[Python-modules-team] Bug#876145:

[Peter Amstutz]

I left out the --system flag in my example.

Just to be clear:

pkg_A (requires pkg_B==1.0)
pkg_C (requires pkg_B>=1.0)

$ pip install --system pkg_A  # installs pkg_B==1.0
$ pip install --system pkg_C  # ignores pkg_B==1.0, installs pkg_B==2.0
$ pkg_A
pkg_resources.ContextualVersionConflict: (pkg_B 2.0
(/usr/local/lib/python2.7/dist-packages),
Requirement.parse('pkg_B==1.0'), set(['pkg_A']))

Because of --ignore-installed, pkg_C ignores the existing pkg_B (which
it is otherwise compatible with) and installs the latest pkg_B==2.0, as
a result this breaks pkg_A.