I just wanted to add another example showing why I was looking for this kind of feature.
Something similar came up here, but our GUI package Kivy supports various backends for text, image, video, etc. But, at least one backend must be installed for Kivy to work.
Currently, pip install kivy does not install any backends so kivy won’t work without additional steps. Previously we just told users to manually install the dependencies along with listing all our backend options (e.g. pip install kivy_deps.sdl2==x.y.z).
This led to many opened issues of failed installations over time due to user confusion. And this seemingly got worse in recent times because many users don’t read install docs and simply install Kivy graphically in PyCharm by searching for Kivy and then clicking install. Naturally this doesn’t install any dependencies.
My improvement was to add base and full keys in extras_require. When specified, base will install a set of per-platform dependencies that I judged a average user would want. So now we just tell users to install it with pip install kivy[base]. But this doesn’t solve e.g. the PyCharm problem.
So I was looking for a way to make pip install kivy “default” to installing base as well, but also a way to say pip install kivy --no-extras or pip install kivy[] so advanced users who want the install_requires dependencies but not the base dependencies can do that. If this doesn’t work out, perhaps we’ll just add our base dependencies to install_requires and have advanced users install kivy with --no-deps and then manually install the “real” install_requires. But ideally that would not be required.