How to Detect Keyboard Input from the Terminal in a Script?

How to detect keyboard input in a script from the terminal

There are several ways to detect keyboard input in a script from the terminal, depending on your needs and operating system.

Synchronous/Blocking key capture

This approach blocks the script until a key is pressed, and then returns the key that was pressed.

• For simple input or raw_input, a blocking function which returns text typed by a user once they press a newline.
• For a simple blocking function that waits for the user to press a single key, then returns that key, use the following code.

<code class="python">class _Getch:
    """Gets a single character from standard input.  Does not echo to the
screen. From http://code.activestate.com/recipes/134892/"""
    def __init__(self):
        try:
            self.impl = _GetchWindows()
        except ImportError:
            try:
                self.impl = _GetchMacCarbon()
            except(AttributeError, ImportError):
                self.impl = _GetchUnix()

    def __call__(self): return self.impl()


class _GetchUnix:
    def __init__(self):
        import tty, sys, termios # import termios now or else you'll get the Unix version on the Mac

    def __call__(self):
        import sys, tty, termios
        fd = sys.stdin.fileno()
        old_settings = termios.tcgetattr(fd)
        try:
            tty.setraw(sys.stdin.fileno())
            ch = sys.stdin.read(1)
        finally:
            termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
        return ch

class _GetchWindows:
    def __init__(self):
        import msvcrt

    def __call__(self):
        import msvcrt
        return msvcrt.getch()

class _GetchMacCarbon:
        """
        A function which returns the current ASCII key that is down;
        if no ASCII key is down, the null string is returned.  The
        page http://www.mactech.com/macintosh-c/chap02-1.html was
        very helpful in figuring out how to do this.
        """
        def __init__(self):
            import Carbon
            Carbon.Evt #see if it has this (in Unix, it doesn't)

        def __call__(self):
            import Carbon
            if Carbon.Evt.EventAvail(0x0008)[0]==0: # 0x0008 is the keyDownMask
                return ''
            else:
                #
                # The event contains the following info:
                # (what,msg,when,where,mod)=Carbon.Evt.GetNextEvent(0x0008)[1]
                #
                # The message (msg) contains the ASCII char which is
                # extracted with the 0x000000FF charCodeMask; this
                # number is converted to an ASCII character with chr() and
                # returned
                #
                (what,msg,when,where,mod)=Carbon.Evt.GetNextEvent(0x0008)[1]
                return chr(msg & 0x000000FF)


def getKey():
    inkey = _Getch()
    import sys
    for i in xrange(sys.maxint):
        k=inkey()
        if k'':break

    return k</code>

Asynchronous key capture

• A callback that is called with the pressed key whenever the user types a key into the command prompt, even when typing things into an interpreter (a keylogger)
• A callback that is called with the typed text after the user presses enter (a less realtime keylogger)

Polling

• The user simply wants to be able to do something when a key is pressed, without having to wait for that key (so this should be non-blocking). Thus they call a poll() function and that either returns a key, or returns None. This can either be lossy (if they take too long to between poll they can miss a key) or non-lossy (the poller will store the history of all keys pressed, so when the poll() function requests them they will always be returned in the order pressed).
• The same as above, except that poll only returns something once the user presses a newline.

Robots

• These are something that can be called to programmatically fire keyboard events. This can be used alongside key captures to echo them back out to the user

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