[Python-Dev] Deleting with setting C API functions

[Nick Coghlan]

On 25 November 2015 at 07:33, Guido van Rossum <guido at python.org> wrote:
> Ooooh, that's probably really old code. I guess for the slots the
> reasoning is to save on slots. For the public functions, alas it will
> be hard to know if anyone is depending on it, even if it's
> undocumented. Perhaps add a deprecation warning to these if the value
> is NULL for one release cycle?

I did a quick scan for "PyObject_SetAttr", and it turns out
PyObject_DelAttr is only a convenience macro for calling
PyObject_SetAttr with NULL as the value argument. bltinmodule.c and
ceval.c also both include direct calls to PyObject_SetAttr with
"(PyObject *)NULL" as the value argument.

Investigating some of the uses that passed a variable as the value
argument, one case is the weakref proxy implementation, which uses
PyObject_SetAttr on the underlying object in its implementation of the
setattr slot in the proxy.

So it looks to me like replicating the NULL-handling behaviour of the
slots in the public Set* APIs was intentional, and it's just the
documentation of that detail that was missed (since most folks
presumably use the Del* convenience APIs instead).

Regards,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia