So you want to populate your virtual environment before you run the unit tests, right?

The command to do so is trivial, but when do you need to actually execute it?

Normal make targets have prerequisites and outcomes (I think they are called goals). These must be files and make works by comparing their modification timestamps to see if a recipe has to be (re-)executed.

Now what is the prerequisite for your virtual environment?

What is the output? venv/bin/python ? What if you need to pip install some stuff (maybe only for testing)? What happens when the installation of a single package fails?

I think having a script (or a phony target for people with strange tastes ;) ) makes at least clear that there is no half-assed dependency tracking ongoing.

I think one way to figure this out would be to have a test script that runs inside the venv and outputs some kind of checksum.