What sets up sys.path with Python, and when?
When I run
import sys
print sys.path
on my Mac (Mac OS X 10.6.5, Python 2.6.1), I get the following results.
/Library/Python/2.6/site-packages/ply-3.3-py2.6.egg ... /Library/Python/2.6/site-packages/ipython-0.10.1-py2.6.egg /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python26.zip /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6 /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-darwin /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac/lib-scriptpackages /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-tk /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-old /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload /Library/Python/2.6/site-packages /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/PyObjC /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/wx-2.8-mac-unicode
They are grouped into 5 categories.
And I can add more paths using the code
sys.path.insert(0, MORE_PATH)
ADDED
Based on Michael's answer, I looked into site.py, and I got the following code.
def addsitepackages(known_paths):
"""Add site-packages (and possibly site-python) to sys.path"""
sitedirs = []
seen = []
for prefix in PREFIXES:
if not prefix or prefix in seen:
continue
seen.append(prefix)
if sys.platform in ('os2emx', 'riscos'):
sitedirs.append(os.path.join(prefix, "Lib", "site-packages"))
elif sys.platform == 'darwin' and prefix == sys.prefix:
sitedirs.append(os.path.join("/Library/Python", sys.version[:3], "site-packages"))
I also think that the directory name that has site.py (/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6 for my Mac) should be built into Python source code.
Most of the stuff is set up in Python's site.py which is automatically imported when starting the interpreter (unless you start it with the -S option). Few paths are set up in the interpreter itself during initialization (you can find out which by starting python with -S ).
Additionally, some frameworks (like Django I think) modify sys.path upon startup to meet their requirements.
The site module has a pretty good documentation, a commented source code and prints out some information if you run it via python -m site .
From Learning Python:
sys.path is the module search path. Python configures it at program startup, automatically merging the home directory of the top-level file (or an empty string to designate the current working directory), any PYTHONPATH directories, the contents of any .pth file paths you've created, and the standard library directories. The result is a list of directory name strings that Python searches on each import of a new file.
site.py is indeed the answers. I wanted to remove any dependencies on the old Python that is installed by default on my mac. This works pretty good, as 'site.py' is called each time the python interpreter is started.
For Mac, I manually added the following line at the end of main() in /System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/site.py:
sys.path = filter (lambda a: not a.startswith('/System'), sys.path)
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