virtualenv

Virtual Environments in Python (Windows and Linux)

Introduction

Virtual environments is a way to separate requirements of different applications and code for specific libraries and environment settings.

Different applications require different dependencies (e.g. libraries) in different versions. By installing everything into global environment of operating system there is very good chance of conflicts. Lets say application A requires logging module version 1.0 but application B requires logging module version 2.0. If package or library is named the same it is not possible to install both versions in one environment. Virtual Environment can help in this situation.

Virtual environment creates separate python environment by linking/copying necessary files into separate folder. This way we can safely install and configure libraries needed for specific application without affecting global or other virtual environment.

The separation is achieved by creating separate directory for each virtual env where python files are copied. Note that virtual env directory where python executables are copied is separate thing from directory where sources are. For example you can store your virtual environments under c:\dev\envs and have projects stored under c:\dev\projects\*

It is important to activate virtual env before working with it. Activating will modify $PATH variable of operating system and makes sure python or python3 executable from right directory is called. Once we are done working in virtual env it is important to deactivate it which will undo changes to $PATH variable.

Creating virtual environment

Navigate into directory that should hold python executables for virtual environment. For example c:\dev\envs\sqlalchemy

Now type: python -m venv

Activating virtual environment

Navitage into directory that holds executables for virtual environment and type: .\scripts\activate

On linux you type source ./bin/activate

Depending if you run the command from cmd or powershell activate.bat or activate1.ps will be executed. %PATH% variable will be adapted.

Deactivate virtual environment

Navitage into directory that holds executables for virtual environment and type: .\scripts\deactivate

On linux you type deactivate (command is created with running activate script).

Managing libraries and versions

Note again that virtual environments with python executables are independent from your source code. Do not commit virtual environment files into source control system like Git or Svn.

To document what libraries and versions your code uses use pip freeze and pip install.

Use pip freeze > requirements.txt to save configured libraries and versions

Use pip install -r requirements.txt to install libraries needed to run code

Helpers to simplify work with virtual environments

Switching between virtual environmetns and source code directories can require multiple commands. There are tools that can simplify those tasks.

VirtualEnvWrapper

Virtualenvwrapper is tool with helper functions to easier switch and activate virtual environments.

Installation

Easiest way is to install using pip into global python:

Windows: pip install virtualenvwrapper-win

Linux: pip install virtualenvwrapper

Setup WORKON_HOME environment variable with directory where you have virtual environments (e.g. c:\dev\envs).

Add following files into your startup file (e.g. .bashrc or .profile) so the scripts are loaded into your shell:

export WORKON_HOME=$HOME/.virtualenvs
export PROJECT_HOME=$HOME/dev
source /usr/local/bin/virtualenvwrapper.sh

See also official installation documentation.

Create virtual environment

Type (myapp is name of your virtual environment):

mkvirtualenv myapp

Show list of available virtual environments

workon

Switch to virtual environment

workon myapp

Delete virtual environment

Just delete folder where virtual environment was created. Path where virtual environments are created is specified by WORKON_HOME or look after default .virtualenvs folder in your home directory.

echo $WORKON_HOME
rm -rf ${WORKON_HOME}/myapp

Useful commands

• workon - lists available environments
• workon <dir> - activate specified env
• deactivate
• mkvirtualenv <name> - creates and activates virtual environment
• rmvirtualenv <name> - removes the environment (uses folder_delete.bat)
• cdproject - If a virtualenv environment is active and a projectdir has been defined, enters the project dir
• cd- - returns you to the last directory before calling cdproject
• setprojectdir <full or relative path> - sets project directory (where code is)

Creating virtual environment using virtualenv

Installation (if not already present):

sudo apt-get install virtualenv

Check where is python executable you want to use for virtual env:

which python3
/usr/bin/python3

Create virtual env using python 3:

mkdir ~/venvs
virtualenv --python=/usr/bin/python3 ~/venvs/myapp

If there is problem with connecting to network check proxy settings (see linux-setup section) or firewall.

Activate: source ~/venv/myapp/bin/activate

Deactivate: deactivate