When I attempt to use the YouCompleteMe plugin in vim, I get the following error:
YouCompleteMe unavailable: This package should not be accessible on Python 3. Either you are trying to run from the python-future src folder or your installation of python-future is corrupted
However, I only get this error when I open vim within my virtual environment. It works fine when a virtualenv is not active. My guess is that YouCompleteMe is written in python 2, and for some reason can't see the python2 interpreter from inside the virtual environment, but I am not sure how to fix it. There does not appear to be a Stack Overflow question on the subject, but a similar issue came up in the ycm-users Google Group and went unanswered.
I'm on Ubuntu 14.04, using vim version 7.4.
Looks like it's a bug:
https://github.com/Valloric/ycmd/pull/578
In PR #448, I made the assumption that the site-packages paths are always placed after the standard library path so that if we insert the python-future module just before the first site-packages path, it would necessary be after the standard library. Turns out that it is not true when a site-packages path is added to the PYTHONPATH environment variable, e.g, when using the software ROS. See issue Valloric/YouCompleteMe#2186. When this happens, the python-future module will raise the following exception on Python 3:
ImportError: This package should not be accessible on Python 3. Either you are trying to run from the python-future src folder or your installation of python-future is corrupted.We prevent this by looking for the standard library path in sys.path and by inserting the python-future module just after it. If we can't find it, we raise an exception since YCM and ycmd cannot work without it.
With the above pull request merged, you should be able to fix the issue by pulling the updates--or cloning the repo if haven't already--and reinstalling.