setting pythonpath variable in environment variables

[Gribouillis]

And what is the difference between

There are two hierarchies that don't coincide: the filesystem with its tree of folders and the python packages and subpackages system. For example xml.etree.ElementTree is a submodule of the xml.etree package which is a subpackage of xml.

The ..one means the subpackage 'one' of the current module's grandparent package. But you can't use it if you simply call python two/sample.py because when you do so the current module is __main__ which has no parent package nor grandparent package. If works if parent is a module with a subpackage two and a submodule sample, so it works if you call python -m parent.two.sample.

The filesystem tree helps to build python modules and packages, but there is no direct correspondance between them.