When a module named :mod:`spam` is imported, the interpreter searches for a

file named :file:`spam.py` in the directory containing the input script and

then in the list of directories specified by the environment variable

:envvar:`PYTHONPATH`. This has the same syntax as the shell variable

:envvar:`PATH`, that is, a list of directory names. When :envvar:`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

:file:`.:/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), :envvar:`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 :ref:`tut-standardmodules` for more information.