Exception ignored in: <function Telnet.__del__ at 0x7faeb8656ee0>
Traceback (most recent call last):
  File "/usr/lib/python3.8/telnetlib.py", line 239, in __del__
    self.close()
  File "/usr/lib/python3.8/telnetlib.py", line 265, in close
    sock = self.sock
AttributeError: 'TelnetSubclass' object has no attribute 'sock'

import telnetlib

ALIAS_TO_HOST = {
    "alias1": "localhost",
    "alias2": "anotherhost"
}

class AliasTelnet(telnetlib.Telnet):

#    sock = None  # this mitigates the problem

    def __init__(self, alias):
        host = ALIAS_TO_HOST.get(alias)
        if not host:
#            self.sock = None  # alternatively this mitigates the problem
            raise ValueError(f'cannot map alias "{alias}" to host name')

        super().__init__(host)


# this works
with AliasTelnet("alias1") as tn:
    print(tn.read_some().decode())

# this fails
with AliasTelnet("thisisnotaknownalias") as tn:
    pass

    def close(self):
        """Close the connection."""
        try:
            sock = self.sock
        except AttributeError:
            sock = None
       ...

class Telnet:

    sock = None