'Reads a single batch of sentences.'

def read(self):

    if self._session:
        (sentences, is_last) = self._session.run([self._source, self._is_last])
        if is_last:
            self._session.close()
            self._session = None
    else:
        (sentences, is_last) = ([], True)
    return (sentences, is_last)
