p1-insta485-static

Python Virtual Environment Tutorial

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

Prerequisites

At this point, you should already have a folder for your project (instructions). In this tutorial, we’ll use project 1 as an example. Your folder location might be different.

$ pwd
/Users/awdeorio/src/eecs485/p1-insta485-static

Restarting this tutorial

If you made a mistake with these Python instructions, here’s how to start over. First, close your shell and reopen it to ensure that environment variables are reset. Then, delete the virtual environment.

$ pwd
/Users/awdeorio/src/eecs485/p1-insta485-static
$ rm -rf env

Create a Python virtual environment

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

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

$ python3 --version
Python 3.9.4

$ which python3
/Users/awdeorio/anaconda/bin/python3

$ conda init --reverse

# Close your shell and open a new shell

$ which python3
/usr/local/bin/python3  # macOS
/usr/bin/python3        # WSL/Linux

$ conda deactivate
$ which python3
/usr/local/bin/python3  # macOS
/usr/bin/python3        # WSL/Linux

$ conda install anaconda-clean
$ anaconda-clean --yes

# Close your shell and open a new shell

$ which python3
/usr/local/bin/python3  # macOS
/usr/bin/python3        # WSL/Linux

$ printenv PYTHONPATH  # Output isn't blank, problem!
/Users/awdeorio/local/lib/python3.9/site-packages/

$ grep -s PYTHONPATH .profile .bashrc .bash_profile .zshrc .zlogin .cshrc .tshrc .login
.bashrc: export PYTHONPATH=/Users/awdeorio/local/lib/python3.9/site-packages/

# Edit the file and remove the line.  Ask us if you need help.

# Close your shell and open a new shell

$ printenv PYTHONPATH  # output should be blank

$ env --unset PYTHONPATH
$ printenv PYTHONPATH  # output should be blank

$ which python3  # Apple-provided Python is wrong!
/usr/bin/python3
$ brew install python3
$ which python3
/usr/local/bin/python3

Create a virtual environment in your project’s root directory. (More on venv and the creation of virtual environments)

$ pwd
/Users/awdeorio/src/eecs485/p1-insta485-static
$ 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
/Users/awdeorio/src/eecs485/p1-insta485-static/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 python     # Default python exectuable
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin/python
$ which -a python  # All python executables
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin/python
/usr/bin/python

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

$ which pip
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin/pip
$ pip --version
pip 9.0.1 from /Users/awdeorio/src/eecs485/p1-insta485-static/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 HTML5 Validator. We’ll use this tool later. Your version might be different.

$ pip install html5validator
$ html5validator --version
html5validator 0.3.1

.gitignore and Python virtual environment

Do not commit virtual environment files (env/) to version control. This is because pip installs binaries inside env/. Those binaries will be different on different operating systems and even different computers running the same OS but using different compilers.

The sample .gitignore file you added ignores directories called env/.

$ ls
README.md  env
$ git status
On branch master
nothing to commit, working tree clean

You made a mistake in your .gitignore if you see your Python virtual environment folder as an untracked file.

$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	env/

nothing added to commit but untracked files present (use "git add" to track)

Pro-tip: cleaning your repo

You can remove all untracked and git-ignored files with git clean.

$ git clean -xdi
Would remove the following item:
  env/
*** Commands ***
    1: clean                2: filter by pattern    3: select by numbers    4: ask each
    5: quit                 6: help
What now> 1
Removing env/

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=/Users/awdeorio/src/eecs485/p1-insta485-static
HOME=/Users/awdeorio
USER=awdeorio
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.

$ 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

A pre-configured pip executable installs third party packages to lib/. Your versions of Python and jinja2 may be different.

$ ./env/bin/pip install jinja2
Successfully installed jinja2-2.11.1
$ ls env/lib/python3.7/site-packages/jinja2/ # Your version may be different
__init__.py
...

A pre-configured python executable in bin/ uses the third party packages in lib/.

$ ./env/bin/python
>>> import jinja2
>>> jinja2.__version__
'2.11.1'

Why virtual environments?

Virtual environments are useful when you want to install different Python programs that have different third party package dependencies. For example, you might have a virtual environment for a web course’s project, and a different one for your machine learning course’s homework assignments. The two assignments have different third party packages and different versions of those packages.

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 /Users/awdeorio/src/eecs485/p1-insta485-static/env/bin is first in the list.

$ source env/bin/activate
$ echo $PATH
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin:/usr/local/bin:/usr/bin:/bin
$ echo $PATH | tr ':' '\n'
/Users/awdeorio/src/eecs485/p1-insta485-static/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
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin/python
/usr/local/bin/python
/usr/bin/python
$ which python
/Users/awdeorio/src/eecs485/p1-insta485-static/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
/Users/awdeorio/src/eecs485/p1-insta485-static/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
/Users/awdeorio/src/eecs485/p1-insta485-static
$ echo $VIRTUAL_ENV
/Users/awdeorio/src/eecs485/p1-insta485-static/env
$ pip list
Package    Version
---------- -------
pip        20.1.1
setuptools 47.3.1
wheel      0.34.2

A requirements.txt file lists the exact third party Python packages and their versions needed to replicate another virtual environment. This is useful for ensuring that developers and production servers have identical packages with identical versions. It’s also useful for ensuring that students and the autograder have identical packages with identical versions.

See the list of package dependencies provided in a requirements.txt file. This requirements.txt file is bundled with project starter files, so please download the appropriate project starter files before proceeding. Your output might be different.

$ cat requirements.txt
astroid==2.4.2
...
zipp==3.1.0

Install the package dependencies. Your output might be different.

$ pip install -r requirements.txt
...
Successfully installed astroid-2.4.2 ... zipp-3.1.0
$ pip list
Package            Version
------------------ ---------
astroid            2.4.2
...
zipp               3.1.0

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 it 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
/Users/awdeorio/src/eecs485/p1-insta485-static
$ source env/bin/activate

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

$ which python
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin/python
$ which pip
/Users/awdeorio/src/eecs485/p1-insta485-static/env/bin/pip