If you use QGIS, you have Python 2.6 (Snow Leopard) or Python 2.7 (Lion,Mountain Lion) preinstalled (like in Linux) and it is theses versions that QGIS uses, not another one (no need to install other version of Python).The pythonic way to install a Python module like shapely is not with aptitude but with the solution of Paul Smith (Shapely For MAC) see also How to install shapely for python 2.6 (Mac) NOT 2.7?How to install shapely for python 2.6 (Mac) NOT 2.7?