Configuring a Python virtualenv in Debian

Python virtual environments are a good idea. Naturally, I had a few problems getting mine to work properly. I found that my globally installed libraries were visible the activated venv. The whole point of a venv is to isolate libraries. The globally installed libraries that are not also explicitly installed in the venv should not be usable.

I use Debian and the bash shell.

So how did I fix what I found is a common problem?


# blank the PYTHONPATH environment variable
virtualenv my_venv
# use python3, not python as a REPL or the interpreter in e.g. VSCode
# use pip install, not python -m pip install

I have this line in my .bashrc file:


This means that when I create my venv, this path gets added to the PYTHONPATH environment variable in the venv. This allows the venv to see the packages in ~/.local/lib/python3.7/site-packages/. Which we do not want.

To prevent this, blank the PYTHONPATH before creating the venv by typing:


We can check on what the PYTHONPATH environment variable is by entering:


This should now be blank.

Go ahead and create a new venv by typing e.g.

virtualenv my_venv

This is where things continued to be a little.... non-standard. I should be able to use:

python -m pip freeze

To see only the packages that I install in the venv. But I see all the global packages.

Using only:


shows me what I should see. I would like to know why.

Similarly, I should get a REPL with access to only the packages installed in the venv using:


but typing this starts a REPL that allows me to import globally installed packages.

To get around this, use:


Now things behave as they should. I cannot import a package that is globally installed but not installed in the venv. Which is the correct behaviour for a venv. I use the path to the python3 executable in the my_venv/bin directory as the interpreter in e.g. VSCode when I want to use a venv.

I found I had several installations of pip in various locations. I deleted all of them except for the one in /usr/bin.

If you want to learn more about virtual environments, I recommend this series of short videos:

The videos recommend using the command:

python -m pip

in the venv, which I found to be unreliable.

Instead I just use:


Check that this is pointing to the correct executable by typing:

which pip