Jan 26, 2026
diff --git a/Lib/test/test_ctypes/test_pointers.py b/Lib/test/test_ctypes/test_pointers.py
        ptr.set_type(c_int)
        self.assertIs(ptr._type_, c_int)

    def test_pointer_proto_missing_argtypes_error(self):
        class BadType(ctypes._Pointer):
            # _type_ is intentionally missing
            pass

        func = ctypes.pythonapi.Py_GetVersion
        func.argtypes = (BadType,)

        with self.assertRaises(ctypes.ArgumentError):
            func(object())


 if __name__ == '__main__':
    unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2026-01-21-19-39-07.gh-issue-144100.hLMZ8Y.rst b/Misc/NEWS.d/next/Library/2026-01-21-19-39-07.gh-issue-144100.hLMZ8Y.rst
 Fixed a crash in ctypes when using a deprecated ``POINTER(str)`` type in
 ``argtypes``. Instead of aborting, ctypes now raises a proper Python
 exception when the pointer target type is unresolved.
diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
    /* If we expect POINTER(<type>), but receive a <type> instance, accept
       it by calling byref(<type>).
    */
    assert(typeinfo->proto);
    if (typeinfo->proto == NULL) {
        PyErr_SetString(
            PyExc_TypeError,
            "cannot convert argument: POINTER _type_ type is not set"
        );
        return NULL;
    }
    switch (PyObject_IsInstance(value, typeinfo->proto)) {
    case 1:
        Py_INCREF(value); /* _byref steals a refcount */
Original file line number	Diff line number	Diff line change
Expand Up		@@ -235,6 +235,17 @@ class C(Structure):
		ptr.set_type(c_int)
		self.assertIs(ptr._type_, c_int)

		def test_pointer_proto_missing_argtypes_error(self):
		class BadType(ctypes._Pointer):
		# _type_ is intentionally missing
		pass

		func = ctypes.pythonapi.Py_GetVersion
		func.argtypes = (BadType,)

		with self.assertRaises(ctypes.ArgumentError):
		func(object())


		if __name__ == '__main__':
		unittest.main()
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		Fixed a crash in ctypes when using a deprecated ``POINTER(str)`` type in
		``argtypes``. Instead of aborting, ctypes now raises a proper Python
		exception when the pointer target type is unresolved.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -1338,7 +1338,13 @@ PyCPointerType_from_param_impl(PyObject type, PyTypeObject cls,
		/* If we expect POINTER(<type>), but receive a <type> instance, accept
		it by calling byref(<type>).
		*/
		assert(typeinfo->proto);
		if (typeinfo->proto == NULL) {
		PyErr_SetString(
		PyExc_TypeError,
		"cannot convert argument: POINTER _type_ type is not set"
		);
		return NULL;
		}
		switch (PyObject_IsInstance(value, typeinfo->proto)) {
		case 1:
		Py_INCREF(value); /* _byref steals a refcount */
Expand Down