Python是如何加载sys.path中的所有值的？

Question

我刚开始学Python，现在在玩自己的Python脚本和项目，比如Django。

我用的是Ubuntu系统，并且我把我的PYTHONPATH设置成了

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

当我进入Python解释器时

import sys
print sys.path

我可以看到

['', '/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']

Python是怎么加载这些不在PYTHONPATH里的路径的呢？有没有什么索引？而且，我的Django安装在/usr/lib/pymodules里，虽然它不在sys.path列表中，但它还是能运行。

非常感谢。

Answer 1

请阅读这个链接：http://docs.python.org/library/site.html#module-site

Answer 2

这是我电脑上Python解释器在开始运行我提供的代码之前打开的所有文件的列表（虽然我的设置和你的不完全一样，但大致相似）：

$ 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

这些文件都有可能影响到系统路径（sys.path）。不过，在site.py之前的那些文件不太可能会有影响（因为它们都是操作系统的库和接口，跟Python没有直接关系）。我建议你仔细看看site.py、sitecustomize.py和各种.pth文件（你的.pth文件列表会和我的不同，这很正常）。