EECS 298: Social Consequences of Computing

Python/Virtual Environment Tutorial

This document will guide you through installing Python and setting up a Python Virtual Environment.

Install Python

Install a recent version of Python.

macOS

You might already have Python installed, although your version might be different.

$ python3 --version
Python 3.10.8

If you do not have Python or your version of Python is older than 3.6, install a recent version of Python using the Homebrew package manager.

$ brew install python3
$ python3 --version
Python 3.10.8

WSL or Linux

$ sudo apt-get update
$ sudo apt-get install python3 python3-pip python3-venv python3-wheel python3-setuptools

Create a Python virtual environment

Virtual environments are used to create containers with separate installed libraries. They are useful when you want to install different Python programs that have different third party package dependencies. For example, you might have different virtual environments for different course assignments, as different courses or assignments may require different third party packages or different versions of those packages. More on venv and the creation of virtual environments can be found here.

This section will help you install the Python tools and packages locally, which won’t affect Python tools and packages installed elsewhere on the machine.

After finishing this section, you’ll have a folder called env/ that contains all the Python packages you need for this project.

Create a virtual environment in your project’s root directory. ())

$ pwd
/creiglas/eecs298
$ python3 -m venv env

Activate virtual environment. You’ll need to do this every time you start a new shell.

$ source env/bin/activate

We now have a complete local environment for Python. Everything lives in one directory. Environment variables point to this virtual environment.

$ echo $VIRTUAL_ENV
/creiglas/eecs298/env

We have a Python interpreter installed inside the virtual environment. which python tells you exactly which python executable file will be used when you type python. Because we’re in a virtual environment, there’s more than one option!

$ which python3     # Default python exectuable
/creiglas/eecs298/env/bin/python
$ which -a python  # All python executables
/creiglas/eecs298/env/bin/python3
/usr/local/bin/python3
/usr/bin/python3

There’s a package manager for Python called pip installed in the virtual environment. That will help us install Python packages later.

$ which pip
/creiglas/eecs298/env/bin/pip
$ pip --version
pip 9.0.1 from /creiglas/eecs298/env/lib/python3.7/site-packages (python 3.7)  # Your version may be different

Python packages live in the virtual environment. We can see that Python’s own tools are already installed (pip and setuptools).

$ ls env/lib/python3.7/site-packages/ # Your version may be different
pip
setuptools
...

Upgrade the Python tools in your virtual environment

$ pip install --upgrade pip setuptools wheel

Install numpy. We’ll use this tool later.

$ pip install numpy

Understanding Virtual Environments

This section will give more detail about virtual environments and how they work. Simply put, a virtual environment is a bunch of files (located in env/ in this tutorial) used by Python.

Environment

An environment is a collection of environment variables that are inputs to your shell and your programs.

Print the names and values of all environment variables using the env command. You’ll see key/value pairs used by the shell and used by programs.

$ env
...
PWD=/creiglas/eecs298
HOME=/creiglas
USER=creiglas
PATH=/usr/local/bin:/usr/bin:/bin
...

An important example of an environment variable is PATH, which tells your shell where to look for commands like ls, cd, python and so on. It’s a colon-separated list (:). You can print the value of one variable using the dollar sign ‘$’.

$ echo $PATH
/usr/local/bin:/usr/bin:/bin
$ printenv PATH  # Alternative
/usr/local/bin:/usr/bin:/bin
$ echo $PATH | tr ':' '\n'
/usr/local/bin
/usr/bin
/bin

Notice that each item in the list is a directory that contains executables, for example /usr/local/bin usually contains the python3 executable on macOS with Homebrew (/opt/homebrew/bin on Apple Silicon M1).

$ ls /usr/local/bin
...
python3
...

Environment variables inside a Python program

You can set any environment variable you want.

$ export MESSAGE="hello world"
$ echo $MESSAGE
hello world

Environment variables are accessible from programs, like this test.py.

"""test.py"""
import os
print(os.environ["MESSAGE"])

Set an environment variable and run the program.

$ export MESSAGE="hello world"
$ python3 test.py
hello world

This example shows that environment variables are simply another way to provide input to a running program.

Virtual environment

A virtual environment is a self-contained directory that contains a Python installation and a number of additional Python packages.

As you saw earlier, the command to create a virtual environment creates a new directory, env in this example.

$ python3 -m venv env  # you ran this earlier
$ ls env/
bin  include  lib  pyvenv.cfg

The virtual environment contains a bin/ directory with executables. It also contains a lib/ directory where Python third party packages live. Your versions might be different.

$ ls env/bin/
...
pip
python
...
$ ls env/lib/python3.7/site-packages/ # Your version may be different
__pycache__      pip-19.2.3.dist-info  setuptools-41.2.0.dist-info
easy_install.py  pkg_resources         pip              setuptools

Activate a virtual environment

In the previous example, we used the virtual environment by calling its Python executable explicitly (e.g., ./env/bin/python). As a convenience, we can temporarily make this version the default.

The bin/activate script adds env/bin to the PATH environment variable, making it the first place to look for commands. Notice that /creiglas/eecs298/env/bin is first in the list.

$ source env/bin/activate
$ echo $PATH
/creiglas/eecs298/env/bin:/usr/local/bin:/usr/bin:/bin
$ echo $PATH | tr ':' '\n'
/creiglas/eecs298/env/bin
/usr/local/bin
/usr/bin
/bin

Ask the shell where all the python executables live, then which one is the default.

$ which -a python
/creiglas/eecs298/env/bin/python
/usr/local/bin/python
/usr/bin/python
$ which python
/creiglas/eecs298/env/bin/python

Finally, the activate script sets a $VIRTUAL_ENV environment variable, which contains the path to the virtual environment directory.

$ echo $VIRTUAL_ENV
/creiglas/eecs298/env

Replicate a virtual environment

In the previous section, we created a Python virtual environment, activated it, and upgraded the Python installer tools (pip, setuptools, wheel). We have not yet installed any new third party Python packages.

$ pwd
/creiglas/eecs298/
$ echo $VIRTUAL_ENV
/creiglas/eecs298/env
$ pip list
Package    Version
---------- -------
pip        20.1.1
setuptools 47.3.1
wheel      0.34.2

Deactivate a virtual environment

The deactivate command simply modifies two environment variables, PATH and VIRTUAL_ENV. First, it unsets VIRTUAL_ENV.

$ deactivate
$ echo $VIRTUAL_ENV  # Variable not set, output is blank

Finally, deactivate changes PATH to its previous value, before the virtual environment was activated.

$ echo $PATH | tr ':' '\n'
/usr/local/bin
/usr/bin
/bin

Summary

A Python virtual environment helps you manage third party packages. A pre-configured python executable in ./env/bin/ uses the third party packages in ./env/lib/ (the name of env/ is your choice).

Activate the virtual environment each time you start a new shell.

$ pwd
/creiglas/eecs298/
$ source env/bin/activate

The activate script changes the PATH environment variable, which temporarily changes the default python and pip executables.

$ which python
/creiglas/eecs298/env/bin/python
$ which pip
/creiglas/eecs298/env/bin/pip

Acknowledgments

Original document written by Andrew DeOrio awdeorio@umich.edu.

This document is licensed under a Creative Commons Attribution-NonCommercial 4.0 License. You’re free to copy and share this document, but not to sell it. You may not share source code provided with this document.