Description

iyume

opened on Aug 9, 2022

Issue body actions

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>

Activity

1. iyume

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   on Aug 9, 2022
2. AlexWaygood

   assigned

   rhettinger

   on Aug 9, 2022
3. rhettinger commented on Aug 10, 2022

   

   rhettinger

   on Aug 10, 2022

   Contributor

   More actions

   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.
4. rhettinger

   changed the title [-]Doc: descriptior HOWTO bad example[/-] [+]Inaccurate emulation of _PyType_Lookup() in the Descriptor HOWTO[/+] on Aug 10, 2022
5. rhettinger

   added a commit that references this issue on Aug 13, 2022

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

   013ab10
6. miss-islington

   added a commit that references this issue on Aug 19, 2022

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

   4272abc
7. rhettinger

   added a commit that references this issue on Aug 19, 2022

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

   6740680
8. miss-islington

   added a commit that references this issue on Aug 19, 2022

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

   2d206e7
9. rhettinger

   added 2 commits that reference this issue on Aug 19, 2022

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

   b6e8304

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

   d699068
10. rhettinger commented on Aug 19, 2022

    

    rhettinger

    on Aug 19, 2022

    Contributor

    More actions

    Thanks for the report. This is fixed now.
11. rhettinger

    closed this as completedon Aug 19, 2022

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