How does python load all the values in sys.path?

Question

I'm a python beginner and I'm in my first days playing with my own python scripts and projects such as django.

I use Ubuntu and I set my PYTHONPATH as

export PYTHONPATH=/usr/lib/python2.6:~/Projects/mypython 

When I run into a python interpreter

import sys
print sys.path

I can see

['', '/usr/bin', '/usr/local/lib/python2.6/dist-packages/ropemode-0.1_rc2-py2.6.egg', '/usr/local/lib/python2.6/dist-packages/rope-0.9.3-py2.6.egg', '/usr/local/lib/python2.6/dist-packages/ropevim-0.3_rc-py2.6.egg', '/usr/lib/python2.6', '/home/jaume/Projects/mypython', '/usr/lib/python2.6/plat-linux2', '/usr/lib/python2.6/lib-tk', '/usr/lib/python2.6/lib-old', '/usr/lib/python2.6/lib-dynload', '/usr/local/lib/python2.6/dist-packages', '/usr/lib/python2.6/dist-packages', '/usr/lib/python2.6/dist-packages/PIL', '/usr/lib/python2.6/dist-packages/gst-0.10', '/usr/lib/pymodules/python2.6', '/usr/lib/python2.6/dist-packages/gtk-2.0', '/usr/lib/pymodules/python2.6/gtk-2.0', '/usr/lib/python2.6/dist-packages/wx-2.8-gtk2-unicode', '/usr/lib/pymodules/python2.6/IPython/Extensions', u'/home/jaume/.ipython']

How does python load all the non-PYTHONPATH paths? Is there any index? Moreover, I have my django installed at /usr/lib/pymodules and, even it's not in the sys.path list, it runs.

Many thanks.

Answer 1

This is a list of every file the Python interpreter on my computer (which is set up similar to, but not the same as, yours) opens before it starts running code provided by me:

$ strace -eopen python -c 1 2>&1 | grep -ve '-1 E'
open("/etc/ld.so.cache", O_RDONLY)      = 3
open("/lib/libpthread.so.0", O_RDONLY)  = 3
open("/lib/libdl.so.2", O_RDONLY)       = 3
open("/lib/libutil.so.1", O_RDONLY)     = 3
open("/usr/lib/libssl.so.0.9.8", O_RDONLY) = 3
open("/usr/lib/libcrypto.so.0.9.8", O_RDONLY) = 3
open("/usr/lib/libz.so.1", O_RDONLY)    = 3
open("/lib/libm.so.6", O_RDONLY)        = 3
open("/lib/libc.so.6", O_RDONLY)        = 3
open("/proc/meminfo", O_RDONLY)         = 3
open("/usr/lib/python2.6/site.py", O_RDONLY) = 3
open("/usr/lib/python2.6/site.pyc", O_RDONLY) = 4
open("/usr/lib/python2.6/os.py", O_RDONLY) = 4
open("/usr/lib/python2.6/os.pyc", O_RDONLY) = 5
open("/usr/lib/python2.6/posixpath.py", O_RDONLY) = 5
open("/usr/lib/python2.6/posixpath.pyc", O_RDONLY) = 6
open("/usr/lib/python2.6/stat.py", O_RDONLY) = 6
open("/usr/lib/python2.6/stat.pyc", O_RDONLY) = 7
open("/usr/lib/python2.6/genericpath.py", O_RDONLY) = 6
open("/usr/lib/python2.6/genericpath.pyc", O_RDONLY) = 7
open("/usr/lib/python2.6/warnings.py", O_RDONLY) = 6
open("/usr/lib/python2.6/warnings.pyc", O_RDONLY) = 7
open("/usr/lib/python2.6/linecache.py", O_RDONLY) = 7
open("/usr/lib/python2.6/linecache.pyc", O_RDONLY) = 8
open("/usr/lib/python2.6/types.py", O_RDONLY) = 7
open("/usr/lib/python2.6/types.pyc", O_RDONLY) = 8
open("/usr/lib/python2.6/UserDict.py", O_RDONLY) = 5
open("/usr/lib/python2.6/UserDict.pyc", O_RDONLY) = 6
open("/usr/lib/python2.6/_abcoll.py", O_RDONLY) = 6
open("/usr/lib/python2.6/_abcoll.pyc", O_RDONLY) = 7
open("/usr/lib/python2.6/abc.py", O_RDONLY) = 7
open("/usr/lib/python2.6/abc.pyc", O_RDONLY) = 8
open("/usr/lib/python2.6/copy_reg.py", O_RDONLY) = 5
open("/usr/lib/python2.6/copy_reg.pyc", O_RDONLY) = 6
open("/usr/local/lib/python2.6/dist-packages", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 4
open("/usr/lib/python2.6/dist-packages", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 4
open("/usr/lib/python2.6/dist-packages/PIL.pth", O_RDONLY) = 4
open("/usr/lib/python2.6/dist-packages/pygst.pth", O_RDONLY) = 4
open("/usr/lib/python2.6/dist-packages/python-support.pth", O_RDONLY) = 4
open("/usr/lib/python2.6/dist-packages/wx.pth", O_RDONLY) = 4
open("/usr/lib/python2.6/dist-packages/zope.interface-3.5.3-nspkg.pth", O_RDONLY) = 4
open("/usr/lib/python2.6/sitecustomize.py", O_RDONLY) = 4
open("/usr/lib/python2.6/sitecustomize.pyc", O_RDONLY) = 5
open("/usr/lib/locale/locale-archive", O_RDONLY) = 3
open("/usr/lib/python2.6/encodings/__init__.py", O_RDONLY) = 3
open("/usr/lib/python2.6/encodings/__init__.pyc", O_RDONLY) = 4
open("/usr/lib/python2.6/codecs.py", O_RDONLY) = 4
open("/usr/lib/python2.6/codecs.pyc", O_RDONLY) = 5
open("/usr/lib/python2.6/encodings/aliases.py", O_RDONLY) = 4
open("/usr/lib/python2.6/encodings/aliases.pyc", O_RDONLY) = 5
open("/usr/lib/python2.6/encodings/utf_8.py", O_RDONLY) = 3
open("/usr/lib/python2.6/encodings/utf_8.pyc", O_RDONLY) = 4

All of these could, at least potentially, have an effect on sys.path. It is extremely unlikely that the stuff that comes before site.py would, however (those are all OS libraries and interfaces not specific to Python). I'd suggest you take a hard look at site.py, sitecustomize.py, and the various .pth files (your list of .pth files will be different than mine; that's normal).