[Erwin]

Virtual envs are actually quite simple -- they contain a bin/ directory with a linked python binary. When the python binary runs, it checks it sibling directories (it knows it was executed as e.g. /home/user/.venv/bin/python) for what to load. You don't need the activate shell scripts or anything, just running that binary within your venv is enough; the shell script is just for convenient of inserting the bin directory into the $PATH so just "python" or "pip" runs the right thing.

[dragonwriter]

> the shell script is just for convenient of inserting the bin directory into the $PATH so just "python" or "pip" runs the right thing.

Or so any reference in the program you run that launches another binary or loads a DLL relying onnthe environment gets the right one, etc. There are some binaries you can run without activating a venv with no problem, and others will crash hard, and others will just subtly do the wrong thing if the conditions are “right” in your normal system environment.

[josephg]

Another implication of this is that its impossible for 2 mutually incompatible copies of the same package to exist in the same environment. If packageA needs numpy 1.20 and packageB needs numpy 1.21, you're stuck.

[throwawaymaths]

> Virtual envs are actually quite simple

You have never trashed your system from virtualenv?

Also, there is a problem when wheels assume they can have everything like tensorflow from years ago -- I don't know about now, since tf used to be tied to cuda versions you could get into trouble installing tf versions, even with venv, conda, etc.

[drdaeman]

> You have never trashed your system from virtualenv?

Unless one have done something they shouldn't have done (in particular, using sudo while working with virtualenv), this shouldn't be possible.

Due to limitations of most commonplace system-wide package managers (like, dpkg, rpm or ebuild, not modern stuff like nix) system packages exist to support other system packages. One installs some program, it needs libraries, dependencies get pulled. And then its distro package managers' job to ensure compatibility and deal with multiple version conflicts (not fun).

But if you start or check out some project, common knowledge was that you shouldn't be using on system packages, even if they're available and could work. With some obligatory exceptions like when you're working on a distribution packaging, or developing something meant to be tightly integrated with a particular distro (like a corporate standard stuff).

That is, unless we're talking about some system libraries/drivers needed for CUDA in particular (which is system stuff) rather than virtualenv itself.