Python Grab Bag: Virtual Environments

Introduction

Python virtual environments are a way of isolating specific Python libraries for a project. Virtual environments have their own Python interpreter, pip, and installed libraries. Virtual envrionments can be activated and deactivated on the command line or terminal to work on projects that need certain library versions.

Creating a Virtual Envrionment

Virtual environments can be created anywhere on the file system, a reasonable location is an venvs directory in your home directory (~/env/virtualenvironments). Let’s change to our ~/venvs directory and create a virtual environment named “searing-frost”.

~$ cd ~/venvs
~/venvs$ python3 -m venv searing-frost
~/venvs/searing-frost$ cd searing-frost
~/venvs/searing-frost$ ls
bin  include  lib  lib64  pyvenv.cfg  share

Activating and Deactivating Virtual Environments

The bin directory contains the script to activate the virtual environment.

~/venvs/searing-frost$ source ./bin/activate

Windows: searing-frost\Scripts\activate.bat

Once the virtual environment is activated, the terminal will show (searingfrost) on the prompt.

(searing-frost)~/venvs/searing-frost$

To leave the virtual environment, just use deactivate

(searing-frost)~/venvs/searing-frost$ deactivate
~/venvs/searing-frost$

Installing and Packaging Libraries in Virtual Environments

The virtual environment has its own pip that you can use to install libraries. Let’s see what’s installed by default with pip list.

(searing-frost)~/venvs/searing-frost$ pip list

Package       Version
------------- -------
pip           20.0.2 
pkg-resources 0.0.0  
setuptools    44.0.0 

If we install a new library, it will install to the virtual environment’s site-packages, not system wide. Installing numpy with pip will add it to ~/venvs/searing-frost/lib/python3.8/site-packages/numpy.

(searing-frost)~/venvs/searing-frost$ pip install numpy
(searing-frost)~/venvs/searing-frost$ pip list

Package       Version
------------- -------
numpy         1.19.1 
pip           20.0.2 
pkg-resources 0.0.0  
setuptools    44.0.0 

A requirements.txt file for the project related to the virtual environment can be created easily with pip freeze. Package the requirements.txt with the project source code so everyone can install the same version of libraries. Check out my blog post on pip for more details.

(searing-frost)~/venvs/searing-frost$ pip freeze > ~/searing-frost-project/requirements.txt

requirements.txt
----------------
numpy==1.19.1