Contradiction in definition of "data descriptor" between (dotted lookup behavior/datamodel documentation) and (inspect lib/descriptor how-to)

[aaronchall]

BPO	26103
Nosy	@rhettinger, @serhiy-storchaka, @1st1, @aaronchall, @Mariatta, @csabella, @chnlior
PRs	bpo-26103 resolve Data descriptor contradiction #1959 bpo-26103: Update the description of isdatadescriptor in inspect.rst #16645 [3.8] bpo-26103: Fix the description of isdatadescriptor in inspect.rst (GH-16645) #17180

^{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 2019-03-20.20:16:37.395>
created_at = <Date 2016-01-13.20:56:52.079>
labels = ['3.7', '3.8', 'type-bug', 'library', 'docs']
title = 'Contradiction in definition of "data descriptor" between (dotted lookup behavior/datamodel documentation) and (inspect lib/descriptor how-to)'
updated_at = <Date 2019-11-15.21:49:05.856>
user = 'https://github.com/aaronchall'

bugs.python.org fields:

activity = <Date 2019-11-15.21:49:05.856>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-20.20:16:37.395>
closer = 'Aaron Hall'
components = ['Documentation', 'Library (Lib)']
creation = <Date 2016-01-13.20:56:52.079>
creator = 'Aaron Hall'
dependencies = []
files = []
hgrepos = []
issue_num = 26103
keywords = []
message_count = 12.0
messages = ['258168', '295212', '295243', '295287', '295288', '295307', '295378', '301206', '302721', '308078', '317211', '338426']
nosy_count = 8.0
nosy_names = ['rhettinger', 'docs@python', 'serhiy.storchaka', 'yselivanov', 'Aaron Hall', 'Mariatta', 'cheryl.sabella', 'chnlior']
pr_nums = ['1959', '16645', '17180']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26103'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[aaronchall]

Based on the data-model documentation (https://docs.python.org/2/reference/datamodel.html#invoking-descriptors) and the dotted lookup behavior, the follow definitions are correct:

"If the descriptor defines __set__() and/or __delete__(), it is a data descriptor; if it defines neither, it is a non-data descriptor."

def has_data_descriptor_attrs(obj):
    return set(['__set__', '__delete__']) & set(dir(obj))

def is_data_descriptor(obj):
    return bool(has_data_descriptor_attrs(obj))

However, the inspect module has the following, which is also reflected in the descriptor how-to (https://docs.python.org/2/howto/descriptor.html#descriptor-protocol):

"If an object defines both __get__() and __set__(), it is considered a data descriptor."

def isdatadescriptor(object):
    """Return true if the object is a data descriptor.

Data descriptors have both a __get__ and a __set__ attribute..."""
if isclass(object) or ismethod(object) or isfunction(object):
    # mutual exclusion
    return False
tp = type(object)
return hasattr(tp, "__set__") and hasattr(tp, "__get__")

I'm willing to sign a contributor release and fix myself.

[aaronchall]

Bumping this - I intend to work on this next, if no objections.

[serhiy-storchaka]

This isn't just a documentation issue since it fixes inspect.isdatadescriptor(). I confirm that the new implementation better matches the C code. LGTM, but needed tests for inspect.isdatadescriptor() and a Misc/NEWS entry.

[aaronchall]

Added news, working on tests

[Mariatta]

Please also add yourself to Misc/ACKS.

[aaronchall]

Please also add yourself to Misc/ACKS.

Done!

[aaronchall]

I tweaked the docs a little more this morning, but I believe I am done making any further changes unless so requested.

This issue doesn't say it's assigned to anyone. Is there anything else that needs to happen here?

[aaronchall]

Serhiy,

Not sure what else needs to be done to wrap this up. All checks are passing on the pull request.

Thoughts?