On Sat, Apr 14, 2012 at 17:12, Eric Snow <ericsnowcurren...@gmail.com>wrote:
> On Sat, Apr 14, 2012 at 2:03 PM, Brett Cannon <br...@python.org> wrote: > > To start off, what I am about to propose was brought up at the PyCon > > language summit and the whole room agreed with what I want to do here, > so I > > honestly don't expect much of an argument (famous last words). > > > > In the "ancient" import.c days, a lot of import's stuff was hidden deep > in > > the C code and in no way exposed to the user. But with importlib > finishing > > PEP 302's phase 2 plans of getting imoprt to be properly refactored to > use > > importers, path hooks, etc., this need no longer be the case. > > > > So what I propose to do is stop having import have any kind of implicit > > machinery. This means sys.meta_path gets a path finder that does the > heavy > > lifting for import and sys.path_hooks gets a hook which provides a > default > > finder. As of right now those two pieces of machinery are entirely > implicit > > in importlib and can't be modified, stopped, etc. > > > > If this happens, what changes? First, more of importlib will get publicly > > exposed (e.g. the meta path finder would become public instead of private > > like it is along with everything else that is publicly exposed). Second, > > import itself technically becomes much simpler since it really then is > about > > resolving module names, traversing sys.meta_path, and then handling > fromlist > > w/ everything else coming from how the path finder and path hook work. > > > > What also changes is that sys.meta_path and sys.path_hooks cannot be > blindly > > reset w/o blowing out import. I doubt anyone is even touching those > > attributes in the common case, and the few that do can easily just stop > > wiping out those two lists. If people really care we can do a warning in > 3.3 > > if they are found to be empty and then fall back to old semantics, but I > > honestly don't see this being an issue as backwards-compatibility would > just > > require being more careful of what you delete (which I have been warning > > people to do for years now) which is a minor code change which falls in > line > > with what goes along with any new Python version. > > > > And lastly, sticking None in sys.path_importer_cache would no longer mean > > "do the implicit thing" and instead would mean the same as NullImporter > does > > now (which also means import can put None into sys.path_importer_cache > > instead of NullImporter): no finder is available for an entry on sys.path > > when None is found. Once again, I don't see anyone explicitly sticking > None > > into sys.path_importer_cache, and if they are they can easily stick what > > will be the newly exposed finder in there instead. The more common case > > would be people wiping out all entries of NullImporter so as to have a > new > > sys.path_hook entry take effect. That code would instead need to clear > out > > None on top of NullImporter as well (in Python 3.2 and earlier this would > > just be a performance loss, not a semantic change). So this too could > change > > in Python 3.3 as long as people update their code like they do with any > > other new version of Python. > > > > In summary, I want no more magic "behind the curtain" for Python 3.3 and > > import: sys.meta_path and sys.path_hooks contain what they should and if > > they are emptied then imports will fail and None in > sys.path_importer_cache > > means "no finder" instead of "use magical, implicit stuff". > > This is great, Brett. About sys.meta_path and sys.path_hooks, I see > only one potential backwards-compatibility problem. > > Those implicit hooks were fallbacks, effectively always at the end of > the list, no matter how you manipulated the them. Code that appended > onto those lists would now have to insert the importers/finders in the > right way. Otherwise the default hooks would be tried first, which > has a good chance of being the wrong thing. > > That concern aside, I'm a big +1 on your proposal. Once again, it's just code that needs updating to run on Python 3.3 so I don't view it as a concern. Going from list.append() to list.insert() (even if its ``list.insert(hook, len(list)-2)``) is not exactly difficult.
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