@assertion
def to_be_null(topic):
    if topic is not None:
        msg = 'Expected topic({0}) to be None'.format(topic)
        raise AssertionError(msg)
Translate datastore value to BlobInfo.
dataset/ETHPy150Open heynemann/preggy/preggy/assertions/types/nullable.py/to_be_null--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/google/appengine/ext/blobstore/blobstore.py/BlobReferenceProperty.make_value_from_datastore