class to avoid code duplication
  • to control subclasses extensions
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/template_method/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/template_method/notebook.ipynb

Visitor

Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

• Usage when
  • an object structure contains many classes of objects with differing interfaces and you want to perform operations on those objects that depend on the concrete class
  • many distinct and unrelated operations need to be performed on objects in an object structure and you want to avoid polluting their classes with these operations.
  • the classes defining the object structure rarely change but you often want to define new operations over the structure.
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/visitor/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/visitor/notebook.ipynb

Structural patterns

Structural patterns are concerned with how classes and objects are composed to form larger structures.

• Structural class patterns use inheritance to compose interfaces or implementations
• Structural object patterns describe ways to compose objects to realize new functionality

Adapter

Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.

Two flavors of adapters

• Class adapter uses multiple inheritance to adapte one interface to another
• Object adapter relies on object composition

• Usage when
  • you want to use an existing class and its interface does not match the one you need
  • you want to create a reusable class that cooperates with unrelated or unforementioned classses, which don’t necessarily have compatible interfaces
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/adapter/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/adapter/notebook.ipynb

Bridge

Decouple an abstraction from its implementation so that the two can vary indepentently.

• Usage when:
  • you want to switch implementations at runtime
  • changes in the implementation of an abstraction should not have impact on clients; that is their code should not have to recompile.
  • you want to divide and organize a monolithic class that has several variants of some functionality.
  • you need to extend a class into several indepentent dimensions.
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/bridge/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/bridge/notebook.ipynb

Composite

Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of object uniformly.

• Usage when:
  • you want to represent part-whole hierarchies of objects
  • you want clients to be able to ignroe the difference between compositions of objects and individual objects. Clietns will treat the objects in the composite structure uniformly.
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/composite/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/composite/notebook.ipynb

Decorator

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

• Usage when
  • want to add responsibilities to individual objects dynamically
  • extension by subclassing is impractical
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/decorator/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/decorator/notebook.ipynb

Facade

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

• Usage when
  • you want to provide a simple interface to a complex subsystem.
  • you have many dependencies between clients and implementation classes of an abstraction, use Facade to decouple the subsystem from clients and other subsystems. Usually one Facade object is required, Facades are often Singletons.
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/facade/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/facade/notebook.ipynb

Flyweight

Use sharing to support large numbers of fine-grained objects efficiently.

• Usage when
  • you have a large number of similar objects that can be separated into intrinsic (shared) and extrinsic (context-specific) state
  • apply when all following are true
    • application uses large number of objects
    • storage costs are high because the sheer quantity of objects
    • most object state can be made external
    • many groups of objects may be replaced by few shared objects once external state is removed
    • the application doesn’t depend on object identity, the flyweight objects can be shared
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/flyweight/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/flyweight/notebook.ipynb

Proxy

Provide a surrogate or placeholder for another object to control access to it.

• Usage when there is a need to have more versatile or sophisticated reference to an object than a simple pointer to it
  • use a remote proxy when you want to provide a local representative for an object in a different space
  • use a virtual proxy to create expensive objects on demand
  • use a protection proxy to control access to the original object
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/proxy/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/proxy/notebook.ipynb

Creational patterns

Abstract Factory

Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

• Usage when
  • system should be independent of how its products are created, composed and represented
  • a system should be configured with one of multiple families of products
  • a family of related product objects is designed to be used together and you need to enforce this constraint
  • you want to provide a class library of products and you want to reveal their interfaces not their implementations
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/abstract_factory/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/abstract_factory/notebook.ipynb

Builder

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

• Usage when
  • the algorithm for creating a complex object should be independent of the parts that make up the object and how they are assembled
  • the construction process must allow different representations for the object that is constructed
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/builder/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/builder/notebook.ipynb

Factory method

Define an interface for creating an object, but let subclasses to decide which class to instantiate. Factory method lets a class defer instantiation to subclasses.

• Usage when
  • a class cannot anticipate the class of objects it must create
  • a class wants its subclasses to specify the objects it creates
  • classes delegate responsibility to one of several helper subclasses and you want to localize the knowledge of which helper subclass is the delegate.
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/factory/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/factory/notebook.ipynb

Prototype

Specify the kinds of objects to create using a prototypical instance and create new objects by copying its prototype.

• Usage when
  • a system should be indepentent of how its products are created
    • when the classes to instantiate are specified at runtime
    • when need to avoid building a class hierarchy of factories that parallels the class hierarchy of products
    • when instances of a class can have one of only a few different combinations of state
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/prototype/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/prototype/notebook.ipynb

Singleton

Ensure a class only has one instance and provide a global point of access to it.

• Usage when
  • need to create one instance of a class and use it throughout the lifecycle of a program
  • you want to limit concurrent access to a shared resource
• Implementation: https://github.com/gdyrrahitis/design-patterns-python/blob/main/singleton/pattern.py
• Execution: https://github.com/gdyrrahitis/design-patterns-python/blob/main/singleton/notebook.ipynb