cpython/Doc/library/telnetlib.rst

:mod:`telnetlib` --- Telnet client
==================================

.. module:: telnetlib
   :synopsis: Telnet client class.
.. sectionauthor:: Skip Montanaro <skip@pobox.com>


.. index:: single: protocol; Telnet

The :mod:`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 ``arpa/telnet.h``, with the leading ``TELOPT_`` removed. For symbolic names
of options which are traditionally not included in ``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).


.. class:: Telnet([host[, port[, timeout]]])

   :class:`Telnet` represents a connection to a Telnet server. The instance is
   initially not connected by default; the :meth:`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. The optional
   *timeout* parameter specifies a timeout in seconds for blocking operations
   like the connection attempt (if not specified, or passed as None, the global
   default timeout setting will be used).

   Do not reopen an already connected instance.

   This class has many :meth:`read_\*` methods.  Note that some of them  raise
   :exc:`EOFError` when the end of the connection is read, because they can return
   an empty string for other reasons.  See the individual descriptions below.


.. seealso::

   :rfc:`854` - Telnet Protocol Specification
      Definition of the Telnet protocol.


.. _telnet-objects:

Telnet Objects
--------------

:class:`Telnet` instances have the following methods:


.. method:: Telnet.read_until(expected[, timeout])

   Read until a given string, *expected*, is encountered or until *timeout* seconds
   have passed.

   When no match is found, return whatever is available instead, possibly the empty
   string.  Raise :exc:`EOFError` if the connection is closed and no cooked data is
   available.


.. method:: Telnet.read_all()

   Read all data until EOF; block until connection closed.


.. method:: Telnet.read_some()

   Read at least one byte of cooked data unless EOF is hit. Return ``''`` if EOF is
   hit.  Block if no data is immediately available.


.. method:: Telnet.read_very_eager()

   Read everything that can be without blocking in I/O (eager).

   Raise :exc:`EOFError` if connection closed and no cooked data available.  Return
   ``''`` if no cooked data available otherwise. Do not block unless in the midst
   of an IAC sequence.


.. method:: Telnet.read_eager()

   Read readily available data.

   Raise :exc:`EOFError` if connection closed and no cooked data available.  Return
   ``''`` if no cooked data available otherwise. Do not block unless in the midst
   of an IAC sequence.


.. method:: Telnet.read_lazy()

   Process and return data already in the queues (lazy).

   Raise :exc:`EOFError` if connection closed and no data available. Return ``''``
   if no cooked data available otherwise.  Do not block unless in the midst of an
   IAC sequence.


.. method:: Telnet.read_very_lazy()

   Return any data available in the cooked queue (very lazy).

   Raise :exc:`EOFError` if connection closed and no data available. Return ``''``
   if no cooked data available otherwise.  This method never blocks.


.. method:: Telnet.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 ``SE`` command.
   This method never blocks.


.. method:: Telnet.open(host[, port[, timeout]])

   Connect to a host. The optional second argument is the port number, which
   defaults to the standard Telnet port (23). The optional *timeout* parameter
   specifies a timeout in seconds for blocking operations like the connection
   attempt (if not specified, or passed as None, the global default timeout
   setting will be used).

   Do not try to reopen an already connected instance.


.. method:: Telnet.msg(msg[, *args])

   Print a debug message when the debug level is ``>`` 0. If extra arguments are
   present, they are substituted in the message using the standard string
   formatting operator.


.. method:: Telnet.set_debuglevel(debuglevel)

   Set the debug level.  The higher the value of *debuglevel*, the more debug
   output you get (on ``sys.stdout``).


.. method:: Telnet.close()

   Close the connection.


.. method:: Telnet.get_socket()

   Return the socket object used internally.


.. method:: Telnet.fileno()

   Return the file descriptor of the socket object used internally.


.. method:: Telnet.write(buffer)

   Write a string to the socket, doubling any IAC characters. This can block if the
   connection is blocked.  May raise :exc:`socket.error` if the connection is
   closed.


.. method:: Telnet.interact()

   Interaction function, emulates a very dumb Telnet client.


.. method:: Telnet.mt_interact()

   Multithreaded version of :meth:`interact`.


.. method:: Telnet.expect(list[, 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 :exc:`EOFError`.  Otherwise,
   when nothing matches, return ``(-1, None, text)`` where *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 ``.*``) or if more
   than one expression can match the same input, the results are indeterministic,
   and may depend on the I/O timing.


.. method:: Telnet.set_option_negotiation_callback(callback)

   Each time a telnet option is read on the input flow, this *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.


.. _telnet-example:

Telnet Example
--------------

.. sectionauthor:: Peter Funk <pf@artcom-gmbh.de>


A simple example illustrating typical use::

   import getpass
   import telnetlib

   HOST = "localhost"
   user = 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())