Book Image

Python Object-Oriented Programming - Fourth Edition

By : Steven F. Lott, Dusty Phillips
2 (1)
Book Image

Python Object-Oriented Programming - Fourth Edition

2 (1)
By: Steven F. Lott, Dusty Phillips

Overview of this book

Object-oriented programming (OOP) is a popular design paradigm in which data and behaviors are encapsulated in such a way that they can be manipulated together. Python Object-Oriented Programming, Fourth Edition dives deep into the various aspects of OOP, Python as an OOP language, common and advanced design patterns, and hands-on data manipulation and testing of more complex OOP systems. These concepts are consolidated by open-ended exercises, as well as a real-world case study at the end of every chapter, newly written for this edition. All example code is now compatible with Python 3.9+ syntax and has been updated with type hints for ease of learning. Steven and Dusty provide a comprehensive, illustrative tour of important OOP concepts, such as inheritance, composition, and polymorphism, and explain how they work together with Python’s classes and data structures to facilitate good design. In addition, the book also features an in-depth look at Python’s exception handling and how functional programming intersects with OOP. Two very powerful automated testing systems, unittest and pytest, are introduced. The final chapter provides a detailed discussion of Python's concurrent programming ecosystem. By the end of the book, you will have a thorough understanding of how to think about and apply object-oriented principles using Python syntax and be able to confidently create robust and reliable programs.
Table of Contents (17 chapters)
15
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16
Index

Metaclasses

As we noted earlier, creating a new class involves work done by the type class. The job of the type class is to create an empty class object so the various definitions and attributes assignment statements will build the final, usable class we need for our application.

Here's how it works:

Figure 6.3: How type creates MyClass

The class statement is used to locate the appropriate metaclass; if no special metaclass= is provided, then the type class is used. The type class will prepare a new, empty dictionary, called a namespace, and then the various statements in the class populate this container with attributes and method definitions. Finally, the "new" step completes creation of the class; this is generally where we can make our changes.

Here's a diagram showing how we can use a new class, SpecialMeta, to tap into the way type builds a new class for us:

Figure 6.4: Extending the type class

If we use the metaclass...