[Python-Dev] Peculiar import code in pickle.py (original) (raw)

Brett Cannon brett at python.org
Tue Jul 13 22:52:29 CEST 2010

On Tue, Jul 13, 2010 at 11:34, Alexander Belopolsky < alexander.belopolsky at gmail.com> wrote:

On Tue, Jul 13, 2010 at 1:57 PM, Benjamin Peterson <benjamin at python.org> wrote: .. > No! That's not recommended and a complete hack. The "dance" or > importlib.importmodule is preferred.

Nevertheless, "a complete hack" is what PyImportImport does: PyObject * PyImportImport(PyObject *modulename) { static PyObject *sillylist = NULL; .. /* Call the import function with the proper argument list * Always use absolute import here. */ r = PyObjectCallFunction(import, "OOOOi", modulename, globals, globals, sillylist, 0, NULL); .. } and pickle.c uses PyImportImport() and thus is different form pickle.py which uses the double-lookup dance. As a result, the two implementations are subtly different. They cannot be both right. It should be easy to "fix" pickle.c to do the sys.modules lookup, but I am not sure this is right.

Pulling from sys.modules is the correct way to do this. There are subtle issues when using a bunk fromlist argument (empty modules, double initialization, etc.). If one does not use importlib.import_module -- written specifically to prevent people from doing the nasty hack with the fromlist -- then you should use the sys.modules approach, period. If import.c is not doing this then it should get fixed. You can assign me the issue if you want.

I say this every time I give an import talk and it has been brought up here before but obviously not everyone catches it (which is understandable as I think when it came up on python-dev it was at the tail end of a discussion), so I am just going to repeat myself:

Do not put junk in fromlist if you call import directly! Use importlib.import_module! Or if you have a really good reason to not use it, then use __import__(name); module = sys.modules[name].

I have stopped fixing bugs related to this in import.c because of the annoying issues it causes and I expect the correct approach to gain traction at some point (plus get importlib bootstrapped in so I don't have to care about import.c anymore). -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20100713/f1558bac/attachment-0001.html>

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