I think this is still inside the remit of feature flags/extras, though. It’s valid for a package to have N optional backends, and nothing works if you don’t install at least one of them. I think the key is that there exists a default backend for standard installs, which can be disabled explicitly [1].
It might not be possible to fail gracefully if users do something very silly (i.e. refuse to install any backend) but they shouldn’t do that.
This isn’t something that can be accomplished in PyPI packages right now but it feels like PEP 735 and the like might get to it eventually.
this isn’t possible now, to my knowledge ↩︎