what memcache stores?

Now after I learned how to store one-to-many collections with AppEngine datastore, I want to use Memcache API to eliminate DB hits between requests. Quick test shows it would not work out of the box:

class C(object):
 def __init__(self):
   self.field='field'

c=C()
c.attr='attr'
c.__dict__['dict']='dict'
print dir(c)
memcache.add('c', c)
c=memcache.get("c")
print dir(c)

I would expect this code to output the same list of public attributes before and after, but it does not happen. We can see that only c.field was saved, while c.attr and c.dict are missing. Need to investigate memcache internals to understand why.