[PATCH 6 of 6 import-refactor V3] mercurial: support loading modules from zipimporter

Brett Cannon brett at python.org
Fri Dec 4 13:45:50 CST 2015


On Fri, 4 Dec 2015 at 10:50 Gregory Szorc <gregory.szorc at gmail.com> wrote:

> On Fri, Dec 4, 2015 at 10:27 AM, Brett Cannon <brett at python.org> wrote:
>
>>
>>
>> On Fri, 4 Dec 2015 at 07:13 Augie Fackler <raf at durin42.com> wrote:
>>
>>> (+brett, who might have some idea about import behavior and is
>>> probably going to be interested in this change anyway.)
>>>
>>> On Thu, Dec 03, 2015 at 09:51:12PM -0800, Gregory Szorc wrote:
>>> > # HG changeset patch
>>> > # User Gregory Szorc <gregory.szorc at gmail.com>
>>> > # Date 1449206705 28800
>>> > #      Thu Dec 03 21:25:05 2015 -0800
>>> > # Node ID 782783ea499c7e03253b55f0b0c221b9946356e2
>>> > # Parent  dd8b23489d2a5f5580905c995333d139fe457d1c
>>> > mercurial: support loading modules from zipimporter
>>> >
>>> > The previous refactor to module importing broke module loading when
>>> > mercurial.* modules were loaded from a zipfile (using a zipimporter).
>>> > This scenario is likely encountered when using py2exe.
>>> >
>>> > Supporting zipimporter and the traditional importer side-by-side
>>> > turns out to be quite a pain. In Python 2.x, the standard, file-based
>>> > import mechanism is partially implemented in C. The sys.meta_path
>>> > and sys.path_hooks hook points exist to allow custom importers in
>>> > Python/userland. zipimport.zipimporter and our "hgimporter" class
>>> > from earlier in this patch series are 2 of these.
>>> >
>>> > In a standard Python installation (no matter if running in py2exe
>>> > or similar or not), zipimport.zipimporter appears to be registered
>>> > in sys.path_hooks. This means that as each sys.path entry is
>>> > consulted, it will ask zipimporter if it supports that path and
>>> > zipimporter will be used if that entry is a zip file. In a
>>> > py2exe environment, sys.path contains an entry with the path to
>>> > the zip file containing the Python standard library along with
>>> > Mercurial's Python files.
>>> >
>>> > The way the importer mechanism works is the first importer that
>>> > declares knowledge of a module (via find_module() returning an
>>> > object) gets to load it. Since our "hgimporter" is registered
>>> > in sys.meta_path and returns an interest in specific mercurial.*
>>> > modules, the zipimporter registered on sys.path_hooks never comes
>>> > into play for these modules. So, we need to be zipimporter aware
>>> > and call into zipimporter to load modules.
>>> >
>>> > This patch teaches "hgimporter" how to call out into zipimporter
>>> > when necessary. We detect the necessity of zipimporter by looking
>>> > at the loader for the "mercurial" module. If it is a zipimporter
>>> > instance, we load via zipimporter.
>>> >
>>> > The behavior of zipimporter is a bit wonky.
>>>
>>> Patch is already queued, but Brett, do you have any insight into why
>>> zipimport works this way? (see below)
>>>
>>> >
>>> > You appear to need separate zipimporter instances for each directory
>>> > in the zip file. I'm not sure why this is.
>>
>>
>> sys.path_importer_cache is the most likely culprit for structuring it
>> this way, but otherwise it's just the way someone chose to implement it
>> (the other approach would have been to cache a single importer per zipfile
>> in the sys.path_hooks object and then have the finders and loaders be
>> basically a single instance per zipfile).
>>
>> FYI we are going to rewrite zipimporter in 3.6 or 3.7.
>>
>> I suspect it has
>>> > something to do with the low-level importing mechanism (implemented
>>> > in C) operating on a per-directory basis. PEP-302 makes some
>>> > references to this. I was not able to get a zipimporter to
>>> > import modules outside of its immediate directory no matter how
>>> > I specified the module name. This is why we use separate
>>> > zipimporter instances for the ".zip/mercurial" and
>>> > ".zip/mercurial/pure" locations.
>>> >
>>> > The zipimporter documentation for Python 2.7 explicitly states that
>>> > zipimporter does not import dynamic modules (C extensions). Yet from
>>> > a py2exe distribution on Windows - where the .pyd files are *not*
>>> > in the zip archive - zipimporter imported these dynamic modules
>>> > just fine!
>>
>>
>> That doesn't work on UNIX which is why the docs say extension modules are
>> not supported.
>>
>>
>>> I'm not sure if dynamic modules can't be imported from
>>> > *inside* the zip archive or whether zipimporter looks for dynamic
>>> > modules outside the zip archive. All I know is zipimporter does
>>> > manage to import the .pyd files on Windows and this patch makes
>>> > our new importer compatible with py2exe.
>>> >
>>> > In the ideal world, We'd probably reimplement or fall back to parts
>>> > of the built-in import mechanism instead of handling zipimporter
>>> > specially. After all, if we're loading Mercurial modules via
>>> > something that isn't the built-in file-based importer or zipimporter,
>>> > our custom importer will likely fail because it doesn't know how to
>>> > call into it. I'd like to think that we'll never encounter this
>>> > in the wild, but you never know. If we do encounter it, we can
>>> > come up with another solution.
>>>
>>
>> Sticking yourselves on sys.meta_path does complicate things since
>> sys.meta_path is explicitly for handling alternative storage solutions for
>> modules, i.e., built-in modules, frozen modules, and file-based storage.
>> What it sounds like you're trying to do is wrap the normal file-based
>> import stuff which is tough in Python 2 since that's all implicitly done.
>>
>>
>>> >
>>> > It's worth nothing that Python 3 has moved a lot of the importing
>>> > code from C to Python. Python 3 gives you near total control over
>>> > the import mechanism. So in the very distant future when Mercurial
>>> > drops Python 2 support, it's likely that our custom importer code
>>> > can be refactored to something a bit saner.
>>>
>>
>> Not sure what your custom importer does, but if it exists for lazy
>> loading then I already implemented it for you in Python 3.5:
>> https://docs.python.org/3/library/importlib.html#importlib.util.LazyLoader
>>
>
> Mercurial has C and Python implementations for a handful of modules. We
> desire to run both forms from the same installation so e.g. CPython and
> PyPy can share the same installation.
>
> The custom importer (
> http://hg.netv6.net/clowncopter/file/30a20167ae29/mercurial/__init__.py)
> basically looks in multiple locations for these modules. Depending on the
> environment, it will try to load modules from mercurial/* and
> mercurial/pure/*.
>
> We could probably move files around and use a custom function to obtain a
> reference to the module (instead of "import" throughout all the code that
> imports these modules). But this would have required rewriting a bunch of
> code. I felt it was easier to employ importer hackery than to rewrite the
> world.
>
> I'm receptive to better ideas.
>

If they are drop-in replacements for each other why can't you just give
them the same name and then simply let the priority of extension modules
over source modules handle it?

The other option is to make the extension modules act as accelerators and
follow the common idiom of always importing the source module and at the
bottom of it doing:

  try:
    from _mod import *
  except ImportError:
    pass

where _mod is the extension module. This is how we make the stdlib not
require extension modules when they are used purely for speed purposes.
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