On 7 September 2016 at 02:41, Tomas Orsava <tors...@redhat.com> wrote: > Hi! > > I'm currently writing a PEP titled "Distributing a Subset of the Standard > Library" to standardize and hopefully improve the behavior of Python without > the its full standard library. This is relevant to Fedora, as we exclude > several standard library modules into separate optional packages (e.g. > python3-tkinter). > > I have a draft of the first two sections: Motivation and Specification. > https://fedora-python.github.io/pep-drafts/pep-A.html
Very interesting, although I see a pragmatic problem with trying to check for explicitly missing packages only after checking for the standard library ones: the default import system doesn't make a clear distinction as to which sys.path entries refer to the standard library and which refer to other directories (like site-packages), so you can't readily intercept processing after the standard library is checked but before the rest of sys.path is processed. However, sys.meta_path *does* let you explicitly block imports before the default machinery is tried by raising ImportError from find_spec: https://docs.python.org/3/reference/import.html#the-meta-path Now, I'm making the assumption that what we need is a model whereby the base install includes files that tells Python "these stdlib pieces might be missing", and then the other packages can install files that mean those "these pieces are missing" markers don't get processed. One possible way to do that as a pre-import check injected into the start of sys.meta_path would be to maintain a set of static "module_name.optional" files in the standard library directory that included: - a relative file path to stat to indicate that the optional module is installed - an import error message to raise if its not found If the new hook either didn't see an optional module entry, or if it checked and the file path was present, it would allow the import to continue as normal. However, if it found the optional declaration file and the file path missing, it would raise ImportError with the given message. CPython by default would provide optional declarations for the modules that are optional in the upstream build process, then downstreams like Fedora may add more for things like tkinter, idlelib, and the test suite. The downside of this approach is that it does mean the initial import of optional modules would be a touch slower (since the file path to stat and the error message would need to be read from file), whereas the version in the draft PEP has the virtue of having no impact on import time for modules that are available, even when starting from a completely cold sys.modules cache. So if we did want to enable the draft proposal in the PEP, we'd need to look at proposing a special directory to hold the "missing" markers that was always placed just after the standard library directory on sys.path, and then defining a custom path_hook to process it: https://docs.python.org/3/reference/import.html#path-entry-finders Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia _______________________________________________ python-devel mailing list python-devel@lists.fedoraproject.org https://lists.fedoraproject.org/admin/lists/python-devel@lists.fedoraproject.org
