Change the priority of python sys.path

Question

How can I change the priority of the path in sys.path in python 2.7? I know that I can use PYTHONPATH environment variable, but it is what I will get:

$ PYTHONPATH=/tmp python
Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> for i in sys.path:
...   print i
... 

/usr/local/lib/python2.7/dist-packages/pycuda-2014.1-py2.7-linux-x86_64.egg
/usr/local/lib/python2.7/dist-packages/pytest-2.6.2-py2.7.egg
/usr/local/lib/python2.7/dist-packages/pytools-2014.3-py2.7.egg
/usr/local/lib/python2.7/dist-packages/py-1.4.24-py2.7.egg
/usr/lib/python2.7/dist-packages
/tmp
/usr/lib/python2.7
/usr/lib/python2.7/plat-x86_64-linux-gnu
/usr/lib/python2.7/lib-tk
/usr/lib/python2.7/lib-old
/usr/lib/python2.7/lib-dynload
/usr/local/lib/python2.7/dist-packages
/usr/lib/python2.7/dist-packages/PILcompat
/usr/lib/python2.7/dist-packages/gtk-2.0
/usr/lib/python2.7/dist-packages/ubuntu-sso-client
>>> 

/tmp is added between /usr/lib/python2.7/dist-packages and /usr/lib/python2.7.

My goal is to make python to load packages from /usr/local/lib/python2.7/dist-packages first.

Here is what I want:

$ python
Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> np.version
<module 'numpy.version' from '/usr/local/lib/python2.7/dist-packages/numpy/version.pyc'>
>>> 

If I install python-numpy by apt-get install python-numpy. Python will try to load from /usr/lib/python2.7 and not the one I compiled.

Answer 1

As you may know, sys.path is initialized from:

• the current directory
• your PYTHONPATH
• an installation-dependent default

However unfortunately that is only part of the story: setuptools creates easy-install.pth files, which also modify sys.path and worst of all they prepend packages and therefore totally mess up the order of directories.

In particular (at least on my system), there is /usr/local/lib/python2.7/dist-packages/easy-install.pth with the following contents:

import sys; sys.__plen = len(sys.path)
/usr/lib/python2.7/dist-packages
import sys; new=sys.path[sys.__plen:]; del sys.path[sys.__plen:]; p=getattr(sys,'__egginsert',0); sys.path[p:p]=new; sys.__egginsert = p+len(new)

This causes /usr/lib/python2.7/dist-packages to be prepended even before your PYTHONPATH!

What you could do is simply change the 2nd line in this file to

/usr/local/lib/python2.7/dist-packages

and you will get your desired priority.

However beware this file might be overwritten or changed again by a future setuptools invocation!