Do not uninstall package on upgrade if new version is not to be installed in the same location

[NiklasRosenstein]

If you install a module with --user --upgrade, and the same module is already installed in the global package site, Pip first attempts to uninstall the module. I think that it should not do so unless the target install directory is the same as the previous module's version install directory.

This can be reproduced with (tested on Windows):

$ pip install six==1.10.0 --upgrade  # an old version of six
$ pip show six
$ pip install six --user --upgrade
$ pip show six

I think that in this case, the six==1.10.0 should be left untouched and the newest version of six should simply be installed to the user package directory.

[NiklasRosenstein]

The --ignore-installed option works around this issue, but if the module does not need to be upgraded, you may as well just use the package from the global package site instead of reinstalling the same package into the user site.

The issue also appears with the --target or --prefix option.

[pradyunsg]

This is the problem that implementing #4575 would address. :)

[pradyunsg]

@NiklasRosenstein if this issue would be resolved as a part of #4575, it'd be nice if you close this issue.

[NiklasRosenstein]

Yes, I think this issue should be resolved as part of #4575. :)