Mastering Flask: Gain expertise in Flask to create dynamic and powerful web applications

Installing Git is very simple. Simply go to http://www.git-scm.com/downloads and click on the Operating System (OS) that is being run. A program will begin to download that will walk you through the basic installation process.

Git is a very complex tool; only the basics that are needed for this book will be covered here.

Git does not track your changes automatically. In order for Git to run properly, we have to give it the following information:

Before we can do anything, we tell Git to create a git instance in our directory. In your project directory, run the following in your terminal:

$ git init

Git will now start to track changes in our project. As git tracks our files, we can see the status of our tracked files, and any files that are not tracked, by typing the following command:

$ git status

Now we can save our first commit, which is a snapshot of your code at the time that you run the commit command.

# In Bash, comments are marked with a #, just like Python
# Add any files that have changes and you wish to save in this commit
$ git add main.py
# Commit the changes, add in your commit message with -m
$ git commit -m"Our first commit"

At any point in the future, we can return to this point in our project. Adding files to be committed is called staging files in Git. Remember to add stage files only if you are ready to commit them. Once the files are staged, any further changes will not be staged as well. For an example of more advanced Git usage, add any text to your main.py file with your text editor and then run the following:

# To see the changes from the last commit
$ git diff
# To see the history of your changes
$ git log
# As an example, we will stage main.py
# and then remove any added files from the stage
$ git add main.py
$ git status
$ git reset HEAD main.py
# After any complicated changes, be sure to run status
# to make sure everything went well
$ git status
# lets delete the changes to main.py, reverting to its state at the last commit
# This can only be run on files that aren't staged
$ git checkout -- main.py

Your terminal should look something like this:

The Git system's checkout command is rather advanced for this simple introduction, but it is used to change the current status of the Git system's HEAD pointer—that is, the current location of our code in the history of our project. This will be shown in the next example.

Now, to see the code in a previous commit, first run this:

$ git log
Fri Jan 23 19:16:43 2015 -0500 f01d1e2 Our first commit  [Jack Stouffer]

The string of characters next to our commit message, f01d1e2, is called the hash of our commit. It is the unique identifier of that commit that we can use to return to the saved state. Now, to take the project back to that state, run this:

$ git checkout f01d1e2

Your Git project is now in a special state where any changes or commits will neither be saved nor affect any commits that were made after the one you checked out. This state is just for viewing old code. To return to the normal mode of Git, run this:

$ git checkout master

If you are on Windows, and your installed Python the current version, you already have pip! If your Python installation is not the most recent, the easiest thing to do is to simply reinstall it. Download the Python Windows installer at https://www.python.org/downloads/.

In Windows, the variable that controls which programs are accessible from the command line is path. To modify your path to include Python and pip, we have to add C:\Python27 and C:\Python27\Tools. Edit the Windows path by opening the Windows menu, right-clicking on Computer and clicking on Properties. Under Advanced system settings, click Environment Variables.... Scroll down until you find Path, double-click it, and add ;C:\Python27;C:\Python27\Tools to the end.

To make sure you have modified your path correctly, close and reopen your terminal and type the following into the command line:

pip --help

pip should have printed its usage message as shown in the following screenshot:

Some Python installations of Linux do not come with pip, and Mac OS X installations don't come with pip by default. To install it, download the get-pip.py file from https://raw.githubusercontent.com/pypa/pip/master/contrib/get-pip.py.

Once you have downloaded it, run it with elevated privileges using the following:

$ sudo python get-pip.py

Then pip will be installed automatically.

To install a package with pip, follow this simple step:

$ pip install [package-name]

On Mac and Linux, because you are installing programs outside the user-owned folders, you might have to prepend sudo to the install commands. To install Flask, simply run this:

$ pip install flask

Then, all requirements of Flask will be installed for you.

If you want to remove a package that you are no longer using, run this:

$ pip uninstall [package-name]

If you wish to explore or find a package but don't know its exact name, you may use the search command:

$ pip search [search-term]

Now that we have a couple of packages installed, it is common courtesy in the Python community to create a list of packages that are required to run the project, so others can quickly install every thing required. This also has the added benefit that any new member of your project will be able to run your code quickly.

This list can be created with pip by running this:

$ pip freeze > requirements.txt

What exactly did this command do? pip freeze run by itself prints out a list of the installed packages and their versions as follows:

Flask==0.10.1
itsdangerous==0.24
Jinja2==2.7.3
MarkupSafe==0.23
Werkzeug==0.10.4
wheel==0.24.0

The > operator tells Bash to take everything printed by the last command and write it to this file. If you look into your project directory, you will see the new file named requirements.txt that contains the output of pip freeze.

To install all the packages from this file, a new project maintainer will have to run this:

$ pip install -r requirements.txt

This tells pip to read all the packages listed in requirements.txt and install them.

Let's initialize virtualenv for our project as follows:

$ virtualenv env

The extra env tells virtualenv to store all the packages into a folder named env. virtualenv requires you to start it before it will sandbox your project:

$ source env/bin/activate
# Your prompt should now look like
(env) $

The source command tells Bash to run the script env/bin/activate in the context of the current directory. Let's reinstall Flask in our new sandbox as follows:

# you won't need sudo anymore
(env) $ pip install flask
# To return to the global Python
(env) $ deactivate

However, it goes against the best practices in Git to track what you don't own, so we should avoid tracking the changes in third-party packages. To ignore specific files in our project, the gitignore file is needed.

$ touch .gitignore

touch is the Bash command to create files, and the dot at the start of a file tells Bash to not list its existence unless specifically told to show hidden files. We will create the simple gitignore file for now:

env/
*.pyc

This tells Git to ignore the entire env directory and ignore all the files that end with .pyc (a compiled Python file). When used in this way, the * character is called a wildcard.

In order to make next chapters easier for the reader, we will use the first of many Flask extensions (packages that extend the functionality of Flask) named Flask Script. Flask Script allows programmers to create commands that act within the Application Context of Flask—that is, the state in Flask that allows modification of the Flask object. Flask Script comes with some default commands to run the server and a python shell in the Application Context. To install Flask Script with pip, run this:

$ pip install flask-script

We will cover more advanced usage of Flask Script in Chapter 10, Useful Flask Extensions; for now, let's start with a simple script named manage.py. First import Flask Script's objects and your app as follows:

from flask.ext.script import Manager, Server
from main import app

Then, pass your app to the Manager object, which will initialize Flask Script:

manager = Manager(app)

Now we add our commands. The server is the same as the normal development server run through main.py. The make_shell_context function will create a Python shell that can be run within the app context. The returned dictionary will tell Flask Script what to import by default:

manager.add_command("server", Server())

@manager.shell
def make_shell_context():
    return dict(app=app)

Then, end the file with the Python standard way of running only if the user ran this file:

if __name__ == "__main__":
    manager.run()

You will now be able to run the development server with:

$ python manage.py server

Use the shell with:

$ python manage.py shell
# Lets check if our app imported correctly
>>> app
<Flask 'main'>