Using virtual environments in Python 3

14 July 2017


flickr photo by Tom Mrazek shared under a Creative Commons (BY) license

The Python programming language supports "virtual environments" that make it easy to install libraries for specific projects. For instance, you can employ the newest version of a library for one project and an older version for another. Since virtual environments ensure that all Python projects are no longer forced to depend on a system-wide library, they support program isolation and thus enable software engineers to manage the library dependencies in their Python projects.

For many years virtualenv was one of the most established and well-known tools for creating a virtual environment. When I first started programming in Python, I investigated this approach and expected that it would be the one that I would adopt. However, since my new projects are exclusively in Python 3, I discovered that there was another tool that was part of the Python 3 language and the most appropriate choice for new projects.

What is this new approach? It is called venv! While there is some great documentation on venv, I decided to write this post to share some of the additional details that I learned about how to use it on a recent version of the Ubuntu operating system.

Even though I had Python 3 installed correctly, I found that I also needed to run sudo apt install python3-venv. Now, for illustrative purposes, let's assume that we are creating a project called lightning.py, a new "lightning fast" sorting algorithm that was recently development. If this project is in the lightning.py directory, then the next command to type in the terminal window is python3 -m venv lightning.py. This command will create a venv-based virtual environment inside of the lightning.py directory.

Once the virtual environment is created, we need to start using it through the process of "activation" and by typing the following command in the directory that contains the new Python 3 project: source lightning.py/bin/activate. Depending on how your terminal window is configured, you should see in your command prompt that the lightning.py project is now active.

At this point, you can install various libraries using the pip install command. For instance, if you wanted to install the requests library you could type the following command in your terminal: pip install requests. Notice that this command will not install requests in a system-wide fashion — instead it will only be available to the lightning.py project.

Okay, that's great! Now, we are ready to design, implement, test, and document the lightning.py project. But, what do we do when we want to stop running this virtual environment? You can accomplish this goal by typing the deactivate command in your terminal window. Now the environment is no longer running!

In my experience with developing various software tools to analyze data for research papers, I have found that it is important to employ good software engineering practices. Using virtual environments with venv is a practice that you should adopt if you are using Python 3 in the field of data science. Interested in learning more about this topic? Or, do you have some ideas to share with me about virtual environments? Please contact with me your questions and insights!

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