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

.. module:: telnetlib
   :synopsis: Telnet client class.

.. sectionauthor:: Skip Montanaro <skip@pobox.com>

**Source code:** :source:`Lib/telnetlib.py`

.. 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=None, port=0[, 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 too, 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, 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.

   A :class:`Telnet` object is a context manager and can be used in a
   :keyword:`with` statement.  When the :keyword:`with` block ends, the
   :meth:`close` method is called::

       >>> from telnetlib import Telnet
       >>> with Telnet('localhost', 23) as tn:
       ...     tn.interact()
       ...

   .. versionchanged:: 3.6 Context manager support added


.. 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=None)

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

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


.. method:: Telnet.read_all()

   Read all data until EOF as bytes; block until connection closed.


.. method:: Telnet.read_some()

   Read at least one byte of cooked data unless EOF is hit. Return ``b''`` 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 ``b''`` 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 ``b''`` 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
   ``b''`` 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
   ``b''`` 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=0[, 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, 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 byte string to the socket, doubling any IAC characters. This can
   block if the connection is blocked.  May raise :exc:`OSError` if the
   connection is closed.

   .. versionchanged:: 3.3
      This method used to raise :exc:`socket.error`, which is now an alias
      of :exc:`OSError`.


.. 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=None)

   Read until one from a list of a regular expressions matches.

   The first argument is a list of regular expressions, either compiled
   (:ref:`regex objects <re-objects>`) or uncompiled (byte 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 bytes read up
   till and including the match.

   If end of file is found and no bytes were read, raise :exc:`EOFError`.
   Otherwise, when nothing matches, return ``(-1, None, data)`` where *data* is
   the bytes received so far (may be empty bytes 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
   non-deterministic, 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(b"login: ")
   tn.write(user.encode('ascii') + b"\n")
   if password:
       tn.read_until(b"Password: ")
       tn.write(password.encode('ascii') + b"\n")

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

   print(tn.read_all().decode('ascii'))

