Dependencies should be installed one-at-a-time
It is a sad fact that many packages have a setup.py which imports other packages. This is sub-optimal, but there is unfortunately no way to avoid it given the current state of python packaging tools. For example, any package which uses the Numpy C API has to import numpy in its setup.py in order to ask it for build information. So, for example, the packages 'scipy' and 'pandas' both have a setup-time requirement that numpy is already installed.
The official word from the 'pip' designers is that the only way to install such packages is by running multiple pip invocations: https://github.com/pypa/pip/issues/25
That is, this does not (and will never) work: pip install numpy scipy pandas
This does work, and is the only supported method: pip install numpy; pip install scipy; pip install pandas
My package that I want to test with tox depends on these other packages. But when I try to run tox, it always generates the first form (which doesn't work), and there's no way to tell it to generate the second form (which does). The result is that tox cannot produce working virtualenvs for testing.
AFAICT the only advantage of the first form is that if there is some problem with the packages, pip can notice earlier and avoid trashing your python install. However, tox only ever installs into throwaway virtualenvs, so this is not really an advantage. Therefore I'd suggest that tox always and unconditionally process the deps= option by calling 'pip install' separately for each entry, and in the order they are listed in the config file.