At 02:45 PM 4/24/2010 +0900, David Cournapeau wrote:
Is there a design document or something else decribing how the namespace mechanism works for setuptools ? I would like to support namespace package in my own packaging project, but it is not clear to me what needs to be done on my side of things.
Namespace packages in *Python* are the result of adding entries to a package's '__path__' attribute. By default, the list only has one entry - if you make it have one for each directory (or zipfile subdirectory) that contains modules for that package, then presto, you have a namespace package.
How you get those entries in __path__ is the only real difference between pkgutil, pkg_resources, and the upcoming PEP 382. (Well, there's also the nspkg.pth hack, which I'd just as soon do away with altogether.)
I'm thinking that in 0.7 I may add a PEP 382 emulation module (separate from pkg_resources) for older versions of Python, as it would allow some odd bits to be dispensed with in pkg_resources.
For example, declare_namespace could simply add a '*' to the __path__ of the right package(s), and the nspkg.pth files could just make sure the PEP 382 emulation was loaded, without needing to set up fake package modules. Other pkg_resources APIs that rely on an internal list of namespace packages could just look in sys.namespace_packages, and so on.
One thing that's not entirely clear at the moment in PEP 382, though, is that it doesn't really specify how the other sys.path entries are found and mapped -- presumably it's something like os.path.join(syspathentry, *pkgname.split('.')) -- but the handling for such path entries isn't really described. In a Python version where NullImporter exists (2.5+), though, the cost of extra nonexistent __path__ entries is negligible, so I suppose filtering the entries for directory existence and the presence of other .pth files isn't necessary.
The PEP should probably be clearer on this point, though - i.e., both on what exactly goes in __path__, as well as whether .pth files are read in the other paths, and whether .pth files are needed on any other paths. (I also wonder if other .pth contents besides '*' really need to be supported anyway; I expect it would depend on whether anybody is using the .pkg files allowed by extend_path()...)
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