Re: BUG #17908: plpython_to_hstore() crashes with a non-dict argument

Dmitry Dolgov <9erthalion6(at)gmail(dot)com> writes:
>> On Tue, Apr 25, 2023 at 01:00:02PM +0000, PG Bug reporting form wrote:
>> As noted in https://bugs.python.org/issue5945, the behaviour of
>> PyMapping_Check() changed in Python 3, so it can't be used anymore to
>> differentiate real dictionaries from strings or alike.

> Thanks for finding this! That's indeed very annoying.

Yuck.

> After a quick
> investigation looks like a proposed solution from [1] is to use
> PyType_HasFeature with Py_TPFLAGS_MAPPING:
> - if (!PyMapping_Check(dict))
> + dict_type = Py_TYPE(dict);
> + if (!PyType_HasFeature(dict_type, Py_TPFLAGS_MAPPING))
> But this flag is available only starting from Python 3.10, so not
> backward compatible :(

Yeah, doesn't sound hugely workable; I don't think we can require 3.10
yet. The issue5945 discussion suggested

>>> And recommend `PyMapping_Check() && !PySequence_Check()` for true mapping test?

I haven't verified this but will go test it shortly. If it's true,
it would explain why our other uses of PyMapping_Check haven't
failed, because (whether by chance or not I dunno) they are not
reached if PySequence_Check is true.

regards, tom lane