# `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.

  Distinto en la versión 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)