def default_empty(shape, dtype):
    """Numpy's empty array can have random values in it. To prevent that, we
    define here a default emtpy array. This default empty is a numpy.zeros
    array, except that objects are set to None, and all ints to ID_NOT_SET.
    """
    default = numpy.zeros(shape, dtype=dtype)
    set_default_empty(default)
    return default