get_object() can't obtain an object body

[cuihaikuo]

Hi,I try to get an object with get_object() and compare the md5, here is the code:

resp_headers, obj_contents = self.conn.get_object(self.swift_conf.container, obj_id)
m = hashlib.md5()
m.update(obj_contents)
Etag = m.hexdigest()
if Etag != resp_headers['etag']:
    raise MismatchException('Digest mismatched')

sometimes the obj_contents was null, but etag in resp_headers was normal, why does it happen?