Fix data descriptor detection in inspect.getattr_static

[davidhalter]

BPO	31184
Nosy	@rhettinger, @serhiy-storchaka
Superseder	bpo-26103: Contradiction in definition of "data descriptor" between (dotted lookup behavior/datamodel documentation) and (inspect lib/descriptor how-to)
Files	0001-Fix-data-descriptor-detection-in-inspect.getattr_sta.patch: Potential patch that fixes the issue

^{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 = None
created_at = <Date 2017-08-11.16:22:11.131>
labels = ['type-bug', 'library', '3.11']
title = 'Fix data descriptor detection in inspect.getattr_static'
updated_at = <Date 2021-12-08.04:22:58.232>
user = 'https://bugs.python.org/davidhalter'

bugs.python.org fields:

activity = <Date 2021-12-08.04:22:58.232>
actor = 'rhettinger'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-08-11.16:22:11.131>
creator = 'davidhalter'
dependencies = []
files = ['47078']
hgrepos = []
issue_num = 31184
keywords = ['patch']
message_count = 4.0
messages = ['300170', '300173', '407976', '407985']
nosy_count = 3.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'davidhalter']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'open'
superseder = '26103'
type = 'behavior'
url = 'https://bugs.python.org/issue31184'
versions = ['Python 3.11']

Linked PRs

• gh-75367: fix data descriptor detection in inspect.getattr_static #104517
• [3.11] gh-75367: Fix data descriptor detection in inspect.getattr_static (GH-104517) #104557

[davidhalter]

inspect.getattr_static is currently not identifying data descriptors the right way.

Data descriptors are defined by having a __get__ attribute and at least one of the __set__ and __delete__ attributes.

Implementation detail: Both __delete__ and __get__ set the same slot called tp_descr_set in CPython.

I have attached a patch that fixes the issue IMO.

[serhiy-storchaka]

See also bpo-26103.

[davidhalter]

This is not a duplicate. It is related to https://bugs.python.org/issue26103, because __get__ is not required anymore for an object to be a data descriptor. The current code on master (of inspect.getattr_static) still thinks a descriptor has both __get__ and __set__ set.

Since issue bpo-26103 has been fixed, it's now clear that my patch is slightly wrong, but I'm happy to fix that if someone is actually going to review it.

[rhettinger]

Can you give an example of where getattr_static() is not doing what you expect?

[iritkatriel]

Since issue bpo-26103 has been fixed, it's now clear that my patch is slightly wrong, but I'm happy to fix that if someone is actually going to review it.

@davidhalter Please do update the patch. And please add a unit test that is currently failing so we can see what bug you are fixing.

[davidhalter]

Thanks for bringing it up again. I unfortunately somehow missed @rhettinger's comment on this issue. Will try to give an example as soon as possible.

[arhadthedev]

@davidhalter ping

[davidhalter]

As soon as possible doesn't mean I have time immediately. It will happen and it's still on my radar. Sorry, but I have been quite busy.

[davidhalter]

Here's my example:

import inspect

class DescriptorGet:
    def __get__(self, instance, klass):
        return "Foo"

class DescriptorGetDelete:
    def __get__(self, instance, klass):
        return "Foo"
    def __delete__(self, instance, klass):
        pass

class DescriptorGetSet:
    def __get__(self, instance, klass):
        return "Foo"
    def __set__(self, instance, klass, value):
        pass

class Foo:
    get = DescriptorGet()
    get_delete = DescriptorGetDelete()
    get_set = DescriptorGetSet()

    def __init__(self):
        self.__dict__['get'] = 42
        self.__dict__['get_delete'] = 42
        self.__dict__['get_set'] = 42

for attr in ['get', 'get_delete', 'get_set']:
    print(f'getattr        {attr}', getattr(Foo(), attr))
    print(f'getattr_static {attr}', inspect.getattr_static(Foo(), attr))

Output:

getattr        get 42
getattr_static get 42
getattr        get_delete Foo
getattr_static get_delete 42
getattr        get_set Foo
getattr_static get_set <__main__.DescriptorGetSet object at 0x7f88b10ab7f0>

The get_delete case is clearly wrong, because if a normal getattr returns the executed descriptor, we should never get the value of the instance's __dict__ as a result in the getattr_static case.

@iritkatriel @rhettinger Does this make sense? In case you think it's a bug as well, I'm happy to create a pull request with my patch & a unit test of the __delete__ case.

[AlexWaygood]

Thanks @davidhalter. I agree that this looks like a bug. Just to give a slightly shorter reproducer:

>>> class DescriptorGetDelete:
...     def __get__(self, instance, klass):
...         return 'foo'
...     def __delete__(self, instance, klass): pass
...
>>> class Foo:
...     get_delete = DescriptorGetDelete()
...     def __init__(self):
...         self.__dict__['get_delete'] = 42
...
>>> foo = Foo()
>>> foo.get_delete
'foo'
>>> import inspect
>>> inspect.getattr_static(foo, 'get_delete')
42

The result of the last call should probably be <__main__.DescriptorGetDelete object at 0x000001D9BF998DA0> rather than 42.

If you file a PR for this issue, feel free to ping me on it -- I'd be happy to review it.

[carljm]

The fix here is simply to also check for __delete__ method, not only __set__ method.

To be clear, despite the resolution of #70291 (which I think was resolved incorrectly), the check for __get__ method here in getattr_static is correct and should remain. In other words, I think @davidhalter 's original patch here remains the correct fix, despite the resolution of #70291.

[furkanonder]

@carljm The issue seems to have been resolved. We can close the issue.