[Python-Dev] Peculiar import code in pickle.py

[Steve Holden]

Brett Cannon wrote:
> 
> 
> On Tue, Jul 13, 2010 at 11:34, Alexander Belopolsky
> <alexander.belopolsky at gmail.com <mailto:alexander.belopolsky at gmail.com>>
> wrote:
> 
>     On Tue, Jul 13, 2010 at 1:57 PM, Benjamin Peterson
>     <benjamin at python.org <mailto:benjamin at python.org>> wrote:
>     ..
>     > No! That's not recommended and a complete hack. The "dance" or
>     > importlib.import_module is preferred.
> 
>     Nevertheless, "a complete hack" is what PyImport_Import does:
> 
>     PyObject *
>     PyImport_Import(PyObject *module_name)
>     {
>        static PyObject *silly_list = NULL;
>        ..
>        /* Call the __import__ function with the proper argument list
>         * Always use absolute import here. */
>        r = PyObject_CallFunction(import, "OOOOi", module_name, globals,
>                                  globals, silly_list, 0, NULL);
>        ..
>     }
> 
>     and _pickle.c uses  PyImport_Import() 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).
> 
It's only a matter of time until someone decides to provide a C
implementation of importlib ;-)

regards
 Steve
-- 
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