Question

I thought that the sys.path was a complete list of all search paths for Python modules.

However, on my Ubuntu machine, ‘/usr/local/lib/python2.6/dist-packages/’ is where almost all my modules are and that path is not in sys.path. And I can still import any module on that path.

EDIT, NOT TRUE: Even if I set the sys.path to the empty list, I can still import from that path.

Where does this implicit knowledge of the dist-packages path come from? And are there any other paths in this implicit group of search paths, or whatever it is?

EDIT: It seems like the second part of my post is not true. Indeed, “sys.path = []”, will mean that I can not import anything, not even from my current working directory. My apologies.

Answer 1

Note the mention of an installation-dependent default in the following:

6.1.2. The Module Search Path

When a module named spam is imported, the interpreter searches for a
file named spam.py in the directory containing the input script and
then in the list of directories specified by the environment variable
PYTHONPATH. This has the same syntax as the shell variable PATH, that
is, a list of directory names. When PYTHONPATH is not set, or when the
file is not found there, the search continues in an
installation-dependent default path; on Unix, this is usually
.:/usr/local/lib/python.

Actually, modules are searched in the list of directories given by the
variable sys.path which is initialized from the directory containing
the input script (or the current directory), PYTHONPATH and the
installation-dependent default. This allows Python programs that know
what they’re doing to modify or replace the module search path. Note
that because the directory containing the script being run is on the
search path, it is important that the script not have the same name as
a standard module, or Python will attempt to load the script as a
module when that module is imported. This will generally be an error.
See section Standard Modules for more information.

edit On my Ubuntu box, /usr/local/lib/python2.6/dist-packages is present in sys.path. If I clear sys.path and then try to import a module from the above directory, that no longer works. This suggests that the interpreter has no implicit knowledge of that directory and finds it via sys.path.

edit When you conduct your experiments, make sure that you modify sys.path right at the start of your Python session. If you import X, then clear sys.path, and then import X again, the latter will not fail even though X is no longer on sys.path.