i see from the below link it says "cannot re-open an already existing telnet instance" using Telnetlib. Can this be fixed in the later versions of python or is there any work around for this. 

I'm surprised that this is something which PERL supports and not python .

When I try to open a new session which is already opened the telnet server asks whether to open in a.read only or b.read/write mode and when i try to write 'a' it doesn't take and program fails.

my code:
tn = telnetlib.Telnet(HOST,PORT)
print("established connection")
print(tn.read_until(">> ".encode('ascii')))
tn.write(("a".encode('ascii')))

output:
established connection
b'\r\n a. Connect to Port read/write\r\n b. Connect to Port read only\r\n c. Do not connect, drop this connection request\r\n d. Look at port log file\r\n>> '

Which link are you referring to?

I don't see any code you've provided where you're reopening a connection. It looks like you're opening a new connection, reading, writing, and nothing else.

It also looks like the output you show is coming from the remote server, not anything on the client side in Python.

And, as the example code in the documentation shows, you should probably be using a "with" statement.

Hi Eric,

Apologize for not providing the link. Below is the link which i was referring to:
https://docs.python.org/3.1/library/telnetlib.html

I have an already opened telnet session(established by some other user). Now if I again try to open a telnet session to the same serve, my server will populate me with some options like below:

a. Connect to Port read/write
b. Connect to Port read only
c. Do not connect, drop this connection request
d. Look at port log file
>> 

So python telnetlib module doesn't handle these options even though i use below code. 
#establishing connectino to the remote host 
tn = telnetlib.Telnet(HOST,PORT)
# remote server will populate the above given options and stop at '>>', so i'm using the below line to read '>>' and write 'a'
print(tn.read_until(">> ".encode('ascii')))
tn.write(("a".encode('ascii')))

i have created this issue after trying all possible ways and searching the web for a whole day. 
Let me know if there is any way to handle this.

This sounds like an application-level issue, not a telnetlib problem. As far as I can tell, you're opening a new connection, not reusing an existing connection.

telnetlib provides a low level interface to the telnet protocol.  The dialog you mention appears to be transmitted on the telnet connection, so it is unlikely there is any bug or missing feature in telnetlib that would affect your problem.

The sentence in the docs you appear to be referring to ("Do not reopen an already connected instance.") is just telling you that once you have used to telnetlib to open a connection, don't call open again.  It has nothing to say about a telnet connection made by some other program.

So, you have an application level problem and should seek help on the python-list mailing list or some other general help forum.  Unless you can identify something *specific* in telnetlib (bug or missing feature) that prevents you from solving your problem, I don't think there's anything more to be done here.

(As an aside, have you tried sending /r/n after the a?)

To clarify: don't call open again *on that telnetlib.Telnet object*.  You can certainly have more than one open connection using different telnetlib.Telnet instances, though it might be a bit challenging to manage them :)