README.md

Design Principles and Patterns

• Design Principles and Patterns
  • Single Responsibility Principle (SRP)
    • SRP Examples
  • Open-Closed Principle (OCP)
    • OCP Examples
  • Liskov Substitution Principle (LSP)
    • LSP Examples
  • Interface Segregation Principle (ISP)
    • ISP Examples
  • Dependency Inversion Principle (DIP)
    • DIP Examples
  • Builder
    • Builder Examples
  • Factory
    • Factory Examples
  • Prototype
    • Prototype Examples
  • Singleton
    • Singleton Examples
  • Adapter
    • Adapter Examples
  • Bridge
    • Intent
    • Motivation
    • Applicability
    • Structure
    • Participants
    • Collaborations
    • Consequences
    • Implementation
    • Related Patterns
    • Bridge Examples
  • Composite
    • Intent
    • Motivation
    • Applicability
    • Structure
    • Participants
    • Collaborations
    • Consequences
    • Implementation
    • Related Patterns
    • Examples
  • Sources

Single Responsibility Principle (SRP)

A class should have only 1 reason to change. Responsibility implies a reason for change.

SRP Examples

Python:

• srp_violate.py
• srp_comply.py
• srp_test.py

Open-Closed Principle (OCP)

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

OCP Examples

Python:

• ocp_violate.py
• ocp_comply.py
• ocp_test.py

Liskov Substitution Principle (LSP)

Subtypes must be substitutable for their base types.

LSP Examples

Python:

• lsp_test.py

Interface Segregation Principle (ISP)

Clients should not be forced to depend on methods that they do not use.

ISP Examples

Python:

• isp_violate.py
• isp_comply.py
• isp_test.py

Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions.

DIP Examples

Python:

• dip_violate.py
• dip_comply.py
• dip_test.py

Builder

• When piecewise object construction is complicated, provide an API for doing it succinctly
• Motivation
  • some objects are simple and can be created in a single initialiser call
  • other objects require a lot of ceremony to create
  • having an object with 10 initialiser arguments is not productive
    • instead, opt of piecewise construction
  • Builder provides an API for constructing an object step-by-step

Builder Examples

Python:

• builder.py
• builder_test.py
• builder_facets.py
• builder_facets_test.py
• builder_inheritance.py
• builder_inheritance_test.py

Factory

• Motivation
  • object creation logic becomes too convoluted
  • initialiser is not descriptive
    • cannot overload with the same sets of arguments with different names
    • can turn into 'optional parameter hell'
• Wholesale object creation (not piecewise like Builder) can be outsourced to:
  • a separate method (Factory Method)
    • a static method that creates objects
  • a separate class (Factory)
    • any entity that can take care of object creation
  • a hierarchy of factories (Abstract Factory)
    • correspond to a hierarchy of types

Factory Examples

Python:

• factory_method.py
• factory_method_test.py
• factory.py
• factory_test.py
• abstract_factory.py
• abstract_factory_test.py

Prototype

• Motivation
  • when it's easier to copy an existing object to fully initialise a new one
• Prototype - a partially or fully initialised object that you copy (clone) and make use of
• We make a copy (clone) of the prototype and customise it
  • requires 'deep copy' support
• We make cloning convenient (e.g., via a Factory)

Prototype Examples

Python:

• prototype_test.py
• prototype_factory_test.py

Singleton

• A component that is instantiated only once
• Motivation
  • for some components it only makes sense to have one in the system
    • database repository
    • object factory
  • the initialiser call is expensive
  • object represents a resource and there is only one instance of the resource
• Provide everyone with the same instance
  • prevent anyone creating additional copies
• Lazy instantiation
  • initialise only when someone actually asks for it

Singleton Examples

Python:

• singleton_decorator_test.py
• singleton_metaclass_test.py
• monostate_test.py

Adapter

• Adapt the interface you are given to the interface that you actually need

Adapter Examples

Python:

• adapter_test.py

Bridge

Intent

• Decouple an abstraction from its implementation so that the two can vary independently

Motivation

• Complex elements in a system can sometimes vary in both their external functionality and their underlying implementation
  • in such cases, inheritance is an undesirable solution
    • the number of classes you must create increases as a function of both these aspects
    • 2 representations and implementations yield 4 classes to develop
    • 3 representations and implementations result in 9 classes
• Inheritance ties a component into a static model, making it difficult to change in the future
• It would be preferable to create a dynamic way to vary both aspects of the component on an as-needed basis
• Bridge solves the problem by decoupling the 2 aspects of the component
  • 2 separate inheritance chains
    • one devoted to functionality (abstraction)
    • the other to implementation
  • it's much easier to mix and match elements from each side
• The coding requirements for Bridge give you an overall savings in the number of classes written as you increase the number of variations
• We refer to the relationship as a bridge, because it bridges the abstraction and its implementation, letting them vary independently

Applicability

• You want to avoid a permanent binding between an abstraction and its implementation
  • when the implementation must be selected or switched at run-time
• Both the abstractions and their implementations should be extensible by subclassing
  • the Bridge pattern lets you combine the different abstractions and implementations and extend them independently
• Changes in the implementation of an abstraction should have no impact on clients
• You want to hide the implementation of an abstraction completely from clients
• You have a proliferation of classes
  • such a class hierarchy indicates the need for splitting an object into two parts

Structure

Participants

• Abstraction
  • defines the abstraction's interface
  • maintains a reference to an object of type Implementor
• RefinedAbstraction
  • extends the interface defined by Abstraction.
• Implementor
  • defines the interface for implementation classes
  • doesn't have to correspond exactly to Abstraction's interface - the two interfaces can be quite different
  • typically the Implementor interface provides only primitive operations, and Abstraction defines higher-level operations based on these primitives
• ConcreteImplementor
  • implements the Implementor interface and defines its concrete implementation

Collaborations

• Abstraction forwards client requests to its Implementor object

Consequences

• Decoupling interface and implementation
  • an implementation is not bound permanently to an interface
    • the implementation of an abstraction can be configured at run-time
    • it's possible for an object to change its implementation at run-time
  • eliminates compile-time dependencies on the implementation
    • changing an implementation class doesn't require recompiling the Abstraction class and its clients
  • encourages layering that can lead to a better-structured system
    • the high-level part of a system only has to know about Abstraction and Implementor
• Improved extensibility
  • you can extend the Abstraction and Implementor hierarchies independently

Implementation

Consider the following implementation issues when applying the Bridge pattern:

• Only one Implementor
  • in situations where there's only one implementation - a degenerate case of the Bridge pattern - creating an abstract Implementor class isn't necessary
  • this separation is still useful when a change in the implementation of a class must not affect its existing clients
• Creating the right Implementor object
  • how, when, and where do you decide which Implementor class to instantiate when there's more than one?
  • if Abstraction knows about all ConcreteImplementor classes, then it can instantiate one of them in its constructor
  • another approach is to choose a default implementation initially and change it later according to usage
  • it's also possible to delegate the decision to another object altogether
    • we can introduce a factory object (see Abstract Factory)
    • the factory knows what kind of object to create
    • a benefit of this approach is that Abstraction is not coupled directly to any of the Implementor classes
• Using multiple inheritance
  • you can use multiple inheritance to combine an interface with its implementation
  • e.g., a class can inherit publicly from Abstraction and privately from a ConcreteImplementor
  • this approach relies on static inheritance - it binds an implementation permanently to its interface
    • you can't implement a true Bridge with multiple inheritance

Related Patterns

• An Abstract Factory can create and configure a particular Bridge
• The Adapter pattern is geared toward making unrelated classes work together
  • it is usually applied to systems after they're designed
  • Bridge, on the other hand, is used up-front in a design to let abstractions and implementations vary independently

Bridge Examples

Python:

• bridge_test.py
• bridge_todo_list_test.py

Composite

Intent

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

Motivation

• Some applications let users build complex entities out of simple components
  • the user can group components to form larger components, which in turn can be grouped to form even larger components
• A simple implementation could define classes for the primitives (simple components) plus other classes that act as containers for these primitives
  • the problem is client code that uses these classes must treat primitive and container objects differently, even if most of the time the user treats them identically
  • makes the application more complex
• The Composite pattern describes how to use recursive composition so that clients don't have to make this distinction
• The key to the Composite pattern is an abstract class that represents both primitives and their containers
  • declares operations that are specific to the primitives
  • declares operations that all composite/container objects share, such as operations for accessing and managing its children
• Primitive objects have no child components, so they don't implement child-related operations
• A container object defines an aggregate of primitive objects
  • implements primitive objects' operations to call the corresponding operations of its children
  • implements child-related operations
  • can compose other container objects recursively

Applicability

• You want to represent part-whole hierarchies of objects
• You want clients to be able to ignore the difference between compositions of objects and individual objects; clients will treat all objects in the composite structure uniformly

Structure

Participants

• Component
  • declares the interface for objects in the composition
  • implements default behaviour for the interface common to all classes, as appropriate
  • declares an interface for accessing and managing its child components
  • (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate
• Leaf
  • represents leaf objects in the composition
  • has no children
  • defines behaviour for primitive objects in the composition
• Composite
  • defines behaviour for components having children
  • stores child components
  • implements child-related operations in the Component interface
• Client
  • manipulates objects in the composition through the Component interface

Collaborations

• Clients use the Component class interface to interact with objects in the composite structure
• If the recipient is a Leaf, then the request is handled directly
• If the recipient is a Composite, then it usually forwards requests to its child components, possibly performing additional operations before and/or after forwarding

Consequences

• Defines class hierarchies consisting of primitive objects and composite objects
• Makes the client simple
  • clients can treat composite structures and individual objects uniformly
• Makes it easier to add new kinds of components
  • clients don't have to be changed for new Component classes
• Can make your design overly general
  • sometimes you want a composite to have only certain components
  • you can't rely on the type system to enforce those constraints for you
  • you'll have to use run-time checks

Implementation

• Explicit parent references
  • maintaining references from child components to their parent can simplify the traversal and management of a composite structure
  • the usual place to define the parent reference is in the Component class
  • Leaf and Composite classes can inherit the reference and the operations that manage it
  • essential to maintain the invariant that all children of a composite have as their parent the composite that in turn has them as children
    • can be implemented once in the add and remove operations of the Composite class, then it can be inherited by all the subclasses
• Sharing components
  • it's often useful to share components, for example, to reduce storage requirements
  • but when a component can have no more than one parent, sharing components becomes difficult
  • the Flyweight pattern shows how to rework a design to avoid storing parents altogether
    • works in cases where children can avoid sending parent requests by externalising some or all of their state
• Maximising the Component interface
  • in order to make clients unaware of the specific Leaf or Composite classes they're using, the Component class should define as many common operations for Composite and Leaf classes as possible
  • sometimes conflict with the principle of class hierarchy design that says a class should only define operations that are meaningful to its subclasses
  • the interface for accessing children is a fundamental part of a Composite class but not necessarily Leaf classes
• Declaring the child management operations
  • an important issue in the Composite pattern is which classes declare these operations in the Composite class hierarchy
  • the decision involves a trade-off between safety and transparency:
    • defining the child management interface at the root of the class hierarchy gives you transparency, because you can treat all components uniformly
      • it costs you safety because clients may try to do meaningless things like add and remove objects from leaves
    • defining child management in the Composite class gives you safety, but you lose transparency because leaves and composites have different interfaces
  • we emphasise transparency over safety in this pattern
    • define default add and remove operations in Component
    • usually it's better to make add and remove fail by default (perhaps by raising an exception) if the component isn't allowed to have children or if the argument of remove isn't a child of the component, respectively
• Should Component implement a list of Components?
  • putting the child pointer in the Component class incurs a space penalty for every leaf, even though a leaf never has children
• Caching to improve performance
  • if you need to traverse or search compositions frequently, the Composite class can cache traversal or search information about its children
  • changes to a component will require invalidating the caches of its parents
    • works best when components know their parents
    • define an interface for telling composites that their caches are invalid

Related Patterns

• Often the component-parent link is used for a Chain of Responsibility
• Decorator is often used with Composite
• Flyweight lets you share components, but they can no longer refer to their parents
• Iterator can be used to traverse composites
• Visitor localises operations and behaviour that would otherwise be distributed across Composite and Leaf classes

Examples

Python:

• composite_shapes_test.py

Sources

• Nesteruk, Dmitri. "Design Patterns in Python for Engineers, Designers, and Architects." Udemy, Udemy, Inc., Aug. 2020, www.udemy.com/course/design-patterns-python/.
• Johnson, Ralph, et al. "Design Patterns CD: Elements of Reusable Object-oriented Software." United Kingdom, Addison-Wesley, 1998.
• Stelting, Stephen, and Olav Maassen. "Applied Java Patterns." Prentice Hall PTR, 2001.