It looks like the issue is known to the author of virtualenv http://bitbucket.org/ianb/virtualenv/issue/63/now-python27-requires-_weakrefset and also that it will be fixed in the newest version of Python 2.7 http://bugs.python.org/issue2521
It looks like the issue is known to the author of virtualenv http:// bitbucket. org/ianb/ virtualenv/ issue/63/ now-python27- requires- _weakrefset and also that it will be fixed in the newest version of Python 2.7 http:// bugs.python. org/issue2521