Decorator design pattern

As explained in Software Design for Python Programmers

The Decorator Design Pattern is a structural design pattern that dynamically attaches additional responsibilities to an object by wrapping it with objects that share its interface.

The Decorator Design Pattern adds responsibilities to an object at run time by wrapping it with objects that share its interface. It provides a flexible alternative to hardcoding every variation into the original class. section 14.0 section 14.3

A ticket class that stores party access, VIP seating, and drink coupons as attributes must know every enhancement and calculate each one explicitly. Adding, removing, or changing an enhancement therefore requires modifying Ticket and can create change leaks. section 14.3.2

The refactored design makes BaseTicket and every enhancement a Ticket. Enhancement stores another Ticket, and its cost is its own price plus the wrapped ticket’s cost. Party, VIP, and Coupon can therefore be chained in any order while client code treats the result as a ticket. section 14.3.3

Building an enhanced ticket

Start with BaseTicket(), then wrap it successively: Party(ticket), VIP(ticket), Coupon(ticket), and Coupon(ticket). Calling cost on the outermost object traverses the chain and produces the combined total. A different chain can add three coupons and party access without changing Ticket or the decorators.

Figure 14.12 The generic model of the Decorator Design Pattern. Compare with figure 14.11. A ConcreteComponent object by itself is a Component. A ConcreteComponent wrapped by Decorator objects is also a Component. The private component instance variable of superclass Decorator is either a ConcreteComponent object or a Decorator object. This chain implements the nested wrapping of the Decorator objects.

Use the generic model to map Component, ConcreteComponent, Decorator, and ConcreteDecorator roles onto a wrapping architecture. section 14.3.4

How Software Design for Python Programmers covers it

How deep it goes

Explains the pattern through a flawed ticket design, wrapper classes, recursive cost delegation, and a generic role mapping.

Examples

Uses base tickets, pregame parties, VIP seating, and repeated drink coupons to demonstrate inflexible attributes and flexible wrapper chains.

What it assumes

Assumes familiarity with class responsibilities, instance variables, methods, inheritance, and aggregation while distinguishing the pattern from Python decorators.

What you can do after

Build it from scratch by defining a shared component, a wrapper holding that component, and decorators that extend delegated behavior.

How it connects

Connects runtime responsibilities with inheritance, aggregation, abstract classes, subclassing, and object composition.

liveBookmanning.com

Read Software Design for Python Programmers on liveBook Get the book at manning.com

Also explained in

• 

  liveBookmanning.com
  Object-Oriented Software Design in C++A structural design pattern that attaches additional responsibilities to an object dynamically through wrapper objects that preserve the wrapped object's interface.

Compare how 2 books explain decorator design pattern →

Related concepts

• design patternkind of · A design pattern is a reusable, general solution structure for a recurring software design problem in a particular context.
• inheritancealternative to · Inheritance is a mechanism by which a child type or element receives properties, values, or behavior from a parent, subject to system rules and possible overrides.
• hardcodingaddresses · Hardcoding is embedding fixed values, rules, configuration, or behavioral choices directly in a program or its implementation rather than supplying or deriving them separately.

This explainer was generated by Manning's knowledge graph from the text of Software Design for Python Programmers. The section links point to the parts of the book it draws on.