New submission from Raymond Hettinger <raymond.hettin...@gmail.com>:
Per PEP 3119, "Overloading works as follows: The call isinstance(x, C) first
checks whether C.__instancecheck__ exists, and if so, calls
C.__instancecheck__(x) instead of its normal implementation."
However, this doesn't work because the isinstance() has a bug introduced in
Python 3.1 and no one ever noticed.
In abstract.c::object_recursive_isinstance(), we have:
PyObject *checker = _PyObject_LookupSpecial(cls, &PyId___instancecheck__);
However, that function expects an instance as an argument rather than a class.
Calling typeobject.c::_PyObject_LookupSpecial() runs:
res = _PyType_LookupId(Py_TYPE(self), attrid);
Note, the Py_TYPE(self) is intended to move up from an instance to a class, but
we're already started with a class, so it moves to the metaclass instead.
This code was correct when originally implemented but it did not have tests:
if (name == NULL) {
name = PyString_InternFromString("__instancecheck__");
if (name == NULL)
return -1;
}
checker = PyObject_GetAttr(cls, name);
if (checker == NULL && PyErr_Occurred())
PyErr_Clear();
------- Demonstration code ---------------
class C:
def __instancecheck__(self, inst):
raise RuntimeError(f'{self=} {inst=}')
class P:
def __instancecheck__(self, inst):
raise RuntimeError(f'{self=} {inst=}')
class C(P):
pass
>>> isinstance(C(), P) # Incorrectly fails to invoke __instancecheck__
True
>>> isinstance(C(), P()) # Incorrectly invokes __instancecheck__
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
isinstance(C(), P())
File "<pyshell#5>", line 3, in __instancecheck__
raise RuntimeError(f'{self=} {inst=}')
RuntimeError: self=<__main__.P object at 0x107586c80> inst=<__main__.C object
at 0x107587100>
----------
components: Interpreter Core
messages: 406187
nosy: rhettinger
priority: normal
severity: normal
status: open
title: __instancecheck__ being checked of type(cls) instead of cls
versions: Python 3.10, Python 3.11, Python 3.9
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue45791>
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