runtime_checkable error in newer versions caused by a protocol subclass

[vnmabus]

Consider the following code:

from typing import Sized

isinstance(1, Sized)

This works well and returns False, as expected.

However, if I create a new protocol that subclass Sized:

from typing_extensions import Protocol

class MySized(Protocol, Sized):
 pass

isinstance(1, Sized)

This now raises the following exception:

Details

TypeError Traceback (most recent call last)
<ipython-input-5-2516c5c4af5f> in <module>
----> 1 isinstance(1, Sized)

.../lib/python3.8/typing.py in __instancecheck__(self, obj)
 764 
 765 def __instancecheck__(self, obj):
--> 766 return self.__subclasscheck__(type(obj))
 767 
 768 def __subclasscheck__(self, cls):

.../lib/python3.8/typing.py in __subclasscheck__(self, cls)
 769 if self._special:
 770 if not isinstance(cls, _GenericAlias):
--> 771 return issubclass(cls, self.__origin__)
 772 if cls._special:
 773 return issubclass(cls.__origin__, self.__origin__)

.../lib/python3.8/abc.py in __subclasscheck__(cls, subclass)
 100 def __subclasscheck__(cls, subclass):
 101 """Override for issubclass(subclass, cls)."""
--> 102 return _abc_subclasscheck(cls, subclass)
 103 
 104 def _dump_registry(cls, file=None):

.../lib/python3.8/site-packages/typing_extensions.py in __subclasscheck__(cls, other)
 581 "Protocols with non-method members don't support issubclass()"
 582 )
--> 583 return super().__subclasscheck__(other)
 584 
 585 def __instancecheck__(cls, instance):

.../lib/python3.8/abc.py in __subclasscheck__(cls, subclass)
 100 def __subclasscheck__(cls, subclass):
 101 """Override for issubclass(subclass, cls)."""
--> 102 return _abc_subclasscheck(cls, subclass)
 103 
 104 def _dump_registry(cls, file=None):

.../lib/python3.8/site-packages/typing_extensions.py in _proto_hook(cls, other)
 637 if _allow_reckless_class_checks():
 638 return NotImplemented
--> 639 raise TypeError("Instance and class checks can only be used with"
 640 " @runtime_checkable protocols")
 641 

TypeError: Instance and class checks can only be used with @runtime_checkable protocols

This non-local behavior did not happen in prior versions, and has caused problems in my code (GAA-UAM/scikit-fda#540).

[AlexWaygood]

typing_extensions's behaviour is correct here. You can see that it matches the behaviour of the typing module on Python >=3.10:

Python 3.11.2 (tags/v3.11.2:878ead1, Feb 7 2023, 16:38:35) [MSC v.1934 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> from typing import Sized, Protocol
>>> class MySized(Sized, Protocol): pass
...
>>> isinstance(1, MySized)
Traceback (most recent call last):
 File "<pyshell#3>", line 1, in <module>
 isinstance(1, MySized)
 File "C:\Users\alexw\AppData\Local\Programs\Python\Python311\Lib\typing.py", line 1960, in __instancecheck__
 raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

Your MySized class is a protocol, but it is not a runtime-checkable protocol. If a protocol is not runtime-checkable, you're not allowed to check it at runtime with isinstance() or issubclass(). This is specified in PEP 544. You can fix your code by decorating your MySized class with @typing_extensions.runtime_checkable.

typing_extensions had a bug in previous versions where TypeError was not always raised if you called isinstance() against a protocol that was not decorated with @runtime_checkable. This bug was fixed in v4.6.0, and the fix was documented in the changelog: #132.

[vnmabus]

I am NOT using isinstance with MySized, please check the code again.

[AlexWaygood]

Yikes, that's not good! Thanks for the report. Apologies for misreading your example.

[jgbos]

@AlexWaygood I'm having similar issues. One thing I noticed in my issue you might look at, if you run debug on the error is the error thrown with cls == Sized or on cls == MySized. I noticed the TypeError was thrown on any of protocol without runtime_checkable.

[AlexWaygood]

@jgbos are you still experiencing this issue with the latest version of typing_extensions (4.6.3)?

[jgbos]

@AlexWaygood I just saw your other comment, checking now.