virtualenv Cheatsheet

A virtualenv is one of the first things a Python programmer learns about. This blog post describes what it is, how to set it up, and examples of some concrete use cases.

What is a virtualenv?

A virtualenv is a way of having separate Python environments. It allows you to have different environments for Python 2 and Python 3 and the 3rd party package dependencies to go along with that environment.

Quickstart

Run the following commands to create and activate your first virtualenv

virtualenv for Python3

# installs PIP globally
curl https://bootstrap.pypa.io/get-pip.py | python

# creates a virtualenv
python3 -m venv venv

# activates the virtualenv
source venv/bin/activate

virtualenv for Python2

# installs PIP globally
curl https://bootstrap.pypa.io/get-pip.py | python

# installs virtualenv globally
pip install virtualenv

# creates a virtualenv
virtualenv -p python2.7 venv

# activates the virtualenv
source venv/bin/activate

Verify it’s working

You should see the command line now starting with (venv). This means that the virtualenv is active.

Run which python and it should point to a Python executable inside the virtualenv at venv/bin/python

Quickstart Explanation

First get pip. pip, which stands for “Pip Installs Packages”, is the Python ecosystem’s package installer. If it is not already installed. This can be done using the linked instructions, or with this command.

curl https://bootstrap.pypa.io/get-pip.py | python

If you already have pip or just want to check, run this command to check and output all installed 3rd party packages:

pip freeze

If you currently have other 3rd party packages, and want to clean up the gloal dependencies, then run this command to remove all 3rd party packages:

pip freeze | xargs pip uninstall -y

This is my prefered configuration. To only have virtualenv as the single global 3rd party package. Other packages are local to the virtualenv.

Now, install virtualenv globally.

pip install virtualenv

Check what version of Python is the default version.

python -V

On my machine, the previous command returned Python 2.7.14. So, when I create a virtualenv this will be the default Python version, unless another version is specified.

Create a new virtualenv

virtualenv venv

Why is it named venv?

By convention, the name venv is used as the name of the virtualenv, but any name can be used.

Now, activate the virtualenv

source venv/bin/activate

venv, the name of the virtualenv should show at the start of the command line prompt.

# before
~/Documents/github/jira (master) $

# after
(venv) ~/Documents/github/jira (master) $

If you run pip freeze at this point, there should be no output because this is a new Python environment. The global virtualenv that was just installed should not appear in the output.

At this point, any 3rd party Python package can be installed, just like virtualenv was installed globally, and as long as the virtualenv is active, it will be private to that virtualenv.

To deactivate the virtualenv, run:

deactivate

CheatSheet

Here are some use cases and common commands.

Create a virtualenv in Python3

The venv standard library package is available as of Python 3.3 and can be used to create a virtualenv

python3 -m venv venv

virtualenv with a specific version of Python

This will create a virtualenv in Python 3.5

virtualenv -p /usr/local/bin/python3.5 venv

Access the virtualenv without it being active

By using the path to the Python executable inside of the venv, the virtualenv and all of it’s 3rd party packages will be loaded:

~/Documents/github/jira (master) $ python
Python 2.7.14 (v2.7.14:84471935ed, Sep 16 2017, 12:01:12)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
~/Documents/github/jira (master) $ venv/bin/python
Python 3.6.2 (v3.6.2:5fd33b5926, Jul 16 2017, 20:11:06)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()

virtualenv with cron

crontab -e
* * * * * /path/to/venv/bin/python /path/to/code.py

Include global Python packages when creating a virtualenv

virtualenv --system-site-packages venv

Delete a virtualenv

rm -rf venv

Store copy of Python package versions

This is done by convention in a file called requirements.txt

pip freeze > requirements.txt

Install all packages from requirements.txt

This command would be used when running another projects code, to install all dependencies of the project.

pip install -r requirements.txt

Conclusion

virtualenv has been very useful. Now days, with Docker, and containers, they’re not always necessary, but if you’re developing locally and have multiple Python projects, they’re a must. You won’t get package conflicts, and can maintain different Python version and environments.

Any questions?

Thank you for reading. If you have any questions or anything that I can add, please let me know. Thanks.