Huawei IndexError: string index out of range

Hello @ktbyers , from time to time (seems like when my Huawei router is little bit slower than usual) I got this error:

 Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/nornir/core/task.py", line 85, in start
    r = self.task(self, **self.params)
  File "/usr/local/lib/python3.8/dist-packages/nornir/plugins/tasks/networking/netmiko_send_command.py", line 26, in netmiko_send_command
    net_connect = task.host.get_connection("netmiko", task.nornir.config)
  File "/usr/local/lib/python3.8/dist-packages/nornir/core/inventory.py", line 294, in get_connection
    self.open_connection(
  File "/usr/local/lib/python3.8/dist-packages/nornir/core/inventory.py", line 345, in open_connection
    self.connections[connection].open(
  File "/usr/local/lib/python3.8/dist-packages/nornir/plugins/connections/netmiko.py", line 58, in open
    self.connection = ConnectHandler(**parameters)
  File "/usr/local/lib/python3.8/dist-packages/netmiko/ssh_dispatcher.py", line 266, in ConnectHandler
    return ConnectionClass(*args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/netmiko/base_connection.py", line 327, in __init__
    self._open()
  File "/usr/local/lib/python3.8/dist-packages/netmiko/base_connection.py", line 333, in _open
    self._try_session_preparation()
  File "/usr/local/lib/python3.8/dist-packages/netmiko/base_connection.py", line 756, in _try_session_preparation
    self.session_preparation()
  File "/usr/local/lib/python3.8/dist-packages/netmiko/huawei/huawei.py", line 13, in session_preparation
    self.set_base_prompt()
  File "/usr/local/lib/python3.8/dist-packages/netmiko/huawei/huawei.py", line 95, in set_base_prompt
    if not prompt[-1] in (pri_prompt_terminator, alt_prompt_terminator):
IndexError: string index out of range

I was trying to debug this and I added few print statement lines into the netmiko/huawei/huawei.py:

        # log.debug("In set_base_prompt")
        delay_factor = self.select_delay_factor(delay_factor)
        print(f'delay_factor: {delay_factor}')
        print(f'Buffer before clear: {self.read_channel()}')
        self.clear_buffer()
        self.write_channel(self.RETURN)
        time.sleep(0.5 * delay_factor)

        prompt = self.read_channel()
        print(f'prompt: "{prompt}"')

When there is a failure indeed the prompt variable is empty ( None ):

delay_factor: 2
Buffer before clear: 
prompt: ""

Because I suspect a delay issue I added a little check and hard coded a 5s wait:

        if not prompt:
            print('No prompt, waiting another 5s')
            time.sleep(5)
            prompt = self.read_channel()
            print(f'prompt after 5s: {prompt}')
            if not prompt:
                raise ValueError('Empty prompt')

And it works, prompt was found:

delay_factor: 3
Buffer before clear: 
prompt: ""
No prompt, waiting another 5s
prompt after 5s: "<my_router>"

More info about my setup:

• netmiko==3.2.0

• I am using Nornir and it’s plugin netmiko_send_command

• my Nornir timers are configured as followed:

  connection_options:
        netmiko:
            extras:
                use_keys: True
                ssh_strict: False
                timeout: 60
                session_timeout: 60
                auth_timeout: 60
                blocking_timeout: 60
                banner_timeout: 60
                global_delay_factor: 2

Proposed solution

• check if variable prompt is not empty before going further in the code (so if not prompt[-1] ... won’t fail with IndexError).

• adjust time.sleep(0.5 * delay_factor) - sometimes it is not the best way to configure the delay_factor to some high value, e.g. if there are multiple connections done at the same time (like via Nornir) - it can delay other operations when a router is really not reachable (and the connection should fait/timeout) -> this would cause long delay before proceeding to other tasks or even failures in other time depended tasks.

[m1009d]

Hi @stvnkem ,
thank you for your reply, I am using this workaround directly in the Huawei.py for now:

...
prompt = self.read_channel()

        if not prompt:
            time.sleep(5)
            prompt = self.read_channel()
            if not prompt:
                raise ValueError('Empty prompt')
...

This is working so far without any problem.

[ktbyers]

I am going to close this as it doesn’t look like there is any additional action to be taken.

If there is still something to do here, just re-open this issue.