Python in Ubuntu/Debian

http://www.debian.org/doc/packaging-manuals/python-policy/ch-python.html

site module

http://docs.python.org/library/site.html

I assume sys.prefix and sys.exec_prefix are /usr. It may be different on your machine.

/usr/lib/pythonX.Y/site-packages

/usr/lib/site-python

"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."

Note: sub-directories are not added.

If .pth files exist in those directories, its contents are additional items (one per line) to be added to sys.path.

local admin

A special directory is dedicated to public Python modules installed by the local administrator, /usr/local/lib/pythonX.Y/dist-packages for python2.6 and later, and /usr/local/lib/pythonX.Y/site-packages for python2.5 and earlier. For a local installation by the administrator of python2.6 and later, a special directory is reserved to Python modules which should only be available to this Python, /usr/local/lib/pythonX.Y/site-packages. Unfortunately, for python2.5 and earlier this directory is also visible to the system Python. Additional information on appending site-specific paths to the module search path is available in the official documentation of the site module.

Central repository

It seems all (not all?) modules are installed into directory /usr/share/pyshared. It's a central module repository for python. Python modules in other system directories are symbolic references to files in this directory.

/usr/lib/pyshared/python2.6/: .so python extensions?

python-central

python-central is a tool for Python module management.

pycentral:  register and build utility for Python packages. It manages python modules you installed.

pyversions  prints  information  about installed, supported python runtimes,

py_compilefiles: compiles Python .py source files into .pyc or .pyo bytecode format.

It adds hooks for runtime change:

/usr/share/python/runtime.d/pycentral.rtinstall
/usr/share/python/runtime.d/pycentral.rtremove
/usr/share/python/runtime.d/pycentral.rtupdate

python-support

python-central is another tool for Python module management.

modules managed by python-support are installed in another directory which is added to the sys.path using the .pth mechanism. The .pth mechanism is documented in the Python documentation of the site module. 

During installation, it adds a file /usr/lib/python2.6/dist-packages/python-support.pth which contains /usr/lib/pymodules/pythonX.Y/. This directory also contains the byte-compiled modules for version pythonX.Y.

update-python-modules can be used to rebuild those modules.

It also adds hooks for runtime change:

    /usr/share/python/runtime.d/python-support.rtinstall
    /usr/share/python/runtime.d/python-support.rtremove
    /usr/share/python/runtime.d/python-support.rtupdate