无法散列的类型列表错误，尽管使用字符串

Question

我正在使用字典，有时会收到以下错误消息：

Traceback (most recent call last):
  File "./process_blasthit.py", line 637, in <module>
    prot_names_dict_path, options)
  File "./process_blasthit.py", line 491, in extract_dists
    prot_names_dict, options)
  File "./process_blasthit.py", line 326, in scan_file
    product, options)
  File "./process_blasthit.py", line 253, in add_to_env_dict
    if product not in env_dict[str(cessor_type)]:
TypeError: unhashable type: 'list'

如您所见，在字典中访问键之前，我已经尝试过将键（cessor_type）转换为字符串，但是它仍然无法正常工作。

此外，当我打印出type（cessor_type）告诉我它是一个字符串实例之前，

此外，前几个登录名（即env_dict [str（censor_type）]）工作正常。

这是我尝试访问字典的功能：

def add_to_env_dict(env_queue, cessor_type, env_dict, succ_counter, 
                    product, options):

  if cessor_type=="succ":
    env_queue_new=env_queue[1:]
  elif cessor_type=="pred":
    env_queue_new=env_queue[max(0,len(env_queue)-options["env_k"]):]
  else:
      stderr.write("Cessor type needs to be either \"succ\" or \"pred\"")
      raise SystemExit

  # the key of env_dict is ["pred"/"succ"][product of blasthit]
  # the values are env_k-tuples (stored as lists) of predecessors/successors 
  # with additional information on each predecessor/successor
  print(type(cessor_type))
  if product not in env_dict[str(cessor_type)]:
    env_dict[cessor_type][product]=[env_queue_new]
  else:
    env_dict[cessor_type][product].append(env_queue_new)

  return env_dict

也许我在这里监督了一些明显的事情，但是我似乎无法弄清楚我在做什么错。我很感谢任何提示。