[Python-Dev] PEP 382: Namespace Packages
mal at egenix.com
Thu Apr 2 22:33:25 CEST 2009
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.
> 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
That's going to slow down Python package detection a lot - you'd
replace an O(1) test with an O(n) scan.
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.
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
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
(Please let me know if the above is not clear, I will then try to
follow up on it.)
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
* Namespace packages are easy to recognize by testing for a single
* 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
* 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.
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