telnet login not reliably working in Netmiko 3.3.2 with recent performance improvements

I am unable to login to a cisco switch using telnet protocol using netmiko 3.3.2, it is throwing AuthenticationException but my credentials seems to be correct as I am able to login using the same credentials using 1.4.0 version of netmiko

Code:

dev = {
     'device_type':'cisco_ios_telnet',
     'host':   'myip',
     'username': 'uuuu',
     'password': 'pppppppp',
     'secret':'pppppp',

conn = ConnectHandler(**dev)
}

Here is the error trace:

Traceback (most recent call last):
  File "nigeria_switch.py", line 12, in <module>
    conn = ConnectHandler(**dev)
  File "/home/sanidhya/Py/email_parsing/env/lib/python3.8/site-packages/netmiko/ssh_dispatcher.py", line 312, in ConnectHandler
    return ConnectionClass(*args, **kwargs)
  File "/home/sanidhya/Py/email_parsing/env/lib/python3.8/site-packages/netmiko/cisco/cisco_ios.py", line 17, in __init__
    return super().__init__(*args, **kwargs)
  File "/home/sanidhya/Py/email_parsing/env/lib/python3.8/site-packages/netmiko/base_connection.py", line 346, in __init__
    self._open()
  File "/home/sanidhya/Py/email_parsing/env/lib/python3.8/site-packages/netmiko/base_connection.py", line 351, in _open
    self.establish_connection()
  File "/home/sanidhya/Py/email_parsing/env/lib/python3.8/site-packages/netmiko/base_connection.py", line 910, in establish_connection
    self.telnet_login()
  File "/home/sanidhya/Py/email_parsing/env/lib/python3.8/site-packages/netmiko/cisco_base_connection.py", line 171, in telnet_login
    raise NetmikoAuthenticationException(msg)
netmiko.ssh_exception.NetmikoAuthenticationException: Login failed: myip

Additional Information: python version: Python 3.8.5

[jtraub91]

Just wanted to mention that I was seeing the same thing until fast_cli: False provided a remedy

[ktbyers]

Okay, let’s leave this open as there is an underlying bug that I need to fix.