Description

AlexWaygood

opened on May 31, 2023

Issue body actions

On main, an isinstance() check against Sized works just the same as it does on Python 3.11:

>>> from typing import Sized
>>> isinstance(1, Sized)
False

However! If you first subclass Sized like this, TypeError is raised on that isinstance() check!

>>> from typing import Sized, Protocol
>>> class Foo(Sized, Protocol): pass
...
>>> isinstance(1, Sized)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1153, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1428, in __subclasscheck__
    return issubclass(cls, self.__origin__)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 123, in __subclasscheck__
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1793, in __subclasscheck__
    return super().__subclasscheck__(other)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 123, in __subclasscheck__
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1876, in _proto_hook
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

This was originally reported by @vnmabus in python/typing_extensions#207.

Note that (because of the abc-module cache), this doesn't reproduce if you do an isinstance() check before subclassing typing.Sized:

>>> import typing
>>> isinstance(1, typing.Sized)
False
>>> class Foo(typing.Sized, typing.Protocol): pass
...
>>> isinstance(1, typing.Sized)
False

Linked PRs

• gh-105144: Runtime-checkable protocols: move all 'sanity checks' to _ProtocolMeta.__subclasscheck__ #105152
• gh-105144: abc: Suppress errors raised by unrelated other subclasses #105159
• [3.12] gh-105144: Runtime-checkable protocols: move all 'sanity checks' to _ProtocolMeta.__subclasscheck__ (GH-105152) #105160

Activity

1. AlexWaygood

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   topic-typing

   3.12only security fixesonly security fixes

   3.13only security fixesonly security fixes

   on May 31, 2023
2. AlexWaygood commented on May 31, 2023

   

   AlexWaygood

   on May 31, 2023

   MemberAuthor

   More actions

   This bisects to b27fe67
3. AlexWaygood

   changed the title [-]Strange interaction between `typing._BaseGenericAlias` and `typing.Protocol`[/-] [+]Strange interaction between `typing._BaseGenericAlias` and `typing.Protocol` with `isinstance()` checks[/+] on May 31, 2023
4. AlexWaygood

   changed the title [-]Strange interaction between `typing._BaseGenericAlias` and `typing.Protocol` with `isinstance()` checks[/-] [+]Strange interaction between `typing.Protocol` and unrelated `isinstance()` checks[/+] on May 31, 2023
5. AlexWaygood commented on May 31, 2023

   

   AlexWaygood

   on May 31, 2023

   MemberAuthor

   More actions

   Also reproduces if you use collections.abc.Sized rather than typing.Sized:

   >>> import collections.abc
   >>> import typing
   >>> class Foo(collections.abc.Sized, typing.Protocol): pass
   ...
   >>> isinstance(1, collections.abc.Sized)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 119, in __instancecheck__
       return _abc_instancecheck(cls, instance)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 123, in __subclasscheck__
       return _abc_subclasscheck(cls, subclass)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1796, in __subclasscheck__
       return super().__subclasscheck__(other)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 123, in __subclasscheck__
       return _abc_subclasscheck(cls, subclass)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1879, in _proto_hook
       raise TypeError("Instance and class checks can only be used with"
   TypeError: Instance and class checks can only be used with @runtime_checkable protocols
6. AlexWaygood

   self-assigned this

   on May 31, 2023
7. JelleZijlstra

   added a commit that references this issue on May 31, 2023

   pythongh-105144: abc: Suppress errors raised by unrelated other subcl…

   2008aa5
8. AlexWaygood

   added a commit that references this issue on May 31, 2023

   gh-105144: Runtime-checkable protocols: move all 'sanity checks' to `…

   c05c31d
9. miss-islington

   added a commit that references this issue on May 31, 2023

   pythongh-105144: Runtime-checkable protocols: move all 'sanity checks…

   d075995
10. JelleZijlstra commented on May 31, 2023

    

    JelleZijlstra

    on May 31, 2023

    Member

    More actions

    With some debugging we found that this is due to code in _abc.c that iterates over all the __subclasses__ of the ABC:

    cpython/Modules/_abc.c

    Line 768 in c05c31d

    subclasses = PyObject_CallMethod(self, "__subclasses__", NULL);

    Alex's PR #105152 works around this problem by re-arranging the code in typing.py. In #105159 I propose a fix on the ABC side instead. We should consider merging that PR too to prevent similar problems with other ABC subclasses.

    Reacted by sunmy2019 and Alex Waygood
11. JelleZijlstra

    added a commit that references this issue on May 31, 2023

    [3.12] gh-105144: Runtime-checkable protocols: move all 'sanity check…

    076f3cd
12. AlexWaygood commented on Jun 16, 2023

    

    AlexWaygood

    on Jun 16, 2023

    MemberAuthor

    More actions

    This issue is fixed, and Jelle's decided not to pursue #105159, so I'll close this
13. AlexWaygood

    closed this as completedon Jun 16, 2023

Issue actions