Python packages - problems, pitfalls.

[Robert Roy]

On Sun, 4 Nov 2001 12:07:50 +0530, Prabhu Ramachandran
<prabhu at aero.iitm.ernet.in> wrote:

>hi,
>
>Recently I discovered that there is some very unfortunate behaviour
>with python packages.
>
>Lets say I have a directory 'pkg' that is the root of a package.
>Inside this directory I have a sub-package, called sub.  So its
>something like:
>
>pkg/
>    __init__.py
>    a.py
>    sub/
>        __init__.py
>        b.py
>
>Now, from b I'd expect to be able to import 'a' straight away.
>

Why would you expect that? a.py has no relationship to b.py. It is not
a sibling module, it is not a parent. 

>Say in b.py I do
>
>import a
>
>This will not work!  I know why it happens but shouldn't Python be
>smart enough to avoid such problems?  Why I consider this a major
>problem is that the importing of a sub-package depends on where its
>parent package is!! 


Its a good thing it doesn't. It would be a maintenance nightmare. 


See Tim Peter's rules of python programming: Explicit is better than
implicit

Frankly if I had a situation as you describe above, I would take a
good look at why I had structured my modules that way. Most likely I
would find that the module structure had outlived its usefulness and
needed to be redefined/refactored.


> So if pkg is no longer the root of the package
>and say its put inside another BigPkg then to make pkg's sub packages
>work you'd have to edit *all* the sub packages and change every
>reference to pkg.a to BigPkg.pkg.a.  This is a huge pain.

Use a ".pth" file to tell it where the package resides. Say you
re-nest pkg into BigPkg, place a file called pkg.pth in the root
directory of your distribution and put the location of pkg in it. See
the documentation on module "site". 

for example (not tested!):
Python root

pkg.pth  (contents would be Lib/BigPkg/pkg)

Lib/
	BigPkg/
		__init__.py
		pkg/
			__init__.py

This will allow old pkg references to continue working. 


>
>Are there better ways to get around this packaging problem?  Is this a
>know problem that folks are working on??  Why is it that Python does
>not deal with this issue more sensibly?
>

Python does, it is not terribly well documented though...

bob