The Python path determines how the Python interpreter locates modules. How exactly does Python construct the path?
Using the official docs on sys.path, with its footnote reference to the site module, I’ll recap the process.
If a script is executed, the interpreter sets the first entry of sys.path to that script’s directory. If Python is launched interactively, the first entry is the empty string (“”), meaning Python will scan the present working directory first. The next entries of sys.path are the contents of the PYTHONPATH environment variable, if it exists. Then, installation-dependent entries are appended (example below).
When initializing, the interpreter normally imports the site module automatically. The module, on import, executes code to find .pth files in known site-packages directory locations, which themselves contain entries which are either paths to add to sys.path, or import calls. If we really want to trace what’s going on, we can launch a Python interpreter with -S to prevent loading the site module automatically, and instead trace the import.
(Note, I am working within a virtualenv called py27.)
(py27) ~$ python -S
Python 2.7.2+ (default, Oct 4 2011, 20:06:09)
[GCC 4.6.1] on linux2
>>> import sys
>>> for p in sys.path:
... print p
I have no PYTHONPATH, so these are just my installation-dependent paths. Now, we need to add the directory where the pdb module lives, so we can import it:
>>> sys.path += ["/usr/lib/python2.7"]
>>> import pdb
>>> pdb.run("import site")
I’ll spare you the debugging session details, and summarize what I see:
- site.py grabs orig-prefix.txt from <VIRTUAL_ENV>/lib/python2.7, which for me contains “/usr”, and extends the sys.path array to contain additional “/usr”-based paths.
- site.py then scans the site-packages (in lib/python2.7). For each .pth file (in alphabetical order), step through its entries. If an entry begins with “import”, call exec() on the line; otherwise append the (absolute) path to sys.path. Then do the same in the user site-packages directory (in local/lib/python2.7).
Note, easy-install.pth contains executable code, eg:
import sys; sys.__plen = len(sys.path)
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)
The executable lines move all the entries (some of which are .egg zipped packages) up to the top of the path.
- After stepping through all .pth files, add the existing site-packages directories themselves.
- Finally, attempt to call “import sitecustomize.py” (which doesn’t do anything on my install).