Allow for base deps, overridden by factors (no duplicate requirements)

[blueyed]

Often you have special factors with specific version constraints for dependencies.

With pytest-cov this then results in tox -e py37 not working, since dependencies are not installed then.

It would be nice if this would be supported:

diff --git i/tox.ini w/tox.ini
index bfe166f..b640c23 100644
--- i/tox.ini
+++ w/tox.ini
@@ -13,6 +13,7 @@ setenv =
 passenv =
     *
 deps =
+    pytest
     t310: pytest==3.10.1
     t40: pytest==4.0.2
     t41: pytest==4.1.1
@@ -20,9 +21,11 @@ deps =
     t44: pytest==4.4.2
     t45: pytest==4.5.0

+    pytest-xdist
     {t310,t40,t41,t43}: pytest-xdist==1.27.0
     {t44,t45}: pytest-xdist==1.28.0

+    coverage
     c44: coverage==4.4.2
     c45: coverage==4.5.3
 pip_pre = true

I.e. the deps are installed without a version constraint by default, but for the specific factors the constraint would be used.

Currently this results in:

GLOB sdist-make: /home/daniel/Vcs/pytest-cov/setup.py
py37-t45 create: /home/daniel/Vcs/pytest-cov/.tox/py37-t45
py37-t45 installdeps: pytest, pytest==4.5.0, pytest-xdist, pytest-xdist==1.28.0, coverage
ERROR: invocation failed (exit code 1), logfile: /home/daniel/Vcs/pytest-cov/.tox/py37-t45/log/py37-t45-1.log
================================================================================= log start ==================================================================================
Double requirement given: pytest==4.5.0 (already in pytest, name='pytest')
You are using pip version 19.0.3, however version 19.2.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

================================================================================== log end ===================================================================================
ERROR: could not install deps [pytest, pytest==4.5.0, pytest-xdist, pytest-xdist==1.28.0, coverage]; v = InvocationError('/home/daniel/Vcs/pytest-cov/.tox/py37-t45/bin/python -m pip install --pre pytest pytest==4.5.0 pytest-xdist pytest-xdist==1.28.0 coverage', 1)
__________________________________________________________________________________ summary ___________________________________________________________________________________
ERROR:   py37-t45: could not install deps [pytest, pytest==4.5.0, pytest-xdist, pytest-xdist==1.28.0, coverage]; v = InvocationError('/home/daniel/Vcs/pytest-cov/.tox/py37-t45/bin/python -m pip install --pre pytest pytest==4.5.0 pytest-xdist pytest-xdist==1.28.0 coverage', 1)

A trick is to use envs for this, but that should not be necessary: pytest-dev/pytest-cov#315

[gaborbernat]

This is non-trivial, as it implies us to do some kind of interpretation of the PEP-440 dependencies. I would say definitely won't do part of the core. But is a really strong candidate for a good plugin someone can write. If you do decide on going ahead and making this a reality please comment the repository URL below.

[jayvdb]

Double requirement given: pytest, pytest==4.5.0

IMO this is best fixed in pip, as many times doubles are not incompatible, in which case there is only one desired outcome. Also pip already has all the relevant dependencies and helpers to make that decision.

[gaborbernat]

Well, this is debatable. Technically pip can say specifying multiple times the same dependency on the command line is not a valid call. Though in an ideal world, yes, pip should handle it. Maybe with the new dependency resolver, it may be able to do so. This was beforehand the biggest stumble block.

[untitaker]

The new resolver has landed and outside of Python 2 this now just seems to work.

[gaborbernat]

This is no longer the case with newer pips, so the recommendation is to use a newer pip.