Required package already installed redundantly added to easy-install.pth

Originally reported by: robind42 (Bitbucket: robind42, GitHub: robind42)

We have a situation where there are already some packages installed in other site-package-like folders (in parent virtual environments in our case, all are on the sys.path). When another package is installed or develop-ed targeting the current site-packages folder and has one of those other packages as a requirement, then the location of the dependency is added to easy-install.pth eventhough it is already available on the sys.path. For example:

#!python
import sys; sys.__plen = len(sys.path)
/Users/robind/projects/tmp/setuptools
/Users/robind/projects/tmp/repeat
./argh-0.26.1-py2.7.egg
<ParentVenv0>/lib/python2.7/site-packages
<ParentVenv1>/lib/python2.7/site-packages
import sys; new=sys.path[sys.__plen:]; del sys.path[sys.__plen:]; p=getattr(sys,'__egginsert',0); sys.path[p:p]=new; sys.__egginsert = p+len(new)

At runtime this causes those additional paths to be searched much earlier than they should be, and potentially having older packages in those parent environments taking precedence over a newer version installed in the current environment. In this case at least removing those two site-packages lines fixes the import order issues because they are searched later after the current site-packges as intended.

Is there a reason why easy_install.process_distribution needs to call self.easy_install for requirement dists that are already installed? Just because their path is not in the easy-install.pth currenlty being worked on does not mean they are not present and active. The earlier call to WorkingSets([]).resolve found them in when it searched self.local_index, so perhaps they can be checked for there before calling self.easy_install for them.

• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/400

Start with easy_install.process_distribution and trace its call to self.easy_install for requirements already found by WorkingSets([]). Check how the current easy-install.pth is updated when a dependency is already active on sys.path. Done means avoiding redundant path entries while preserving the intended import order; the issue does not name a regression test or file.