An error in classmethod example in the documentation of descriptor

[titanolodon]

BPO	43917
Nosy	@rhettinger, @miss-islington, @titanolodon
PRs	bpo-43917: Fix descriptor documentation example error #25541 bpo-43917: Fix pure python equivalent for classmethod #25544 [3.9] bpo-43917: Fix pure python equivalent for classmethod (GH-25544) #25546

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2021-04-23.01:17:06.552>
created_at = <Date 2021-04-22.22:46:38.093>
labels = ['type-bug', '3.9', '3.10', 'docs']
title = 'An error in classmethod example in the documentation of descriptor'
updated_at = <Date 2021-04-23.01:17:06.548>
user = 'https://github.com/titanolodon'

bugs.python.org fields:

activity = <Date 2021-04-23.01:17:06.548>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2021-04-23.01:17:06.552>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2021-04-22.22:46:38.093>
creator = 'titanolodon'
dependencies = []
files = []
hgrepos = []
issue_num = 43917
keywords = ['patch']
message_count = 5.0
messages = ['391638', '391639', '391644', '391646', '391647']
nosy_count = 5.0
nosy_names = ['rhettinger', 'docs@python', 'python-dev', 'miss-islington', 'titanolodon']
pr_nums = ['25541', '25544', '25546']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43917'
versions = ['Python 3.9', 'Python 3.10']

[titanolodon]

In this section https://docs.python.org/3/howto/descriptor.html#class-methods in the example of python implementation of ClassMethod inside __get__ method I think that we should check hasattr(self.f, "__get__") instead of hasattr(obj, "__get__").

[rhettinger]

It looks like it should be:

    if hasattr(type(self.f), "__get__"):

Here's the relevant C code:

static PyObject *
cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    classmethod *cm = (classmethod *)self;

    if (cm->cm_callable == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "uninitialized classmethod object");
        return NULL;
    }
    if (type == NULL)
        type = (PyObject *)(Py_TYPE(obj));
    if (Py_TYPE(cm->cm_callable)->tp_descr_get != NULL) {
        return Py_TYPE(cm->cm_callable)->tp_descr_get(cm->cm_callable, type,
                                                      NULL);
    }
    return PyMethod_New(cm->cm_callable, type);
}

[rhettinger]

New changeset 14092b5 by Raymond Hettinger in branch 'master':
bpo-43917: Fix pure python equivalent for classmethod (GH-25544)
14092b5

[rhettinger]

New changeset 34be484 by Miss Islington (bot) in branch '3.9':
bpo-43917: Fix pure python equivalent for classmethod (GH-25544) (GH-25546)
34be484

[rhettinger]

Okay, it's fixed. Thanks for the report.