This arises from the behavior of pickle (which is used by default in multiprocessing to serialize objects sent to / received from other processes in data exchanges), as seen with Python 3.6:

>>> import pickle
>>> x = pickle.dumps(mock.sentinel.foo)
>>> x
b'\x80\x03cunittest.mock\n_SentinelObject\nq\x00)\x81q\x01}q\x02X\x04\x00\x00\x00nameq\x03X\x03\x00\x00\x00fooq\x04sb.'
>>> pickle.loads(x)
sentinel.foo
>>> pickle.loads(x) == mock.sentinel.foo
False