21.19. "telnetlib" --- Telnet client
************************************

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

======================================================================

The "telnetlib" module provides a "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 telnetlib.Telnet(host=None, port=0[, timeout])

   "Telnet" represents a connection to a Telnet server. The instance
   is initially not connected by default; the "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 "read_*()" methods.  Note that some of them
   raise "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 "Telnet" object is a context manager and can be used in a "with"
   statement.  When the "with" block ends, the "close()" method is
   called:

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

   3.6 版更變: Context manager support added

也參考:

  **RFC 854** - Telnet Protocol Specification
     Definition of the Telnet protocol.


21.19.1. Telnet Objects
=======================

"Telnet" instances have the following methods:

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 "EOFError" if the connection is closed
   and no cooked data is available.

Telnet.read_all()

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

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.

Telnet.read_very_eager()

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

   Raise "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.

Telnet.read_eager()

   Read readily available data.

   Raise "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.

Telnet.read_lazy()

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

   Raise "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.

Telnet.read_very_lazy()

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

   Raise "EOFError" if connection closed and no data available. Return
   "b''" if no cooked data available otherwise.  This method never
   blocks.

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.

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.

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.

Telnet.set_debuglevel(debuglevel)

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

Telnet.close()

   Close the connection.

Telnet.get_socket()

   Return the socket object used internally.

Telnet.fileno()

   Return the file descriptor of the socket object used internally.

Telnet.write(buffer)

   Write a byte string to the socket, doubling any IAC characters.
   This can block if the connection is blocked.  May raise "OSError"
   if the connection is closed.

   3.3 版更變: This method used to raise "socket.error", which is now
   an alias of "OSError".

Telnet.interact()

   Interaction function, emulates a very dumb Telnet client.

Telnet.mt_interact()

   Multithreaded version of "interact()".

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 (regex 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 "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.

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.


21.19.2. Telnet Example
=======================

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'))
