__slots__ and copy again: why does it work?

[fortepianissimo]

I remember from painful experience that copy.copy() won't really copy
__slots__ members. But I have trouble explaning why the following code
works:

--- START---
#!/usr/bin/env python

import copy


class Foo (object):
  __slots__ = 'i'

  def __init__ (self):
    self.i = 10


class Bar (Foo):
  __slots__ = 'j'

  def __init__ (self):
    self.j = 20


f1 = Foo()
f2 = copy.copy(f1)

print f2.i   # why does it work?


b1 = Bar()
b2 = copy.copy(b1)

print b2.j   # why does it work?
print b2.i   # doesn't work, as expected

--- END---


Any insight is welcome!