Python + Testing + Mock

I am trying to mock an import during testing

Test.py looks like

# Store original __import__
orig_import = __import__

b_mock = mock.Mock()

def import_mock(name, *args):
    if name == 'B':
        return b_mock
   return orig_import(name, *args)

with mock.patch('__builtin__.__import__', side_effect=import_mock):
    import A

A.py looks like

import B

def a():
    return B.func()

Now

b_mock.func.return_value = 'spam'

Therefore, A.a() should return 'spam' However, A.a() returns the entire mock object b_mock like this: < Mock name='mock.B.func()' id='139854736039632' >

Everytime B.func() is called, how do I get the return value which I have set(i.e 'spam') instead of getting the entire mock object?