A good solid understanding of virtual environments is critical to any serious Python developer. The purpose of a virtual env is “isolation of dependencies”. This will probably sound confusing but below is an explanation with an example.
Let’s say you are working on two separate python projects on your computer.
Your first project is a legacy project. It’s pretty old and you are trying to get it to work with modern python. It relies heavily on a really old version of requests. Your second project is hip with the times, it also relies on requests but it is using the latest and greatest version.
If you could only install one version of requests then you would only be able to run one of those two projects on your computer, which sucks.
Virtual environments let you isolate your projects from each other so they each have their own separate copies of dependencies.
Please read this primer: https://realpython.com/python-virtual-environments-a-primer/
A virtualenv is really just a directory with a bunch of python stuff inside it.
Every virtualenv has it’s own instance of the Python executable. So if you have a python3.8 venv and it is active and you type in python it will run python3.8. If you type python3 then it will also run python3.8. Same thing with pip.
Whenever you activate a virtualenv it runs a script called activate, this is a plain simple bash script. This means you can put extra stuff in there. It’s very useful for setting up environmental variables, you can export your variables near the top of your activate script to make sure they are always available on your development machine.
For example, let’s say you are writing a program that sends emails, and in order to send the emails the program needs to be able to authenticate using a keyfile.
Your python code could maybe be something like this:
import os
from some_cool_email_service import Client
PATH_TO_EMAIL_KEY = os.environ['PATH_TO_EMAIL_KEY'] # if the variable is not set then this will raise a KeyError
client = Client()
client.authenticate_with_key_file(PATH_TO_EMAIL_KEY)
client.send_email(
subject="winning!",
message="something really quite witty",
etc
)
To get this code to work you would need to activate your venv, and make sure that the environmental variable is set up.
source /path/to/your_environment/bin/activate
python your_script.py
# This gives us a KeyError
export PATH_TO_EMAIL_KEY=`/something/local/on/your/machine`
python your_script.py
# yay! it works
You can also just add this to the top of your virtualenv bin/activate script.
export PATH_TO_EMAIL_KEY=`/something/local/on/your/machine`
Then you’ll be able to do this:
source /path/to/your_environment/bin/activate
python your_script.py
# yay! it works
Ideally in serious projects you should be using a tool called pipenv. This probably won’t make sense until you understand virtualenvironments. It feels very similar to npm and yarn for those of you with a JavaScript background.
This guide explains it nicely: https://realpython.com/pipenv-guide/