GAE: Why are subclasses NOT instances of db.Model (parent)?

Question

Given:

class A:
 pass
class B(A):
 pass

isinstance(B(), A) will return True.

BUT

class MyModel(db.Model):
    pass

isinstance(MyModel(), db.Model) returns False(surely True?).

What am I missing?

Edit: Ok, simplest test that fails- created a blank GAE project. Inside main.py I've defined:

from google.appengine.ext import db  

class MyModel(db.Model):  
    detail = db.StringProperty()  

Then I've created a test file (test_ami.py) which contains the following:

import unittest  
from main import MyModel  
from google.appengine.ext import db  

class TestAmI(unittest.TestCase):  
    def test_whatami(self):  
        m = MyModel()  
        self.assertEquals(True, isinstance(m, db.Model));  
        self.assertEquals(True, isinstance(MyModel(), db.Model));  

On the command line: nosetests --with-gae results in:
File "test_ami.py", line 8, in test_whatami self.assertEquals(True, isinstance(m, db.Model)) AssertionError: True != False

Line 8 is: self.assertEquals(True, isinstance(m, db.Model));

Answer 1

isinstance(B(), A) will return True.

That seems unlikely, since in your class definition, B doesn't extend A.

isinstance(MyModel(), db.Model) returns False(surely True?).

Testing this on shell.appspot.com, it returns True, as expected.