[Python-Dev] PEP 382: Namespace Packages

David Cournapeau cournape at gmail.com
Tue Apr 7 17:29:02 CEST 2009


On Tue, Apr 7, 2009 at 11:58 PM, M.-A. Lemburg <mal at egenix.com> wrote:

>>
>> This means your proposal actually doesn't add any benefit over the
>> status quo, where you can have an __init__.py that does nothing but
>> declare the package a namespace.  We already have that now, and it
>> doesn't need a new filename.  Why would we expect OS vendors to start
>> supporting it, just because we name it __pkg__.py instead of __init__.py?
>
> I lost you there.
>
> Since when do we support namespace packages in core Python without
> the need to add some form of magic support code to __init__.py ?

I think P. Eby refers to the problem that most packaging systems don't
like several packages to have the same file - be it empty or not.
That's my main personal grip against namespace packages, and from this
POV, I think it is fair to say the proposal does not solve anything.
Not that I have a solution, of course :)

cheers,

David
>
> My suggestion basically builds on the same idea as Martin's PEP,
> but uses a single __pkg__.py file as opposed to some non-Python
> file yaddayadda.pkg.
>
> Here's a copy of the proposal, with some additional discussion
> bullets added:
>
> """
> 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.
>
> * The search does not depend on the order of directories in sys.path.
> There's no requirement for the defining package to appear first
> on sys.path.
>
> * Namespace packages are easy to recognize by testing for a single
> resource.
>
> * There's no conflict with existing files using the .pkg extension
> such as Mac OS X installer files or Solaris packages.
>
> * 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.
>
> * There's no need for a package directory scan, allowing the
> mechanism to also work with resources that do not permit to
> (easily and efficiently) scan the contents of a package "directory",
> e.g. frozen packages or imports from web resources.
>
> 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
>
> Professional Python Services directly from the Source  (#1, Apr 07 2009)
>>>> Python/Zope Consulting and Support ...        http://www.egenix.com/
>>>> mxODBC.Zope.Database.Adapter ...             http://zope.egenix.com/
>>>> mxODBC, mxDateTime, mxTextTools ...        http://python.egenix.com/
> ________________________________________________________________________
> 2009-03-19: Released mxODBC.Connect 1.0.1      http://python.egenix.com/
>
> ::: Try our new mxODBC.Connect Python Database Interface for free ! ::::
>
>
>   eGenix.com Software, Skills and Services GmbH  Pastor-Loeh-Str.48
>    D-40764 Langenfeld, Germany. CEO Dipl.-Math. Marc-Andre Lemburg
>           Registered at Amtsgericht Duesseldorf: HRB 46611
>               http://www.egenix.com/company/contact/
> _______________________________________________
> Python-Dev mailing list
> Python-Dev at python.org
> http://mail.python.org/mailman/listinfo/python-dev
> Unsubscribe: http://mail.python.org/mailman/options/python-dev/cournape%40gmail.com
>



More information about the Python-list mailing list