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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: Thu, 4 Jan 2018 04:22:22 +0100

On 04.01.2018 02:48, Troy Curtis Jr wrote:
>
>
> On Tue, Jan 2, 2018 at 3:12 AM Branko Čibej <brane_at_apache.org
> <mailto:brane_at_apache.org>> wrote:
>
> On 31.12.2017 03:05, Troy Curtis Jr wrote:
> >
> >     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.
> >
> >
> > That is a reasonable concern.  I definitely preferred the cleaner
> > single implementation, but honestly the code necessary to
> continue to
> > use classic classes in python 2 is not large.  I've attached a
> working
> > patch for reference/discussion.  It is a bit more code and some
> > conditional definitions, but perhaps it is the more preferred course
> > to take?
> >
> > [[[
> > On branch swig-py3: Go back to using classic classes for Python
> 2 swig
> > bindings.
> >
> > Add some additional clarifying comments for the reasons behind
> overriding
> > __getattr__ and __getattribute__.
> >
> > * build/ac-macros/swig.m4
> >   (SVN_FIND_SWIG): Add the '-classic' flag to swig when python 2 is
> > detected.
> >
> > * subversion/bindings/swig/include/proxy.py
> >    (_retrieve_swig_value): Factor out metadata retrieval from
> > __getattribute__ to a new function.  
> >    (__getattribute__): Only define __getattribute__ for new
> style classes.
> >    (__getattr__): Add back implementation for classic classes.
> > ]]]
> >
> > Troy
>
> [...]
>
> > +· # SWIG classes generated with -classic do not define this
> variable,
> > +· # so set it to 0 when it doesn't exist
> > +· try: _newclass
> > +· except NameError: _newclass = 0
>
> I prefer to break the try/except blocks onto separate lines, and
> to use
> None for the tristate idiom value:
>
>     try:
>       _newclass
>     except NameError:
>       _newclass = None
>
>
> Using None here is certainly more Pythonic, but in this case I was
> trying to match up with what swig generates:
>
>  try:
>    _object = object
>    _newclass = 1
>  except __builtin__.Exception:
>    class _object:
>      pass
>    _newclass = 0
>
> In this case we only need the _newclass variable defined, and not the
> "empty" class definition.  In all the conditional cases which use that
> value, either way should work, but I think it is likely better to
> stick with 0 for consistency in this case.
>
> However, I can understand the formatting request.
>
> Other than that, is the consensus that we should continue with classic
> classes in Python 2 with the conditional logic, or use a common
> implementation for the python2/python3 code like is currently in the
> swig-py3 branch?

The differences between old-style and new-style classes is tricky.
Offhand I can think of differences in invocation of base class
constructors, for example. I'd say leave old style classes for Python 2
precisely because the change would not be 100% backwards compatible.

-- Brane
Received on 2018-01-04 04:22:35 CET

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