In this chapter, we will use virtualenv to isolate the environment we will work in. As the name indicates, virtualenv is a tool that creates a virtual environment. It can keep the dependencies required by different projects in separate places while keeping the global site-packages clean. In other words, when you install Flask in the virtual environment, it is only installed in the local virtualenv project directory, not the global site-packages.

The chances are you may have already come across virtualenv while working with Python before, so we will run through this process quickly. If you have not, feel free to pick up one of many excellent tutorials online, such as http://docs.python-guide.org/en/latest/dev/virtualenvs/. We will need to install virtualenv first:

# Python 3
$ sudo apt-get install python3-venv
$ python3 -m venv venv

# Python 2
$ sudo apt-get install python-virtualenv
$ virtualenv venv-python2

Then, activate and deactivate it in order to be in and out of the environment:

$ source venv/bin/activate
(venv) $ python
Python 3.5.2 (default, Nov 17 2016, 17:05:23)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
(venv) $ deactivate

In this chapter, we will install quite a few Python packages. To make life easier, I have included a requirements.txt file on the book's GitHub repository; we can use it to install all the necessary packages (remember to activate your virtualenv):

(venv) $ pip install -r requirements.txt

For our network topology, we will use a simple four-node network, as shown here:

Let's take a look at Flask in the next section.