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Learning Python Application Development
Learning Python Application Development

Learning Python Application Development: Take Python beyond scripting to build robust, reusable, and efficient applications

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Learning Python Application Development

Chapter 2. Dealing with Exceptions

In the previous chapter, we started with a simple command-line script and gradually transformed it into an object-oriented code. Several new features were added in the process. So far, we have paid little attention to the application quality. We neglected to look for any obvious errors encountered during the program execution. Such errors detected during the application runtime are referred to as exceptions. In this chapter, you will learn techniques to make the application more robust by handling exceptions.

Specifically, we will cover the following topics:

  • What are the exceptions in Python?
  • Controlling the program flow with the try…except clause
  • Dealing with common problems by handling exceptions
  • Creating and using custom exception classes

Let's start by reviewing the feedback you received from the users.

Revisiting Attack of the Orcs v1.0.0

The heal feature added in v1.0.0 became a hit among the core users. The OOP approach put you in a better position to implement new features (or so you thought!). As the feature requests started pouring in, so did the reported bugs.

The game is OK, but there are several annoyances. For example, when prompted to choose a hut, sometimes I input a number greater than 5 or input a character by mistake. After this, it just prints some weird error message and the application terminates. Can you fix this?

Revisiting Attack of the Orcs v1.0.0

Debugging the problem

Let's try to reproduce the reported problem. Run the example from Chapter 1, Developing Simple Applications:

$ python ch01_ex03.py

When prompted for the hut number, enter any character, as shown in the following screenshot:

Debugging the problem

The application is terminated with an error traceback in the console. A traceback is a snapshot of the call stack at the point where the exception (the error) occurred. In this particular example, the _process_user_choice...

Exceptions

Before jumping straight into the code and fixing these issues, let's first understand what an exception is and what we mean by handling an exception.

What is an exception?

An exception is an object in Python. It gives us information about an error detected during the program execution. The errors noticed while debugging the application were unhandled exceptions as we didn't see those coming. Later in the chapter, you will learn the techniques to handle these exceptions.

The ValueError and IndexError exceptions seen in the earlier tracebacks are examples of built-in exception types in Python. In the following section, you will learn about some other built-in exceptions supported in Python.

Most common exceptions

Let's quickly review some of the most frequently encountered exceptions. The easiest way is to try running some buggy code and let it report the problem as an error traceback! Start your Python interpreter and write the following code:

Most common exceptions

Here are a few more exceptions...

Back to the game – Attack of the Orcs v1.1.0

With this knowledge of the exception handling, let's work on the next incremental version of the application.

Preparatory work

Before writing any code, let's first understand how the rest of the section is organized. In a nutshell, we will start with v1.0.0 of the code from Chapter 1, Developing Simple Applications, progressively add the exception handling code, and call the new version v1.1.0.

Note

The Python files in the supporting code bundle already include the exception handling code to be discussed in this section as well as in a later section of this chapter, Defining custom exceptions

The following points elaborate further details:

  • We will start by downloading the v1.0.0 of the game from Chapter 1, Developing Simple Applications. The file name is ch01_ex03_AbstractBaseClass.py (recall that this was provided as a solution to an exercise in Chapter 1, Developing Simple Applications). You can find this file in this chapter&apos...

Defining custom exceptions

You can define your own exception class by inheriting from the Exception base class or any other exception class. Why do we need such customization? Firstly, you can create an exception class with a descriptive name. This allows us to identify the purpose of the exception just by looking at the descriptive name. For example, instead of ValueError, a custom exception named ValueGreaterThanFiveError will immediately help identify the problem. There are other advantages as well. You can use such classes to add customized messages based on error subcategories, writing error logs, and so on. Let's learn how to define custom exceptions next.

Preparatory work

Here is a list of files we will use:

  • attackoftheorcs_v1_1.py: This is the file from the previous section that we will use. As mentioned earlier, the supporting code bundle already has a file by the same name. It includes all the modifications we will discuss.
  • gameuniterror.py: This is a new module to hold a custom...

Exercise

Identify any code that can benefit from exception handling. For example, create a new HutError exception, and use it to raise errors related to the Hut class. Here is a cheat sheet:

Exercise

Instead of using error_dict, you can also create subclasses, such as:

class HutNumberGreaterThanFiveError(HutError): pass
class NegativeHutNumberError(HutError): pass 

Summary

This chapter served as an introduction to the basics of exception handling in Python. We saw how the exceptions occur, learned about some common built-in exception classes, and wrote simple code to handle these exceptions using the try…except clause. By handling exceptions, we fixed some obvious bugs in the Attack of the Orcs game.

The chapter also demonstrated techniques, such as raising and re-raising exceptions, using the finally clause, and so on. The later part of the chapter focused on implementing custom exception classes. We defined a new exception class and used it for raising custom exceptions for our application.

With exception handling, the code is in a better shape. However, we still have the majority of the code squished inside a single file (attackoftheorcs_v1_1.py). In the next chapter, you will learn how to package the application code and release it to a broader audience.

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Key benefits

  • Get to grips with Python techniques that address commonly encountered problems in general application development.
  • Develop, package, and deploy efficient applications in a fun way.
  • All-practical coverage of the major areas of application development, including best practices, exception handling, testing, refactoring, design patterns, performance, and GUI application development.

Description

Python is one of the most widely used dynamic programming languages, supported by a rich set of libraries and frameworks that enable rapid development. But fast paced development often comes with its own baggage that could bring down the quality, performance, and extensibility of an application. This book will show you ways to handle such problems and write better Python applications. From the basics of simple command-line applications, develop your skills all the way to designing efficient and advanced Python apps. Guided by a light-hearted fantasy learning theme, overcome the real-world problems of complex Python development with practical solutions. Beginning with a focus on robustness, packaging, and releasing application code, you’ll move on to focus on improving application lifetime by making code extensible, reusable, and readable. Get to grips with Python refactoring, design patterns and best practices. Techniques to identify the bottlenecks and improve performance are covered in a series of chapters devoted to performance, before closing with a look at developing Python GUIs.

Who is this book for?

Do you know the basics of Python and object oriented programming? Do you want to go an extra mile and learn techniques to make your Python application robust, extensible, and efficient? Then this book is for you.

What you will learn

  • Build a robust application by handling exceptions.
  • Modularize, package, and release the source distribution.
  • Document the code and implement coding standards.
  • Create automated tests to catch bugs in the early development stage.
  • Identify and re-factor badly written code to improve application life.
  • Detect recurring problems in the code and apply design patterns.
  • Improve code efficiency by identifying performance bottlenecks and fixing them.
  • Develop simple GUI applications using Python.
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Table of Contents

11 Chapters
1. Developing Simple Applications Chevron down icon Chevron up icon
2. Dealing with Exceptions Chevron down icon Chevron up icon
3. Modularize, Package, Deploy! Chevron down icon Chevron up icon
4. Documentation and Best Practices Chevron down icon Chevron up icon
5. Unit Testing and Refactoring Chevron down icon Chevron up icon
6. Design Patterns Chevron down icon Chevron up icon
7. Performance – Identifying Bottlenecks Chevron down icon Chevron up icon
8. Improving Performance – Part One Chevron down icon Chevron up icon
9. Improving Performance – Part Two, NumPy and Parallelization Chevron down icon Chevron up icon
10. Simple GUI Applications Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

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1 star 0%
Will Ware Oct 13, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Disclosure: I am the technical reviewer for this book and I received a complimentary copy from the author. That said, I'm delighted with the book, and to have in some small way contributed to its excellence. While the fantasy game theme might suggest this is a book for beginners, it is densely packed with resources that will serve the needs of advanced Python developers. After two chapters of basics, Ninad moves on to creating and distributing packages, code documentation and quality monitoring, unit testing and mocking, refactoring, measuring and improving performance, and GUI programming. Each topic is a deep dive, examining multiple approaches and making informed recommendations, far beyond the casual web searching that software developers (myself included) routinely do in our haste to accomplish specific goals. I look forward to using the book in my daily work and plan to recommend it to friends and co-workers. After ten-plus years of professional Python development, I fully expect that working with this book will improve the quality of my output.
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Qi Feb 11, 2019
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I bought the Chinese translation version of this book a year ago and read less than 2 chapters after I bought it. I started to read it again since yesterday and spent a day and finished 6 chapters and found it’s very helpful for me to better understand not only python as a language but also many aspects for software development. Many concepts were introduced in a simple way and easily connected with actual corresponding code.
Amazon Verified review Amazon
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