Issue 26103: Contradiction in definition of "data descriptor" between (dotted lookup behavior/datamodel documentation) and (inspect lib/descriptor how-to)

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.

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

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.

Added news, working on tests

Please also add yourself to Misc/ACKS.

> Please also add yourself to Misc/ACKS.

Done!

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?

Serhiy,

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

Thoughts?

The only question is remained -- should *data descriptors* be *descriptors*? I.e. is the __get__ method required for data descriptors?

Joining @Serhiy Storchaka last question.
Is the __get__ method existance is a must be a data descriptor?

According to the C implementation in descrobject.h
```
#define PyDescr_IsData(d) (Py_TYPE(d)->tp_descr_set != NULL)
#endif
```
the answer is No.
Does this C code reflect the true definition?

New changeset 4054b172ab59054715a2aaf4979bd84ac23e3ada by Serhiy Storchaka (Aaron Hall, MBA) in branch 'master':
bpo-26103: Fix inspect.isdatadescriptor() and a data descriptor definition. (GH-1959)
https://github.com/python/cpython/commit/4054b172ab59054715a2aaf4979bd84ac23e3ada

It looks like this issue can be closed now that it's merged?

nosy: + rhettinger

messages: + msg295243

assignee: docs@python -> (no value)
stage: needs patch -> patch review

assignee: docs@python
components: + Documentation, Library (Lib)
stage: needs patch