cmake invocation shouldn't install dependencies

[jagerman]

(@dean0x7d)

One of the changes in PR #321 (18319d5) invokes pip during cmake to install pytest if it isn't found, but this seems conceptually wrong: a cmake invocation really shouldn't be modifying anything outside the project dir.

I propose that this should be changed to produce a fatal error if pytest isn't found (perhaps with a message to invoke pip install pytest or pip install --user pytest), or at the very least protected behind a disabled-by-default PYBIND11_INSTALL_DEPS option. Otherwise we're installing something outside of the pybind11 directory without asking just because someone decides to configure the pybind11 test code build.

(As a side note, even with the option approach, I think the debian builds should go back to install python$PY-pytest instead of python$PY-pip python$PY-setuptools—part of the point of those builds was to install all deps via native system packages rather than via pip; it also ensures that everything works as expected without pip/setuptools available.)

[aldanor]

Agreed, I was surprised by this as well. I'd probably prefer it to just fail if py.test executable is not found (or if ${PYTHON} -m pytest --version fails).

[dean0x7d]

a cmake invocation really shouldn't be modifying anything outside the project dir.

True. In trying to make it more convenient, it may have become too intrusive. I guess failing with a nice error message should be the way to go then. And yeah, at that point the docker build on Travis should switch to the system pytest package.

Slightly off-topic: pip and setuptools are bundled with Python but Linux distributions specifically remove them from their Python packages. Seeing a pure-Python package like pytest as a system package is baffling. System packages used to be a necessary evil for difficult to install stuff like numpy and scipy, but having the entire PyPI catalogue recreated with every distribution's package manager seems counterproductive.