code: def blob_size(self, digest):
          
          r = self._request('head', 'blobs/' + digest)
          return long(r.headers['content-length'])
docs: Return the size of a blob in the registry given the hash of its content.
  
          :param digest: Hash of the blob's content (prefixed by ``sha256:``).
          :type digest: str
  
          :rtype: long
          :returns: Whether the blob exists.