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Re: svn commit: r1818995 - in /subversion/branches/swig-py3: build/ac-macros/swig.m4 subversion/bindings/swig/include/proxy.py subversion/bindings/swig/include/proxy.swg

From: Branko Čibej <brane_at_apache.org>
Date: Sat, 30 Dec 2017 19:49:51 +0100

On 30.12.2017 16:11, Troy Curtis Jr wrote:
>
>
> On Fri, Dec 29, 2017 at 11:46 PM Daniel Shahaf <d.s_at_daniel.shahaf.name
> <mailto:d.s_at_daniel.shahaf.name>> wrote:
>
> Good morning Troy,
>
> troycurtisjr_at_apache.org <mailto:troycurtisjr_at_apache.org> wrote on
> Fri, Dec 22, 2017 at 03:52:59 -0000:
> > On branch swig-py3: Correctly manage swig objects with Python
> new style classes.
> >
> > * build/ac-macros/swig.m4
> >   (SVN_FIND_SWIG):
> >   Request that swig generate new style classes, even for Python
> 2, to reduce
> >   differences with Python 3.
> >
> > Modified: subversion/branches/swig-py3/build/ac-macros/swig.m4
> > URL:
> http://svn.apache.org/viewvc/subversion/branches/swig-py3/build/ac-macros/swig.m4?rev=1818995&r1=1818994&r2=1818995&view=diff
> >
> ==============================================================================
> > --- subversion/branches/swig-py3/build/ac-macros/swig.m4 (original)
> > +++ subversion/branches/swig-py3/build/ac-macros/swig.m4 Fri Dec
> 22 03:52:59 2017
> > @@ -143,7 +143,7 @@ AC_DEFUN(SVN_FIND_SWIG,
> >          if test "$ac_cv_python_is_py3" = "yes"; then
> >             SWIG_PY_OPTS="-python -py3"
> >          else
> > -           SWIG_PY_OPTS="-python -classic"
> > +           SWIG_PY_OPTS="-python"
> >          fi
>
> What are the compatibility implications of this change?  I
> couldn't find any
> documentation for the "-classic" option.
>
> > * subversion/bindings/swig/include/proxy.py
> >   (__getattr__): Replace __getattr__ with __getattribute__ to
> correctly
> >   intercept attribute access, even when swig uses descriptors
> for new style
> >   classes (which are the only type available in Python 3).
>
> Could you help me understand what's going on here and why it's
> correct?  I've
> read the documentation of object.__getattr__ and
> object.__getattribute__¹ but I
> don't follow how the fact that the class into which this code is
> included
> became a new-style class (rather than a classic class) brought on
> the need to
> migrate from __getattr__ to __getattribute__, nor why the order of
> precedence
> of lookup (first __dict__, then object.__getattribute__, then
> _swig_getattr) is
> correct.
>
> I'm asking because I'm trying to review the branch (to +1 its
> merge) and this
> is one of the open questions I have.
>
> ¹
> https://docs.python.org/3/reference/datamodel.html#object.__getattr__
> et seq.
>
>
> Sure thing.  This is the change that took me so long to find, and then
> fix the right way, so I certainly understand the non-obviousness of
> it.  Perhaps some portion of this explanation could go in an
> appropriate place in the code for future reference?  Though much of it
> only applies to the motivation for the change and probably wouldn't be
> beneficial just given the final implementation.
>
> Giving swig the '-classic' option forces it to generate only classic
> classes.  Without that flag it will produce code that works as either
> an old style or new style class, dependent on a try/except block
> around 'import object'.  In practice, this means that for any Python
> past 2.2 (when new-style classes were introduced), the classes are in
> the new style and inherit from 'object'. 

This all makes sense and seems nice on the surface, but I'm not sure we
can just change the behaviour of the bindings from old-style to
new-style classes in a Python 2.x build. There are enough subtle
differences in behaviour between the two that existing scripts could
break after an upgrade of the bindings.

Python 3.x has only new-style (or rather, even-newer-style) classes and
there's no backward-compatibility consideration, since our bindings
currently don't work with Python3.

-- Brane
Received on 2017-12-30 19:50:00 CET

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