On one server I'm able to import a library, on a second one I'm not. The library I need to import is shared via NFS and reachable by both servers.
I found out that the Python path is different between the two servers, but the Python binary is the same, installed from standard ubuntu 16.04 repos, and $PYTHONPATH is unset on both servers.
Server 1:
$ echo $PYTHONPATH
$ python
>>> import sys; sys.path
['', '/usr/lib/python2.7', '...', '/usr/local/lib/python2.7/dist-packages', '/home/user/app/src/python', '/usr/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages/wx-3.0-gtk2']
Server 2:
$ echo $PYTHONPATH
$ python
>>> import sys; sys.path
['', '/usr/lib/python2.7', '...', '/usr/local/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages']
How do I understand where is this library defined, in order to replicate the same behaviour to server 2?
The curious fact is that the library is placed between the paths /usr/local/lib and /usr/lib.
Edit: The python binary is the same (same version 2.7.12, and the checksums match) and it's installed from the ubuntu repos in the standard location /usr/bin/python.
I've played with the library site and all variables are the same (site.ENABLE_USER_SITE returns True).
I know I can manipulate the paths inside the script, or specify PYTHONPATH in /etc/profile.d/ for all servers, but I'd like to know where is the difference.
I've discovered the existence of path configuration files in Python. From the doc of the site module on Python 3:
It starts by constructing up to four directories from a head and a tail part. For the head part, it uses sys.prefix and sys.exec_prefix; empty heads are skipped. For the tail part, it uses the empty string and then lib/site-packages (on Windows) or lib/pythonX.Y/site-packages (on Unix and Macintosh). For each of the distinct head-tail combinations, it sees if it refers to an existing directory, and if so, adds it to sys.path and also inspects the newly added path for configuration files.
...
A path configuration file is a file whose name has the form name.pth and exists in one of the four directories mentioned above; its contents are additional items (one per line) to be added to sys.path. Non-existing items are never added to sys.path, and no check is made that the item refers to a directory rather than a file. No item is added to sys.path more than once. Blank lines and lines beginning with # are skipped. Lines starting with import (followed by space or tab) are executed.
In my case there was a file in /usr/local/lib/python2.7/dist-packages/easy-install.pth which was missing on the second server.
Disabling the automatic import of those files with python -S may turn out to be useful while debugging.
Unfortunately I haven't found a way to retrieve a list of directories where the path configuration files are read from.