Looking at the contents of sphinx-build, you could probably get away with something like:

import sphinx
sphinx.build_main(['sphinx-build'] + input_args)

I don't understand your concern; why would it pick up sphinx-build outside the environment? If the environment didn't install the executable and modify PATH, then something is wrong with the environment.

There is a problem on Python 3 though if using system packages, because they generally install both sphinx-build and sphinx-build-3. This is handled by the Makefile using a modifiable variable SPHINXBUILD which can be set when calling make: make SPHINXBUILD=sphinx-build-3.

It's quite common in my experience for me to have sphinx-build installed on the system, or in my user installs, and for that to get picked up when I am working inside a virtualenv, on which I have not installed sphinx. That can be pretty confusing.

Oh yes, not having it installed at all, good point. We can fix this now, but I think there's not really much of a way to fix this when switching to the Makefile, though.

Actually, as of sphinx 1.5.3, you can just do

python -msphinx -bhtml $srcdir $destdir

Perhaps we can just check since when this form is supported and update the requirement. I will open an issue upstream to fix their Makefile/make.bat as well.

xref sphinx-doc/sphinx#3523

Closed by #8269.