[Python-Dev] PEP 382: Namespace Packages

[M.-A. Lemburg]

On 2009-04-02 17:32, Martin v. Löwis wrote:
> I propose the following PEP for inclusion to Python 3.1.

Thanks for picking this up.

I'd like to extend the proposal to Python 2.7 and later.

> Please comment.
> 
> Regards,
> Martin
> 
> Specification
> =============
> 
> Rather than using an imperative mechanism for importing packages, a
> declarative approach is proposed here, as an extension to the existing
> ``*.pkg`` mechanism.
> 
> The import statement is extended so that it directly considers ``*.pkg``
> files during import; a directory is considered a package if it either
> contains a file named __init__.py, or a file whose name ends with
> ".pkg".

That's going to slow down Python package detection a lot - you'd
replace an O(1) test with an O(n) scan.

Alternative Approach:
---------------------

Wouldn't it be better to stick with a simpler approach and look for
"__pkg__.py" files to detect namespace packages using that O(1) check ?

This would also avoid any issues you'd otherwise run into if you want
to maintain this scheme in an importer that doesn't have access to a list
of files in a package directory, but is well capable for the checking
the existence of a file.

Mechanism:
----------

If the import mechanism finds a matching namespace package (a directory
with a __pkg__.py file), it then goes into namespace package scan mode and
scans the complete sys.path for more occurrences of the same namespace
package.

The import loads all __pkg__.py files of matching namespace packages
having the same package name during the search.

One of the namespace packages, the defining namespace package, will have
to include a __init__.py file.

After having scanned all matching namespace packages and loading
the __pkg__.py files in the order of the search, the import mechanism
then sets the packages .__path__ attribute to include all namespace
package directories found on sys.path and finally executes the
__init__.py file.

(Please let me know if the above is not clear, I will then try to
follow up on it.)

Discussion:
-----------

The above mechanism allows the same kind of flexibility we already
have with the existing normal __init__.py mechanism.

* It doesn't add yet another .pth-style sys.path extension (which are
difficult to manage in installations).

* It always uses the same naive sys.path search strategy. The strategy
is not determined by some file contents.

* The search is only done once - on the first import of the package.

* It's possible to have a defining package dir and add-one package
dirs.

* Namespace packages are easy to recognize by testing for a single
resource.

* Namespace __pkg__.py modules can provide extra meta-information,
logging, etc. to simplify debugging namespace package setups.

* It's possible to freeze such setups, to put them into ZIP files,
or only have parts of it in a ZIP file and the other parts in the
file-system.

Caveats:

* Changes to sys.path will not result in an automatic rescan for
additional namespace packages, if the package was already loaded.
However, we could have a function to make such a rescan explicit.

-- 
Marc-Andre Lemburg
eGenix.com

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