Relative Imports, why the hell is it so hard?

[Maxim Khitrov]

On Tue, Mar 24, 2009 at 5:05 AM, CinnamonDonkey
<CinnamonDonkey at googlemail.com> wrote:
> Thanx Max - your explanation sorted it :-), and a big thank you to
> everyone else also!
>
> >From the various posts, Python considers any directory containing the
> __init__.py file to be a package. The top level package is the highest
> directory (closest to root) with a __init__.py file.
>
> Inter-package communication is not allowed unless the packages
> themselves are contained by a parent package.
>
> How does this relate to the site-packages folder? Is it a top level
> package for all installed packages?
>
> Let's say I have installed the "Trac" system which uses "Genshi", they
> are both packages. They are also both installed at the same level and
> I know "Trac" uses "Genshi" to work. \Python25\Lib\site-packages does
> not contain a __init__.py file so it is not a package (i.e. not a
> parent package to "Trac" and "Genshi") :0.

Trac does not use relative imports to access genshi. When relative
imports are not used, python goes through sys.path list to find
modules (with a small exception made when absolute_imports are not
enabled, but that should be default in 2.7). The site-packages
directory is added to sys.path, so when trac executes something like
"from genshi import some_module", python will look in site-packages,
among other directories, for a directory called "genshi" that contains
an __init__.py file.

When you execute a script, the directory of that script is
automatically added to sys.path, so with your example you could have
used absolute imports between subpack1 and subpack2, with the \App
directory performing the same function as site-packages (Gabriel's
suggestion). This is for your original version of the code when
main.py was under App.

Once you moved main.py outside of \App, running "import sybpack2"
would no longer work. You can, however, append directories to
sys.path, so by doing the following in main.py you could again allow
non-relative imports between subpack1 and subpack2:

import os
import sys

sys.path.append(os.path.realpath('App'))

- Max