code: def upload_token(
              self,
              bucket,
              key=None,
              expires=3600,
              policy=None,
              strict_policy=True):
          
          if bucket is None or bucket == '':
              raise ValueError('invalid bucket name')
  
          scope = bucket
          if key is not None:
              scope = '{0}:{1}'.format(bucket, key)
  
          args = dict(
              scope=scope,
              deadline=int(time.time()) + expires,
          )
  
          if policy is not None:
              self.__copy_policy(policy, args, strict_policy)
  
          return self.__upload_token(args)
docs: 生成上传凭证
  
          Args:
              bucket:  上传的空间名
              key:     上传的文件名，默认为空
              expires: 上传凭证的过期时间，默认为3600s
              policy:  上传策略，默认为空
  
          Returns:
              上传凭证