The hg_local script uses "/usr/bin/env python" to run python. This causes problems if the default version of python is python 3.x (mercurial doesn't work with py3k). I think that GUI apps ignored the $PATH setting of the user in OS X 10.8, but in OS X 10.9, it seems to use my $PATH and picks /opt/local/bin/python, which is a symlink to python3.3. Just hard-coding python2.7 might be an acceptable solution, but with a bit of extra code you could set it up so that python2.7 is only used as a fallback if python corresponds to python3.