Inaccurate emulation of _PyType_Lookup() in the Descriptor HOWTO

Documentation

Doc link: https://docs.python.org/3.10/howto/descriptor.html#invocation-from-an-instance

Source: https://github.com/python/cpython/blame/eb81c1aea16914347919745e843c982ed831a9fb/Doc/howto/descriptor.rst#L585-L601

The code example is bad at https://github.com/python/cpython/blame/eb81c1aea16914347919745e843c982ed831a9fb/Doc/howto/descriptor.rst#L589

getattr(objtype, name) will go through descriptor.__get__.

Code Snippet:

class A:
    def __get__(self, obj, objtype):
        return obj, objtype

class B:
    a = A()

print(B().a)
# (<__main__.B object at 0x7ff56d0915e0>, <class '__main__.B'>)
print(B.a)  # same as getattr
# (None, <class '__main__.B'>)

Solution

Using __dict__ (or vars) instead of getattr:

print(vars(B)['a'])
# <__main__.A object at 0x7ff56cfef2b0>

Thanks for the bug report. Here's a proposed update that replaces getattr(objtype, name) with find_name_in_mro(objtype, name, null):

def find_name_in_mro(cls, name, default):
    "Emulate _PyType_Lookup() in Objects/typeobject.c"
    for base in cls.__mro__:
        if name in vars(base):
            return vars(base)[name]
    return default

def object_getattribute(obj, name):
    "Emulate PyObject_GenericGetAttr() in Objects/object.c"
    null = object()
    objtype = type(obj)
    cls_var = find_name_in_mro(objtype, name, null)    
    descr_get = getattr(type(cls_var), '__get__', null)
    if descr_get is not null:
        if (hasattr(type(cls_var), '__set__')
            or hasattr(type(cls_var), '__delete__')):
            return descr_get(cls_var, obj, objtype)     # data descriptor
    if hasattr(obj, '__dict__') and name in vars(obj):
        return vars(obj)[name]                          # instance variable
    if descr_get is not null:
        return descr_get(cls_var, obj, objtype)         # non-data descriptor
    if cls_var is not null:
        return cls_var                                  # class variable
    raise AttributeError(name)

Also, I want to add tests to cover your example and to cover why the fix proposed in the PR was incorrect.

pythonGH-95822: Need _PyType_Lookup() in descriptor howto code equiva…

GH-95822: Need _PyType_Lookup() in descriptor howto code equivalent. (G…

pythonGH-95822: Need _PyType_Lookup() in descriptor howto code equiva…

GH-95822: Need _PyType_Lookup() in descriptor howto code equivalent. (G…

Thanks for the report. This is fixed now.

Navigation Menu

Repository navigation

Inaccurate emulation of _PyType_Lookup() in the Descriptor HOWTO #95822

Documentation

Solution

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Inaccurate emulation of _PyType_Lookup() in the Descriptor HOWTO #95822

Description

Documentation

Solution

Activity

rhettinger commented on Aug 10, 2022

rhettinger commented on Aug 19, 2022

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions