Book Image

The Python Apprentice

By : Robert Smallshire, Austin Bingham
Book Image

The Python Apprentice

By: Robert Smallshire, Austin Bingham

Overview of this book

Experienced programmers want to know how to enhance their craft and we want to help them start as apprentices with Python. We know that before mastering Python you need to learn the culture and the tools to become a productive member of any Python project. Our goal with this book is to give you a practical and thorough introduction to Python programming, providing you with the insight and technical craftsmanship you need to be a productive member of any Python project. Python is a big language, and it’s not our intention with this book to cover everything there is to know. We just want to make sure that you, as the developer, know the tools, basic idioms and of course the ins and outs of the language, the standard library and other modules to be able to jump into most projects.
Table of Contents (21 chapters)
Title Page
Credits
About the Authors
www.PacktPub.com
Customer Feedback
Preface
12
Afterword – Just the Beginning

Validation and invariants


It's good practice for the initializer of an object to establish so-called class invariants. The invariants are truths about objects of that class that should endure for the lifetime of the object. One such invariant for flights is that the flight number always begins with an upper case two-letter airline code followed by a three or four digit route number.

In Python, we establish class invariants in the __init__() method and raise exceptions if they can't be attained:

class Flight:

    def __init__(self, number):
        if not number[:2].isalpha():
            raise ValueError("No airline code in '{}'".format(number))

        if not number[:2].isupper():
            raise ValueError("Invalid airline 
                             code'{}'".format(number))

        if not (number[2:].isdigit() and int(number[2:]) <= 9999):
            raise ValueError("Invalid route 
                             number'{}'".format(number))

        self._number = number

   ...