[Python-Dev] Draft PEP: "Simplified Package Layout and Partitioning"

[Glenn Linderman]

On 7/19/2011 8:58 PM, P.J. Eby wrote:
> Standard Library Changes/Additions
> ----------------------------------
>
> The ``pkgutil`` module should be updated to handle this
> specification appropriately, including any necessary changes to
> ``extend_path()``, ``iter_modules()``, etc.
>
> Specifically the proposed changes and additions to ``pkgutil`` are:
>
> * A new ``extend_virtual_paths(path_entry)`` function, to extend
>   existing, already-imported virtual packages' ``__path__`` attributes
>   to include any portions found in a new ``sys.path`` entry.  This
>   function should be called by applications extending ``sys.path``
>   at runtime, e.g. when adding a plugin directory or an egg to the
>   path.
>
>   The implementation of this function does a simple top-down traversal
>   of ``sys.virtual_packages``, and performs any necessary
>   ``get_subpath()`` calls to identify what path entries need to
>   be added to each package's ``__path__``, given that `path_entry`
>   has been added to ``sys.path``.  (Or, in the case of sub-packages,
>   adding a derived subpath entry, based on their parent namespace's
>   ``__path__``.) 

When I read about creating __path__ from sys.path, I immediately thought 
of the issue of programs that extend sys.path, and the above is the 
"workaround" for such programs.  but it requires such programs to do 
work, and there are a lot of such programs (I, a relative newbie, have 
had to write some).  As it turns out, I can't think of a situation where 
I have extended sys.path that would result in a problem for fancy 
namespace packages, because so far I've only written modules, not 
packages, and only modules are on the paths that I add to sys.path.  But 
that does not make for a general solution.

Is there some way to create a new __path__ that would reflect the fact 
that it has been dynamically created, rather than set from __init__.py, 
and then when it is referenced, calculate (and cache?) a new value of 
__path__ to actually search?
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