A gevent-based script that copies input to output, but interrupts every second to paint the current time at the top of the screen. May change the time format with an optional argument.

clock_overlay.py

"""
A gevent-based script that copies input to output, but interrupts every second to paint the current time at the top of the screen.
May change the time format with an optional argument.
Time format is as per the date(1) command, as well as the following escapes:
\e -> ESC character
\t -> Tab
\\ -> Literal \

For example, you could color your time output bold blue with the argument '\e[1;34m%X\e[m'
"""

import sys, os
from time import strftime
from time import sleep
from signal import signal, SIGALRM, setitimer, ITIMER_REAL
from errno import EINTR

format_escapes = {
	r'\e': '\x1b',
	r'\t': '\t',
	r'\n': '\x1b[B\x1b[G',
	r'\\': '\\'
}

def main(progname, _time_format='%X'):
	global time_format
	time_format = replace_all(_time_format, format_escapes)
	signal(SIGALRM, on_alarm)
	setitimer(ITIMER_REAL, 1, 1)
	patrick()

def on_alarm(signum, frame):
	s = "\x1b[s\x1b[H\x1b[2K" + strftime(time_format) + "\x1b[B\x1b[2K\x1b[u"
	while s:
		n = os.write(sys.stdout.fileno(), s)
		s = s[n:]

def patrick():
	c = None
	while 1:
		try:
			n = 0
			c = sys.stdin.read(1) if not c else c
			if not c:
				break
			n = os.write(sys.stdout.fileno(), c)
		except (OSError, IOError), ex:
			if ex.errno != EINTR:
				raise
		if n: c = None

def replace_all(s, replacements):
	for item in replacements.items():
		s = s.replace(*item)
	return s

if __name__=='__main__':
	sys.exit(main(*sys.argv))