LocalProxy breaking isinstance() for abc.ABC

[douglas-raillard-arm]

When tested with isinstance() against an ABC, LocalProxy instances raise an exception rather than returning False.

import abc
from werkzeug import local

ls=local.LocalStack()
x=ls()

class C(abc.ABC):
	pass

isinstance(x, C)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/abc.py", line 119, in __instancecheck__
    return _abc_instancecheck(cls, instance)
  File "/usr/lib/python3.9/abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class

The call to isinstance() should have returned False rather than an exception.

It seems to be related to the fact that x.__class__ is not a class as expected, but an unbound method. Calling it with x.__class__() returns the expected werkzeug.local.LocalProxy class. While it's not documented in Python except for builtin types, most of the ecosystem will assume type(x) is x.__class__, including it appears the standard library.

Environment:

• Python version: Python 3.9.6
• Werkzeug version: 2.0.1

[davidism]

It looks like the C implementation of abc doesn't trigger the descriptor protocol when inspecting the __class__ attribute. This seems like it should be reported to CPython.

[douglas-raillard-arm]

If something is not triggered, it's not only in abc though:

It seems to be related to the fact that x.__class__ is not a class as expected, but an unbound method

This can be observed with:

import abc
from werkzeug import local

ls=local.LocalStack()
x=ls()

print(x.__class__)
assert isinstance(x.__class__, type)

Which prints:

<bound method LocalProxy.<lambda> of <LocalProxy unbound>>
Traceback (most recent call last):
  File "testwzeug.py", line 8, in <module>
    assert isinstance(x.__class__, type)
AssertionError

It would be expected to print:

<class 'werkzeug.local.LocalProxy'>

which can be obtained with x.__class__().

The descriptor protocol seems to be followed by Python:

(Pdb) type(x).__dict__['__class__'].__get__(x, type(x))
<bound method LocalProxy.<lambda> of <LocalProxy unbound>>

In local.py:

class _ProxyLookup:
    ....
    def __get__(self, instance: "LocalProxy", owner: t.Optional[type] = None) -> t.Any:
        ....
            return self.fallback.__get__(instance, owner)  # type: ignore

Replacing with return self.fallback.__get__(instance, owner)() fixes the problem by actually calling the bound method, rather than just returning it