[Python-Dev] making the import machinery explicit

Brett Cannon brett at python.org
Tue Apr 17 17:58:26 CEST 2012

The only people to bring up worries about this thread were Eric and Nick
and they both seem fine with making stuff explicit and changing the meaning
of None in sys.path_importer_cache, so I have created
http://bugs.python.org/issue14605 and will plan on implementing the ideas
for it before Python 3.3 goes out.

On Sat, Apr 14, 2012 at 16:03, Brett Cannon <brett at 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".
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