:mod:`pty` --- Pseudo-terminal utilities

.. module:: pty
   :platform: Unix
   :synopsis: Pseudo-Terminal Handling for Unix.

.. moduleauthor:: Steen Lumholt

.. sectionauthor:: Moshe Zadka <moshez@zadka.site.co.il>

Source code: :source:`Lib/pty.py`

The :mod:`pty` module defines operations for handling the pseudo-terminal concept: starting another process and being able to write to and read from its controlling terminal programmatically.

Pseudo-terminal handling is highly platform dependent. This code is mainly tested on Linux, FreeBSD, and macOS (it is supposed to work on other POSIX platforms but it's not been thoroughly tested).

The :mod:`pty` module defines the following functions:

.. function:: fork(mode=None, winsz=None)

   Fork. Connect the child's controlling terminal to a pseudo-terminal. Return
   value is ``(pid, fd)``. Note that the child  gets *pid* 0, and the *fd* is
   *invalid*. The parent's return value is the *pid* of the child, and *fd* is a
   file descriptor connected to the child's controlling terminal (and also to the
   child's standard input and output). The *mode* argument, which is expected
   to be a termios attribute list like the one returned by
   :func:`termios.tcgetattr`, will be applied to the slave end. The *winsz*
   argument, which is expected to be a winsize pair like the one returned by
   :func:`termios.tcgetwinsize`, will be applied to the slave end.

.. function:: openpty(mode=None, winsz=None, name=False)

   Open a new pseudo-terminal pair, using :func:`os.openpty` if possible, or
   emulation code for generic Unix systems. The *mode* argument, which is
   expected to be a termios attribute list like the one returned by
   :func:`termios.tcgetattr`, will be applied to the slave end. The *winsz*
   argument, which is expected to be a winsize pair like the one returned by
   :func:`termios.tcgetwinsize`, will be applied to the slave end. If *name*
   is false, return a pair of file descriptors ``(master, slave)``, for the
   master and the slave end, respectively. Otherwise, return
   ``(master, slave, name)``, where the additional value is the filename of
   the slave.

.. function:: spawn(argv[, master_read[, stdin_read]], slave_echo=True, \
                    handle_winch=False)

   Spawn a process, and connect its controlling terminal with the current
   process's standard io. This is often used to baffle programs which insist on
   reading from the controlling terminal. It is expected that the process
   spawned behind the pty will eventually terminate, and when it does *spawn*
   will return.

   A loop copies STDIN of the current process to the child and data received
   from the child to STDOUT of the current process. It is not signaled to the
   child if STDIN of the current process closes down.

   The functions *master_read* and *stdin_read* are passed a file descriptor
   which they should read from, and they should always return a byte string. In
   order to force spawn to return before the child process exits an
   empty byte array should be returned to signal end of file.

   The default implementation for both functions will read and return up to 1024
   bytes each time the function is called. The *master_read* callback is passed
   the pseudoterminal’s master file descriptor to read output from the child
   process, and *stdin_read* is passed file descriptor 0, to read from the
   parent process's standard input.

   Returning an empty byte string from either callback is interpreted as an
   end-of-file (EOF) condition, and that callback will not be called after
   that. If *stdin_read* signals EOF the controlling terminal can no longer
   communicate with the parent process OR the child process. Unless the child
   process will quit without any input, *spawn* will then loop forever. If
   *master_read* signals EOF the same behavior results (on linux at least).

   The ECHO termios attribute of the slave end is turned on or off based on
   the value of the argument *slave_echo* being true or false respectively.

   If *spawn* is called from the main thread, then a handler for
   :const:`signal.SIGWINCH` will be installed if *handle_winch* is true, if
   the pair of constants
   (:const:`termios.TIOCGWINSZ`, :const:`termios.TIOCSWINSZ`) is defined, and
   if STDIN of the current process is a terminal.

   Return the exit status value from :func:`os.waitpid` on the child process.

   :func:`waitstatus_to_exitcode` can be used to convert the exit status into
   an exit code.

   .. audit-event:: pty.spawn argv pty.spawn

   .. versionchanged:: 3.4
      :func:`spawn` now returns the status value from :func:`os.waitpid`
      on the child process.

Example

.. sectionauthor:: Steen Lumholt

The following program acts like the Unix command :manpage:`script(1)`, using a pseudo-terminal to record all input and output of a terminal session in a "typescript".

import argparse
import os
import pty
import sys
import time

parser = argparse.ArgumentParser()
parser.add_argument('-a', dest='append', action='store_true')
parser.add_argument('-p', dest='use_python', action='store_true')
parser.add_argument('filename', nargs='?', default='typescript')
options = parser.parse_args()

shell = sys.executable if options.use_python else os.environ.get('SHELL', 'sh')
filename = options.filename
mode = 'ab' if options.append else 'wb'

with open(filename, mode) as script:
    def read(fd):
        data = os.read(fd, 1024)
        script.write(data)
        return data

    print('Script started, file is', filename)
    script.write(('Script started on %s\n' % time.asctime()).encode())

    pty.spawn(shell, read)

    script.write(('Script done on %s\n' % time.asctime()).encode())
    print('Script done, file is', filename)

Uh oh!