inspect.isdatadescriptor false negative

[chnlior]

BPO	32400
Nosy	@serhiy-storchaka, @aaronchall, @corona10, @chnlior
Superseder	bpo-26103: Contradiction in definition of "data descriptor" between (dotted lookup behavior/datamodel documentation) and (inspect lib/descriptor how-to)

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-06-10.11:24:32.055>
created_at = <Date 2017-12-21.18:57:04.972>
labels = ['type-bug', 'library', 'docs']
title = 'inspect.isdatadescriptor  false negative'
updated_at = <Date 2018-06-10.11:24:32.054>
user = 'https://github.com/chnlior'

bugs.python.org fields:

activity = <Date 2018-06-10.11:24:32.054>
actor = 'berker.peksag'
assignee = 'docs@python'
closed = True
closed_date = <Date 2018-06-10.11:24:32.055>
closer = 'berker.peksag'
components = ['Documentation', 'Library (Lib)']
creation = <Date 2017-12-21.18:57:04.972>
creator = 'chnlior'
dependencies = []
files = []
hgrepos = []
issue_num = 32400
keywords = ['patch']
message_count = 2.0
messages = ['308895', '319204']
nosy_count = 5.0
nosy_names = ['docs@python', 'serhiy.storchaka', 'Aaron Hall', 'corona10', 'chnlior']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '26103'
type = 'behavior'
url = 'https://bugs.python.org/issue32400'
versions = ['Python 3.6']

[chnlior]

According to the c code in Include/descrobject.h

#define PyDescr_IsData(d) (Py_TYPE(d)->tp_descr_set != NULL)

and according to the "data model" chapter,
a data descriptor is an object who has __set__ and /or __delete__.

the "inspect.isdatadescriptor" checks for existence of __get__ and __set__ which IMHO is wrong. an object with __set__/delete only will return falsely False (should be True).

This is related to @serhiy Storchaka comment in bpo-26103 opened by @aaron Hall.

[corona10]

@serhiy Storchaka
This issue can be closed due to PR 1959