Upon trying to create a proxy I stumbled upon this exception:

Traceback (most recent call last):
  File "foo.py", line 11, in <module>
    p.__class__.__call__(p)
SystemError: PyEval_EvalCodeEx: NULL globals

Investigating further led me to this code which reproduces the exception:

class Proxy(object):
    def __init__(self, proxied):
        self.proxied = proxied
    @property
    def __class__(self):
        return self.proxied.__class__
    def __call__(self):
        return self.proxied.__call__()

p = Proxy(lambda: 1)
p.__class__.__call__(p)

I understand that `p.__class__.__call__()` expects an argument of a different type however this is not obvious from the exception itself. PyPy on the other hand raises this exception:

Traceback (most recent call last):
  File "app_main.py", line 53, in run_toplevel
  File "foo.py", line 11, in <module>
    p.__class__.__call__(p)
TypeError: 'function' object expected, got 'Proxy' instead

Which explains the issue and is expected (at least by me) and apparently Jython raises an exception at least similar to PyPy's.

I think this is a duplicate of issue #9756: `methoddescr_call()` checks whether the given argument is acceptable as "self" argument and does so using `PyObject_IsInstance()`. As the class in the given code returns the type of the proxied object for the `__class__` attribute, that check will return true.

As a quick fix, the attached patch (against release27-maint branch) will raise a TypeError as expected by the OP, but the real issue is much broader.

> I think this is a duplicate of issue #9756

Yes it is and the issue is now fixed. The example now fails with the correct exception:

Traceback (most recent call last):
  File " x.py", line 12, in <module>
    p.__class__.__call__(p)
TypeError: descriptor '__call__' requires a 'function' object but received a 'Proxy'