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Mastering Python Design Patterns
Mastering Python Design Patterns

Mastering Python Design Patterns: Start learning Python programming to a better standard by mastering the art of Python design patterns

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Mastering Python Design Patterns

Chapter 2. The Builder Pattern

Imagine that we want to create an object that is composed of multiple parts and the composition needs to be done step by step. The object is not complete unless all its parts are fully created. That's where the Builder design pattern can help us. The Builder pattern separates the construction of a complex object from its representation. By keeping the construction separate from the representation, the same construction can be used to create several different representations [GOF95, page 110], [j.mp/builderpat].

A practical example can help us understand what the purpose of the Builder pattern is. Suppose that we want to create an HTML page generator, the basic structure (construction part) of an HTML page is always the same: it begins with <html> and finishes with </html>; inside the HTML section are the <head> and </head> elements, inside the head section are the <title> and </title> elements, and so forth...

A real-life example

The Builder design pattern is used in fast-food restaurants. The same procedure is always used to prepare a burger and the packaging (box and paper bag), even if there are many different kinds of burgers (classic, cheeseburger, and more) and different packages (small-sized box, medium-sized box, and so forth). The difference between a classic burger and a cheeseburger is in the representation, and not in the construction procedure. The director is the cashier who gives instructions about what needs to be prepared to the crew, and the builder is the person from the crew that takes care of the specific order. The following figure provided by www.sourcemaking.com shows a Unified Modeling Language (UML) sequence diagram of the communication that takes place between the customer (client), the cashier (director), and the crew (builder) when a kid's menu is ordered [j.mp/builderpat].

A real-life example

A software example

The HTML example that was mentioned at the beginning of the chapter is actually used by django-widgy, a third-party tree editor for Django that can be used as a Content Management System (CMS). The django-widgy editor contains a page builder that can be used for creating HTML pages with different layouts [j.mp/widgypb].

The django-query-builder library is another third-party Django library that relies on the Builder pattern. The django-query-builder library can be used for building SQL queries dynamically. Using this, we can control all aspects of a query and create a different range of queries, from simple to very complex [j.mp/djangowidgy].

Use cases

We use the Builder pattern when we know that an object must be created in multiple steps, and different representations of the same construction are required. These requirements exist in many applications such as page generators (like the HTML page generator mentioned in this chapter), document converters [GOF95, page 110], and User Interface (UI) form creators [j.mp/pipbuild].

Some resources mention that the Builder pattern can also be used as a solution to the telescopic constructor problem [j.mp/wikibuilder]. The telescopic constructor problem occurs when we are forced to create a new constructor for supporting different ways of creating an object. The problem is that we end up with many constructors and long parameter lists, which are hard to manage. An example of the telescopic constructor is listed at the stackoverflow website [j.mp/sobuilder]. Fortunately, this problem does not exist in Python, because it can be solved in at least two ways:

Implementation

Let's see how we can use the Builder design pattern to make a pizza ordering application. The pizza example is particularly interesting because a pizza is prepared in steps that should follow a specific order. To add the sauce, you first need to prepare the dough. To add the topping, you first need to add the sauce. And you can't start baking the pizza unless both the sauce and the topping are placed on the dough. Moreover, each pizza usually requires a different baking time, depending on the thickness of its dough and the topping used.

We start with importing the required modules and declaring a few Enum parameters [j.mp/pytenum] plus a constant that are used many times in the application. The STEP_DELAY constant is used to add a time delay between the different steps of preparing a pizza (prepare the dough, add the sauce, and so on) as follows:

from enum import Enum

PizzaProgress = Enum('PizzaProgress', 'queued preparation baking ready')
PizzaDough...

Summary

In this chapter, we have seen how to use the Builder design pattern. We use the Builder pattern for creating an object in situations where using the Factory pattern (either a Factory Method or an Abstract Factory) is not a good option. A Builder pattern is usually a better candidate than a Factory pattern when:

  • We want to create a complex object (an object composed of many parts and created in different steps that might need to follow a specific order).
  • Different representations of an object are required, and we want to keep the construction of an object decoupled from its representation
  • We want to create an object at one point in time but access it at a later point

We saw how the Builder pattern is used in fast-food restaurants for preparing meals, and how two third-party Django packages, django-widgy and django-query-builder, use it for generating HTML pages and dynamic SQL queries, respectively. We focused on the differences between a Builder pattern and a Factory pattern, and gave...

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Description

This book is for Python programmers with an intermediate background and an interest in design patterns implemented in idiomatic Python. Programmers of other languages who are interested in Python can also benefit from this book, but it would be better if they first read some introductory materials that explain how things are done in Python.

Who is this book for?

This book is for Python programmers with an intermediate background and an interest in design patterns implemented in idiomatic Python. Programmers of other languages who are interested in Python can also benefit from this book, but it would be better if they first read some introductory materials that explain how things are done in Python.

What you will learn

  • Explore Factory Method and Abstract Factory for object creation
  • Clone objects using the Prototype pattern
  • Make incompatible interfaces compatible using the Adapter pattern
  • Secure an interface using the Proxy pattern
  • Choose an algorithm dynamically using the Strategy pattern
  • Extend an object without subclassing using the Decorator pattern
  • Keep the logic decoupled from the UI using the MVC pattern

Product Details

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Publication date : Jan 28, 2015
Length: 212 pages
Edition : 1st
Language : English
ISBN-13 : 9781783989331
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Product Details

Publication date : Jan 28, 2015
Length: 212 pages
Edition : 1st
Language : English
ISBN-13 : 9781783989331
Category :
Languages :

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Table of Contents

17 Chapters
1. The Factory Pattern Chevron down icon Chevron up icon
2. The Builder Pattern Chevron down icon Chevron up icon
3. The Prototype Pattern Chevron down icon Chevron up icon
4. The Adapter Pattern Chevron down icon Chevron up icon
5. The Decorator Pattern Chevron down icon Chevron up icon
6. The Facade Pattern Chevron down icon Chevron up icon
7. The Flyweight Pattern Chevron down icon Chevron up icon
8. The Model-View-Controller Pattern Chevron down icon Chevron up icon
9. The Proxy Pattern Chevron down icon Chevron up icon
10. The Chain of Responsibility Pattern Chevron down icon Chevron up icon
11. The Command Pattern Chevron down icon Chevron up icon
12. The Interpreter Pattern Chevron down icon Chevron up icon
13. The Observer Pattern Chevron down icon Chevron up icon
14. The State Pattern Chevron down icon Chevron up icon
15. The Strategy Pattern Chevron down icon Chevron up icon
16. The Template Pattern Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Half star icon Empty star icon 3.6
(5 Ratings)
5 star 20%
4 star 60%
3 star 0%
2 star 0%
1 star 20%
Richard Hazey Mar 04, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
This book is well organized and concisely written. Each chapter stands on its own and can be read independently, which fits my style and allows for reading it as time permits. The chapters each contain some theory or explanation as well as copious amounts of code and real world examples. The use cases were also helpful in understanding situations where the design patterns could be implemented.I purchased the eBook version which had links embedded in the text for additional detail and in-depth explanations on related topics. I like to explore as I read and learn, so this was a welcome feature.The only con for me was that it's written for Python 3, however, the concepts apply to Python 2 and other languages. Eventually I will move to Python 3 and this was a gave me a chance to try Python 3.I recommend if you want to learn about design patterns and you're a Python user.
Amazon Verified review Amazon
Matteo Mar 01, 2015
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
If you need to find new ideas on how to optimize or make your code more easy to interpret this book will give you several examples of code structures to make it more efficient or more simple to interpret. The book writing style is a bit dry and it's not for beginners(you need to know what python is about and how to code with it or you will get lost) but the core content is sound so if you know your stuff and you want to learn new coding styles this book is for you.
Amazon Verified review Amazon
GEORGE G Mar 22, 2015
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
I really enjoy reading this book. It was very useful to learn how to properly apply certain design patterns to real-life examples.
Amazon Verified review Amazon
Arto Mujunen Sep 14, 2019
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
Straight forward explanations for each design patterns without academic jargon
Amazon Verified review Amazon
vytas315 Jul 30, 2016
Full star icon Empty star icon Empty star icon Empty star icon Empty star icon 1
There is simple and then there is simplistic. The examples in this book may be structured in the style of the design patterns discussed, but they lack the complexity to truly communicate the purpose of these patterns. If you need general ideas about patterns, this book is fine. However, I'm finding myself googling every section to learn more practical information about when and why to use these patterns. This book is very light on content and does nothing to convey deeper understanding of when certain principles should be used. I encourage you to google each chapter title rather than buying this book.
Amazon Verified review Amazon
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