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 Professional Python Services directly from the Source (#1, Apr 02 2009)
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