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Practical Python Design Patterns: Pythonic Solutions to Common

W. Badenhorst Practical Python Design Patterns



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Practical Python Design Patterns - Springer

Practical Python Design Patterns: Pythonic Solutions to Common Problems ISBN-13 (pbk): 978-1-4842-2679-7 ISBN-13 (electronic): 978-1-4842-2680-3 https://doi org/10 1007/978-1-4842-2680-3 Library of Congress Control Number: 2017957538 Copyright © 2017 by Wessel Badenhorst This work is subject to copyright



Python Design Patterns - Tutorialspoint

Python Design Patterns – Introduction Design patterns are used to represent the pattern used by developers to create software or web application These patterns are selected based on the requirement analysis The patterns describe the solution to the problem when and where to apply the solution and the consequences of the implementation



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CHAPTER 1 Contributors List of contributors Note: This needs some thought I want to include everyone who makes a contribution but I’d also like to indicate people who have made larger contributions – there’s no automatic way to do this now that we

What are design patterns in Python?

    Python Design Patterns 1 Design patterns are used to represent the pattern used by developers to create software or web application. These patterns are selected based on the requirement analysis. The patterns describe the solution to the problem, when and where to apply the solution and the consequences of the implementation.

How many chapters are in the book design patterns in Python?

    Chapter 1. Creational Design Patterns in Python................ 5 Chapter 2. Structural Design Patterns in Python................ 29 Chapter 3. Behavioral Design Patterns in Python.............. 73 Chapter 4. High-Level Concurrency in Python.................. 141 Chapter 5. Extending Python.................................... 179

What is Python in practice?

    Python in Practicelooks at all of the design patterns in the context of Python, providing Python examples of those that are useful, as well as explaining why some are irrelevant to Python programmers. These patterns are covered in Chapter 1,Chapter 2,and Chapter 3.

What is an example of the Builder pattern?

    We will show an example of the Builder Pattern in a program that can produce forms—either webformsusing HTML,or GUIformsusing Pythonand Tkinter. Both forms work visually and support text entry; however, their buttons are non-functional.?The forms are shown in Figure 1.2; the source code is in

Python Design Patterns

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Python Design Patterns

i This tutorial explains the various types of design patterns and their implementation in Python scripting language. This tutorial will take you through a roller coaster ride with different approaches and examples using Python concepts. This tutorial is aimed to benefit both basic and intermediate levels of programmers and developers. Before you proceed with this tutorial, it is assumed that the user is already aware about basic python programming concepts.

Copyright 2018 by Tutorials Point (I) Pvt. Ltd.

All the content and graphics published in this e-book are the property of Tutorials Point (I) Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish any contents or a part of contents of this e-book in any manner without written consent of the publisher. We strive to update the contents of our website and tutorials as timely and as precisely as possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I) Pvt. Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our website or its contents including this tutorial. If you discover any errors on our website or in this tutorial, please notify us at contact@tutorialspoint.com

Python Design Patterns

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About the Tutorial ............................................................................................................................................ i

Audience ........................................................................................................................................................... i

Prerequisites ..................................................................................................................................................... i

Copyright & Disclaimer ..................................................................................................................................... i

Table of Contents ............................................................................................................................................ ii

1. Python Design Patterns - Introduction ..................................................................................................... 1

Structure of a design pattern .......................................................................................................................... 1

Why Python? ................................................................................................................................................... 2

2. Python Design Patterns - Gist of Python ................................................................................................... 4

Features of Python Language .......................................................................................................................... 4

Important Points.............................................................................................................................................. 4

How to download python language in your system? ...................................................................................... 6

The Important Tools in Python ........................................................................................................................ 6

What constitutes a design pattern in Python? ................................................................................................ 7

3. Python Design Patterns - Model View Controller Pattern ......................................................................... 8

4. Python Design Patterns - Singleton Pattern ............................................................................................ 12

How to implement a singleton class? ............................................................................................................ 12

5. Python Design Patterns - Factory Pattern ............................................................................................... 15

How to implement a factory pattern? ........................................................................................................... 15

6. Python Design Patterns - Builder Pattern ............................................................................................... 17

How to implement builder pattern? ............................................................................................................. 17

7. Python Design Patterns - Prototype Pattern ........................................................................................... 21

How to implement a prototype pattern? ...................................................................................................... 21

8. Python Design Patterns - Facade Pattern ............................................................................................... 25

How to design a facade pattern? .................................................................................................................. 25

9. Python Design Patterns - Command Pattern .......................................................................................... 32

Python Design Patterns

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How to implement the command pattern? .................................................................................................. 32

10. Python Design Patterns - Adapter Pattern .............................................................................................. 35

How to implement the adapter pattern? ...................................................................................................... 35

11. Python Design Patterns - Decorator Pattern ........................................................................................... 39

How to implement decorator design pattern ............................................................................................... 39

12. Python Design Patterns - Proxy Pattern ................................................................................................. 44

How to implement the proxy pattern?.......................................................................................................... 44

13. Python Design Patterns - Chain of Responsibility Pattern....................................................................... 46

How to implement the chain of responsibility pattern? ............................................................................... 46

14. Python Design Patterns - Observer Pattern ............................................................................................ 49

How to implement the observer pattern? .................................................................................................... 49

15. Python Design Patterns - State Pattern .................................................................................................. 52

How to implement the state pattern? .......................................................................................................... 52

16. Python Design Patterns - Strategy Pattern ............................................................................................. 55

How to implement the strategy pattern? ..................................................................................................... 55

17. Python Design Patterns - Template Pattern ........................................................................................... 57

18. Python Design Patterns - Flyweight Pattern ........................................................................................... 60

How to implement the flyweight pattern? .................................................................................................... 60

19. Python Design Patterns - Abstract Factory Pattern................................................................................. 62

How to implement the abstract factory pattern? ......................................................................................... 62

20. Python Design Patterns - Object Oriented Pattern ................................................................................. 66

How to implement the object oriented pattern? .......................................................................................... 66

21. Python Design Patterns - Object Oriented Concepts Implementation .................................................... 68

How to implement classes and object variables? ......................................................................................... 68

22. Python Design Patterns - Iterator Pattern .............................................................................................. 71

How to implement the iterator pattern? ...................................................................................................... 71

23. Python Data Structure - Dictionaries ...................................................................................................... 74

How to implement dictionaries in Python? ................................................................................................... 74

Python Design Patterns

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24. Python Design Pattern - The Lists Data Structure ................................................................................... 76

How to implement lists? ................................................................................................................................ 76

25. Python Design Patterns - Sets................................................................................................................. 78

How to implement sets? ............................................................................................................................... 78

26. Python Design Patterns - Queues ........................................................................................................... 82

How to implement the FIFO procedure? ....................................................................................................... 82

How to implement the LIFO procedure? ....................................................................................................... 83

What is a Priority Queue?.............................................................................................................................. 84

27. Python Design Patterns - Strings and Serialization ................................................................................. 86

28. Python Design Patterns - Concurrency in Python ................................................................................... 88

29. Python Design Patterns - Anti-Patterns .................................................................................................. 91

Important features of anti-patterns .............................................................................................................. 91

30. Python Design Patterns - Exception Handling ......................................................................................... 94

Why use exceptions? ..................................................................................................................................... 94

Python Design Patterns

1 Design patterns are used to represent the pattern used by developers to create software or web application. These patterns are selected based on the requirement analysis. The patterns describe the solution to the problem, when and where to apply the solution and the consequences of the implementation. The documentation of design pattern is maintained in a way that focuses more on the technology that is used and in what ways. The following diagram explains the basic structure of design pattern documentation.

Pattern Name

It describes the pattern in short and effective manner.

Intent/Motive

It describes what the pattern does.

Applicability

It describes the list of situations where pattern is applicable.

1. Python Design Patterns - Introduction

Python Design Patterns

2

Participants and consequences

Participants include classes and objects that participate in the design pattern with a list of consequences that exist with the pattern. Python is an open source scripting language. It has libraries that support a variety of design patterns. The syntax of python is easy to understand and uses English keywords. Python provides support for the list of design patterns that are mentioned below. These design patterns will be used throughout this tutorial:

Model View Controller Pattern

Singleton pattern

Factory pattern

Builder Pattern

Prototype Pattern

Facade Pattern

Command Pattern

Adapter Pattern

Prototype Pattern

Decorator Pattern

Proxy Pattern

Chain of Responsibility Pattern

Observer Pattern

State Pattern

Strategy Pattern

Template Pattern

Flyweight Pattern

Abstract Factory Pattern

Object Oriented Pattern

Benefits of using design pattern

Following are the different benefits of design pattern: Patterns provide developer a selection of tried and tested solutions for the specified problems.

All design patterns are language neutral.

Python Design Patterns

3 Patterns help to achieve communication and maintain well documentation. It includes a record of accomplishment to reduce any technical risk to the project. Design patterns are highly flexible to use and easy to understand.

Python Design Patterns

4 Python is an open source scripting language, which is high-level, interpreted, interactive and object-oriented. It is designed to be highly readable. The syntax of Python language is easy to understand and uses English keywords frequently. In this section, we will learn about the different features of Python language.

Interpreted

Python is processed at runtime using the interpreter. There is no need to compile program before execution. It is similar to PERL and PHP.

Object-Oriented

Python follows object-oriented style and design patterns. It includes class definition with various features like encapsulation, polymorphism and many more.

Portable

Python code written in Windows operating system and can be used in Mac operating system. The code can be reused and portable as per the requirements.

Easy to code

Python syntax is easy to understand and code. Any developer can understand the syntax of Python RLPOLQ IHR ORXUVB 3\PORQ ŃMQ NH GHVŃULNHG MV ³SURJUMPPHU-IULHQGO\´

Extensible

If needed, a user can write some of Python code in C language as well. It is also possible to put python code in source code in different languages like C++. This makes Python an extensible language. Consider the following important points related to Python programming language: It includes functional and structured programming methods as well as object- oriented programming methods. It can be used as scripting language or as a programming language.

It includes automatic garbage collection.

2. Python Design Patterns - Gist of Python

Python Design Patterns

5 It includes high-level dynamic data types and supports various dynamic type checking. Python includes a feature of integration with C, C++ and languages like Java.

Python Design Patterns

6 To download Python language in your system, follow this link: https://www.python.org/downloads/ It includes packages for various operating systems like Windows, MacOS and Linux distributions. In this section, we will learn in brief about a few important tools in Python.

Python Strings

The basic declaration of strings is as follows:

str = 'Hello World!'

Python Lists

The lists of python can be declared as compound data types separated by commas and enclosed within square brackets ([]). list = [ 'abcd', 786 , 2.23, 'john', 70.2 ] tinylist = [123, 'john']

Python Tuples

A tuple is dynamic data type of Python, which consists of number of values separated by commas. Tuples are enclosed with parentheses. tinytuple = (123, 'john')

Python Design Patterns

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Python Dictionary

Python dictionary is a type of hash table. A dictionary key can be almost any data type of Python. The data types are usually numbers or strings. tinydict = {'name': 'omkar','code':6734, 'dept': 'sales'} Python helps in constituting a design pattern using the following parameters:

Pattern Name

Intent

Aliases

Motivation

Problem

Solution

Structure

Participants

Constraints

Sample Code

Python Design Patterns

8 Model View Controller is the most commonly used design pattern. Developers find it easy to implement this design pattern. Following is a basic architecture of the Model View Controller:

Let us now see how the structure works.

Model It consists of pure application logic, which interacts with the database. It includes all the information to represent data to the end user. View

3. Python Design Patterns - Model View Controller Pattern

Python Design Patterns

9 data to user.

Controller

It acts as an intermediary between view and model. It listens to the events triggered by view and queries model for the same.

Python code

Let us consider a basic object ŃMOOHG ³3HUVRQ´ and create an MVC design pattern.

Model.py

import json class Person(object): def __init__(self, first_name = None, last_name = None): self.first_name = first_name self.last_name = last_name #returns Person name, ex: John Doe def name(self): return ("%s %s" % (self.first_name,self.last_name)) @classmethod #returns all people inside db.txt as list of Person objects def getAll(self): database = open('db.txt', 'r') result = [] json_list = json.loads(database.read()) for item in json_list: item = json.loads(item) person = Person(item['first_name'], item['last_name']) result.append(person) return result It calls for a method, which fetches all the records of the Person table in database. The records are presented in JSON format.

Python Design Patterns

10 View It displays all the records fetched within the model. View never interacts with model; controller does this work (communicating with model and view). from model import Person def showAllView(list): print 'In our db we have %i users. Here they are:' % len(list) for item in list: print item.name() def startView(): print 'MVC - the simplest example' print 'Do you want to see everyone in my db?[y/n]' def endView(): print 'Goodbye!'

Controller

Controller interacts with model through the getAll() method which fetches all the records displayed to the end user. from model import Person import view def showAll(): #gets list of all Person objects people_in_db = Person.getAll() #calls view return view.showAllView(people_in_db) def start(): view.startView() input = raw_input() if input == 'y': return showAll() else:

Python Design Patterns

11 return view.endView() if __name__ == "__main__": #running controller function start()

Python Design Patterns

12 This pattern restricts the instantiation of a class to one object. It is a type of creational pattern and involves only one class to create methods and specified objects. It provides a global point of access to the instance created. The following program demonstrates the implementation of singleton class where it prints the instances created multiple times. class Singleton: __instance = None @staticmethod def getInstance(): """ Static access method. """ if Singleton.__instance == None:

Singleton()

4. Python Design Patterns - Singleton Pattern

Python Design Patterns

13 return Singleton.__instance def __init__(self): """ Virtually private constructor. """ if Singleton.__instance != None: raise Exception("This class is a singleton!") else:

Singleton.__instance = self

s = Singleton() print s s = Singleton.getInstance() print s s = Singleton.getInstance() print s

Output

The above program generates the following output:

Python Design Patterns

14 The number of instances created are same and there is no difference in the objects listed in output.

Python Design Patterns

15 The factory pattern comes under the creational patterns list category. It provides one of the best ways to create an object. In factory pattern, objects are created without exposing the logic to client and referring to the newly created object using a common interface. Factory patterns are implemented in Python using factory method. When a user calls a method such that we pass in a string and the return value as a new object is implemented through factory method. The type of object used in factory method is determined by string which is passed through method. In the example below, every method includes object as a parameter, which is implemented through factory method. Let us now see how to implement a factory pattern. class Button(object): html = "" def get_html(self): return self.html class Image(Button): html = "" class Input(Button): html = "" class Flash(Button): html = "" class ButtonFactory(): def create_button(self, typ): targetclass = typ.capitalize() return globals()[targetclass]() button_obj = ButtonFactory()

5. Python Design Patterns - Factory Pattern

Python Design Patterns

16 button = ['image', 'input', 'flash'] for b in button: print button_obj.create_button(b).get_html() The button class helps to create the html tags and the associated html page. The client will not have access to the logic of code and the output represents the creation of html page.

Python Design Patterns

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