[Python-Dev] Deleting with setting C API functions

[M.-A. Lemburg]

On 02.12.2015 13:29, Serhiy Storchaka wrote:
> On 02.12.15 12:06, Victor Stinner wrote:
>> 2015-12-02 9:42 GMT+01:00 Serhiy Storchaka <storchaka at gmail.com>:
>>> You have enough time to update your projects, and you can update them
>>> uniformly for all versions. And may be you will found few weird bugs related
>>> to misuse of Set* API.
>>
>> Did you check popular projects using C extensions to check if they
>> call Set*() functions to delete attributes/items?
> 
> I have checked following projects.
> 
> regex, simplejson, Pillow, PyQt4, LibreOffice, PyGTK, PyICU, pyOpenSSL, libxml2, Boost, psutil,
> mercurial don't use PyObject_SetAttr at all.
> 
> NumPy, pgobject don't use PyObject_SetAttr for deleting.
> 
> PyYAML and lxml use PyObject_SetAttr only in code generated by Cython and never use it for deleting.

While I don't think deleting attributes is a very common thing
to do in any Python code base (unless you need to break circular
references or explicitly want to free resources), the
fact that PyObject_DelAttr() itself is implemented as macro
using the NULL attribute value clearly creates an API incompatibility
when removing this functionality or generating warnings, since
all code using the correct PyObject_DelAttr() at the moment,
would then trigger the warning as well.

As a result, the deprecation would have to be extended across
more releases than the usual cycle.

A first step would be to replace the macro with a proper function
to avoid false positive warnings, even when using the correct API.

Then we could add a warning to the PyObject_SetAttr() function and
hope that not too many projects use the stable ABI as basis to
have C extensions work across several releases.

Overall, I'm not sure whether it's worth the trouble. Documenting
the feature and adding a deprecation notice to just the documentation
would likely be better. We could then remove the functionality
in Python 4.

-- 
Marc-Andre Lemburg
eGenix.com

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