"pty" — Pseudo-terminal utilities
*********************************

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

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

The "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.

Because pseudo-terminal handling is highly platform dependent, there
is code to do it only for Linux. (The Linux code is supposed to work
on other platforms, but hasn’t been tested yet.)

The "pty" module defines the following functions:

pty.fork()

   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).

pty.openpty()

   Open a new pseudo-terminal pair, using "os.openpty()" if possible,
   or emulation code for generic Unix systems. Return a pair of file
   descriptors "(master, slave)", for the master and the slave end,
   respectively.

pty.spawn(argv[, master_read[, stdin_read]])

   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.

   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 "OSError" should be thrown.

   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).

   If both callbacks signal EOF then *spawn* will probably never
   return, unless *select* throws an error on your platform when
   passed three empty lists. This is a bug, documented in issue 26228.

   Raises an auditing event "pty.spawn" with argument "argv".

   Changed in version 3.4: "spawn()" now returns the status value from
   "os.waitpid()" on the child process.


Example
=======

The following program acts like the Unix command *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)
