Chapter 1, Starting on the Right Foot – Using Virtualenv, kicks off our dive into Python web application development with the basics of using and managing virtual environments to isolate the application dependencies. We will look at the setup tools, pip, libraries, and utilities that are used to install and distribute reusable packages of Python code, and virtualenv, a tool to create isolated environments for the Python-based software requirements of a project. We will also discuss what these tools are not able to do, and look at the virtualenvwrapper abstraction to augment the functionality that virtualenv provides.

Chapter 2, Small to Big – Growing the Flask Application Structure, explores the various baseline layouts and configurations that you might consider for a Flask application. The pros and cons of each approach are outlined as we progress from the simplest one-file application structure to the more complex, multipackage Blueprint architecture.

Chapter 3, Snap – the Code Snippet Sharing Application, builds our first simple Flask application centered around learning the basics of one of the most popular relational database abstractions, SQLAlchemy, and several of the most popular Flask extensions: Flask-Login to handle authenticated user login sessions, Flask-Bcrypt to ensure that account passwords are stored in a secure manner, and Flask-WTF to create and process form-based input data.

Chapter 4, Socializer – the Testable Timeline, builds a very simple data model for a social web application where the main focus is on unit and functional testing using pytest, the Python testing framework and tools. We will also explore the use of the application factory pattern, which allows us to instantiate separate versions of our application for the purposes of simplifying testing. Additionally, the use and creation of often-omitted (and forgotten) signals, provided by the Blinker library, are described in detail.

Chapter 5, Shutterbug, the Photo Stream API, builds a skeleton of an application around a JSON-based API, which is a requirement for any modern web application these days. One of the many API-based Flask extensions, Flask-RESTful, is used to prototype the API, where we also delve into simple authentication mechanisms for stateless systems and even write a few tests along the way. A short detour is made into the world of Werkzeug, the WSGI toolkit that Flask is built upon, to build a custom WSGI middleware that allows the seamless handling of URI-based version numbers for our nascent API.

Chapter 6, Hublot – Flask CLI Tools, covers a topic that is often omitted from most web application framework discussions: command-line tools. The use of Flask-Script is explained, and several CLI-based tools are created to interact with the data models of our application. Additionally, we will build our very own custom Flask extension that wraps an existing Python library to fetch the repository and issue information from the GitHub API.

Chapter 7, Dinnerly – Recipe Sharing, introduces the somewhat intimidating concept of the OAuth authorization flow that many large web applications, such as Twitter, Facebook, and GitHub, implement in order to allow third-party applications to act on behalf of the account owners without compromising basic account security credentials. A barebones data model is constructed for a recipe-sharing application that allows the so-called social sign in and the ability to cross-post the data from our application to the feeds or streams of the services that a user has connected. Finally, we will introduce the concept of database migrations using Alembic, which allow you to synchronize your SQLAlchemy model metadata with the schemas of the underlying relational database tables in a reliable manner.

To work through most of the examples in this book, all you need is your favorite text editor or IDE, access to the Internet (to install the various Flask extensions, not to mention Flask itself), a relational database (one of SQLite, MySQL, or PostgreSQL), a browser, and some familiarity with the command line. Care has been taken to indicate when additional packages or libraries are required to complete the examples in each chapter.

This book was created for the new Python developers who wish to dive into the world of web application development, or for the seasoned Python web application professional who is interested in learning about Flask and the extension-based ecosystem behind it. To get the most out of each chapter, you should have a solid understanding of the Python programming language, a basic knowledge of relational database systems, and fluency with the command line.

In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "This will create a blank app1 environment and activate it. You should see an (app1) tag in your shell prompt."

A block of code is set as follows:

[default]
  <div>{{ form.password.label }}: {{ form.password }}</div>
  {% if form.password.errors %}
  <ul class="errors">{% for error in form.password.errors %}<li>{{ error }}</li>{% endfor %}</ul>
  {% endif %}

  <div><input type="submit" value="Sign up!"></div>
</form>

{% endblock %}

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

    from application.users.views import users
    app.register_blueprint(users, url_prefix='/users')

    from application.posts.views import posts
    app.register_blueprint(posts, url_prefix='/posts')

        # …

Any command-line input or output is written as follows:

$ source ~/envs/testing/bin/activate
(testing)$ pip uninstall numpy

New terms and important words are shown in bold. Words that you see on the screen, in menus or dialog boxes for example, appear in the text like this: "Then it asserts that the Sign up! button text appears in the returned HTML".

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