This lesson is being piloted (Beta version)

Environments and task runners


Teaching: 10 min
Exercises: 10 min
  • How do you install and manage packages?

  • How can you ensure others run the same code you do?

  • Learn about virtual environments

  • Use a task runner to manage environments and run code

You will see two very common recommendations when installing a package:

pip install <package>         # Use only in virtual environment!
pip install --user <package>  # Almost never use

Don’t use them unless you know exactly what you are doing! The first one will try to install globally, and if you don’t have permission, will install to your user site packages. In global site packages, you can get conflicting versions of libraries, you can’t tell what you’ve installed for what, packages can update and break your system; it’s a mess. And user site packages are worse, because all installs of Python on your computer share it, so you might override and break things you didn’t intend to. And with pip’s new smart solver, updating packages inside a global environment can take many minutes and produce unexpectedly solves that are technically “correct” but don’t work because it backsolved conflicts to before issues were discovered.

There is a solution: virtual environments (libraries) or pipx (applications).

There are likely a few libraries (ideally just pipx) that you just have to install globally. Go ahead, but be careful (and always use your system package manager instead if you can, like brew on macOS or the Windows ones – Linux package managers tend to be too old to use for Python libraries).

Virtual Environments

The following uses the standard library venv module. The virtualenv module can be installed from PyPI, and works identically, though is a bit faster and provides newer pip by default.

Python 3 comes with the venv module built-in, which supports making virtual environments. To make one, you call the module with

python3 -m venv .venv

This creates links to Python and pip in .venv/bin, and creates a site-packages directory at .venv/lib. You can just use .venv/bin/python if you want, but many users prefer to source the activation script:

. .venv/bin/activate

(Shell specific, but there are activation scripts for all common shells here). Now .venv/bin has been added to your PATH, and usually your shell’s prompt will be modified to indicate you are “in” a virtual environment. You can now use python, pip, and anything you install into the virtualenv without having to prefix it with .venv/bin/.

Check the version of pip installed! If it’s old, you might want to run pip install -U pip or, for modern versions of Python, you can add --upgrade-deps to the venv creation line.

To “leave” the virtual environment, you undo those changes by running the deactivate function the activation added to your shell:


What about conda?

The same concerns apply to Conda. You should avoid installing things to the base environment, and instead make environments and use those above. Quick tips:

conda config --set auto_activate_base false  # turn off the default environment
conda env create -n some_name  # or use paths with `-p`
conda activate some_name
conda deactivate


There are many Python packages that provide a command line interface and are not really intended to be imported (pip, for example, should not be imported). It is really inconvenient to have to set up venvs for every command line tool you want to install, however. pipx, from the makers of pip, solves this problem for you. If you pipx install a package, it will be created inside a new virtual environment, and just the executable scripts will be exposed in your regular shell.

Pipx also has a pipx run <package> command, which will download a package and run a script of the same name, and will cache the temporary environment for a week. This means you have all of PyPI at your fingertips in one line on any computer that has pipx installed!

Task runner (nox)

A task runner, like make (fully general), rake (Ruby general), invoke (Python general), tox (Python packages), or nox (Python simi-general), is a tool that lets you specify a set of tasks via a common interface. These can be a crutch, allowing poor packaging practices to be employed behind a custom script, and they can hide what is actually happening.

Nox has two strong points that help with this concern. First, it is very explicit, and even prints what it is doing as it operates. Unlike the older tox, it does not have any implicit assumptions built-in. Second, it has very elegant built-in support for both virtual and Conda environments. This can greatly reduce new contributor friction with your codebase.

A daily developer is not expected to use nox for simple tasks, like running tests or linting. You should not rely on nox to make a task that should be made simple and standard (like building a package) complicated. You are not expected to use nox for linting on CI, or sometimes even for testing on CI, even if those tasks are provided for users. Nox is a few seconds slower than running directly in a custom environment - but for new users and rarely run tasks, it is much faster than explaining how to get setup or manually messing with virtual environments. It is also highly reproducible, creating and destroying the temporary environment each time by default.

You should use nox to make it easy and simple for new contributors to run things. You should use nox to make specialized developer tasks easy. You should use nox to avoid making single-use virtual environments for docs and other rarely run tasks.

Since nox is an application, you should install it with pipx. If you use Homebrew, you can install nox with that (Homebrew isolates Python apps it distributes too, just like pipx).

Running nox

If you see a in a repository, that means nox is supported. You can start by checking to see what the different tasks (called sessions in nox) are provided by the noxfile author. For example, if we do this on’s repository:

nox -l  # or --list-sessions
Sessions defined in /github/pypa/

- translation -> Build the gettext .pot files.
- build -> Make the website.
- preview -> Make and preview the website.
- linkcheck -> Check for broken links.

sessions marked with * are selected, sessions marked with - are skipped.

You can see that there are several different sessions. You can run them with -s:

nox -s preview

Will build and start up a preview of the site.

If you need to pass options to the session, you can separate nox options with and the session options with --.

Writing a Noxfile

For this example, we’ll need a minimal test file for pytest to run. Let’s make this file in a local directory:


def test_runs():
    assert True

Let’s write our own noxfile. If you are familiar with pytest, this should look familiar as well; it’s intentionally rather close to pytest. We’ll make a minimal session that runs pytest:

import nox

def tests(session):

A noxfile is valid Python, so we import nox. The session decorator tells nox that this function is going to be a session. By default, the name will be the function name, the description will be the function docstring, it will run on the current version of Python (the one nox is using), and it will make a virtual environment each time the session runs, though all of this is changeable via keyword arguments to session.

The session function will be given a nox.Session object that has various useful methods. .install will install things with pip, and .run will run a command in a sesson. The .run command will print a warning if you use an executable outside the virtual environment unless external=True is passed. Errors will exit the session.

Let’s expand this a little:

import nox

def tests(session: nox.Session) -> None:
    Run our tests.
    session.install("pytest")"pytest", *session.posargs)

This adds a type annotation to the session object, so that IDE’s and type checkers can help you write the code in the function. There’s a docstring, which will print out nice help text when a user lists the sessions. And we pass through to pytest anything the user passes in via session.posargs.

Let’s try running it:

nox -s tests
nox > Running session tests
nox > Creating virtual environment (virtualenv) using python3.10 in .nox/tests
nox > python -m pip install pytest
nox > pytest
==================================== test session starts ====================================
platform darwin -- Python 3.10.5, pytest-7.1.2, pluggy-1.0.0
rootdir: /Users/henryschreiner/git/teaching/packaging
collected 1 item .                                                                          [100%]

===================================== 1 passed in 0.05s =====================================
nox > Session tests was successful.

Passing arguments through

Try passing -v to pytest.


nox -s tests -- -v

Virtual environments

Nox is really just doing the same thing we would do manually (and printing all the steps except the exact details of creating the virtual environment. You can see the virtual environment in .nox/tests! How would you activate this environment?


. .nox/tests/bin/activate

Key Points

  • Virtual environments isolate software

  • Virtual environments solve the update problem

  • A task runner makes it easier to contribute to software