\section{\module{telnetlib} ---
         Telnet client}

\declaremodule{standard}{telnetlib}
\modulesynopsis{Telnet client class.}
\sectionauthor{Skip Montanaro}{skip@mojam.com}

\index{protocol!Telnet}

The \module{telnetlib} module provides a \class{Telnet} class that
implements the Telnet protocol.  See \rfc{854} for details about the
protocol. In addition, it provides symbolic constants for the protocol
characters (see below), and for the telnet options. The
symbolic names of the telnet options follow the definitions in
\code{arpa/telnet.h}, with the leading \code{TELOPT_} removed. For
symbolic names of options which are traditionally not included in
\code{arpa/telnet.h}, see the module source itself.

The symbolic constants for the telnet commands are: IAC, DONT, DO,
WONT, WILL, SE (Subnegotiation End), NOP (No Operation), DM (Data
Mark), BRK (Break), IP (Interrupt process), AO (Abort output), AYT
(Are You There), EC (Erase Character), EL (Erase Line), GA (Go Ahead),
SB (Subnegotiation Begin).


\begin{classdesc}{Telnet}{\optional{host\optional{, port}}}
\class{Telnet} represents a connection to a Telnet server. The
instance is initially not connected by default; the \method{open()}
method must be used to establish a connection.  Alternatively, the
host name and optional port number can be passed to the constructor,
to, in which case the connection to the server will be established
before the constructor returns.

Do not reopen an already connected instance.

This class has many \method{read_*()} methods.  Note that some of them 
raise \exception{EOFError} when the end of the connection is read,
because they can return an empty string for other reasons.  See the
individual descriptions below.
\end{classdesc}


\begin{seealso}
  \seerfc{854}{Telnet Protocol Specification}{
          Definition of the Telnet protocol.}
\end{seealso}



\subsection{Telnet Objects \label{telnet-objects}}

\class{Telnet} instances have the following methods:


\begin{methoddesc}{read_until}{expected\optional{, timeout}}
Read until a given string, \var{expected}, is encountered or until
\var{timeout} seconds have passed.

When no match is found, return whatever is available instead,
possibly the empty string.  Raise \exception{EOFError} if the connection
is closed and no cooked data is available.
\end{methoddesc}

\begin{methoddesc}{read_all}{}
Read all data until \EOF; block until connection closed.
\end{methoddesc}

\begin{methoddesc}{read_some}{}
Read at least one byte of cooked data unless \EOF{} is hit.
Return \code{''} if \EOF{} is hit.  Block if no data is immediately
available.
\end{methoddesc}

\begin{methoddesc}{read_very_eager}{}
Read everything that can be without blocking in I/O (eager).

Raise \exception{EOFError} if connection closed and no cooked data
available.  Return \code{''} if no cooked data available otherwise.
Do not block unless in the midst of an IAC sequence.
\end{methoddesc}

\begin{methoddesc}{read_eager}{}
Read readily available data.

Raise \exception{EOFError} if connection closed and no cooked data
available.  Return \code{''} if no cooked data available otherwise.
Do not block unless in the midst of an IAC sequence.
\end{methoddesc}

\begin{methoddesc}{read_lazy}{}
Process and return data already in the queues (lazy).

Raise \exception{EOFError} if connection closed and no data available.
Return \code{''} if no cooked data available otherwise.  Do not block
unless in the midst of an IAC sequence.
\end{methoddesc}

\begin{methoddesc}{read_very_lazy}{}
Return any data available in the cooked queue (very lazy).

Raise \exception{EOFError} if connection closed and no data available.
Return \code{''} if no cooked data available otherwise.  This method
never blocks.
\end{methoddesc}

\begin{methoddesc}{read_sb_data}{}
Return the data collected between a SB/SE pair (suboption begin/end).
The callback should access these data when it was invoked with a
\code{SE} command. This method never blocks.

\versionadded{2.3}
\end{methoddesc}

\begin{methoddesc}{open}{host\optional{, port}}
Connect to a host.
The optional second argument is the port number, which
defaults to the standard Telnet port (23).

Do not try to reopen an already connected instance.
\end{methoddesc}

\begin{methoddesc}{msg}{msg\optional{, *args}}
Print a debug message when the debug level is \code{>} 0.
If extra arguments are present, they are substituted in the
message using the standard string formatting operator.
\end{methoddesc}

\begin{methoddesc}{set_debuglevel}{debuglevel}
Set the debug level.  The higher the value of \var{debuglevel}, the
more debug output you get (on \code{sys.stdout}).
\end{methoddesc}

\begin{methoddesc}{close}{}
Close the connection.
\end{methoddesc}

\begin{methoddesc}{get_socket}{}
Return the socket object used internally.
\end{methoddesc}

\begin{methoddesc}{fileno}{}
Return the file descriptor of the socket object used internally.
\end{methoddesc}

\begin{methoddesc}{write}{buffer}
Write a string to the socket, doubling any IAC characters.
This can block if the connection is blocked.  May raise
\exception{socket.error} if the connection is closed.
\end{methoddesc}

\begin{methoddesc}{interact}{}
Interaction function, emulates a very dumb Telnet client.
\end{methoddesc}

\begin{methoddesc}{mt_interact}{}
Multithreaded version of \method{interact()}.
\end{methoddesc}

\begin{methoddesc}{expect}{list\optional{, timeout}}
Read until one from a list of a regular expressions matches.

The first argument is a list of regular expressions, either
compiled (\class{re.RegexObject} instances) or uncompiled (strings).
The optional second argument is a timeout, in seconds; the default
is to block indefinitely.

Return a tuple of three items: the index in the list of the
first regular expression that matches; the match object
returned; and the text read up till and including the match.

If end of file is found and no text was read, raise
\exception{EOFError}.  Otherwise, when nothing matches, return
\code{(-1, None, \var{text})} where \var{text} is the text received so
far (may be the empty string if a timeout happened).

If a regular expression ends with a greedy match (such as \regexp{.*})
or if more than one expression can match the same input, the
results are indeterministic, and may depend on the I/O timing.
\end{methoddesc}

\begin{methoddesc}{set_option_negotiation_callback}{callback}
Each time a telnet option is read on the input flow, this
\var{callback} (if set) is called with the following parameters :
callback(telnet socket, command (DO/DONT/WILL/WONT), option).  No other
action is done afterwards by telnetlib.
\end{methoddesc}


\subsection{Telnet Example \label{telnet-example}}
\sectionauthor{Peter Funk}{pf@artcom-gmbh.de}

A simple example illustrating typical use:

\begin{verbatim}
import getpass
import sys
import telnetlib

HOST = "localhost"
user = raw_input("Enter your remote account: ")
password = getpass.getpass()

tn = telnetlib.Telnet(HOST)

tn.read_until("login: ")
tn.write(user + "\n")
if password:
    tn.read_until("Password: ")
    tn.write(password + "\n")

tn.write("ls\n")
tn.write("exit\n")

print tn.read_all()
\end{verbatim}