like pv, but for existing files

#!/usr/bin/env python3

filename = "/var/tmp/test"
interval = 10
targetsize = 2*1000*1000*1000*1000 # 2 TB

import os, time, datetime

file = os.open(filename, os.O_RDONLY)
startsize = os.lseek(file, 0, os.SEEK_END)
lastsize = startsize

starttime = time.perf_counter()
lasttime = starttime

time.sleep(interval/10)
while True:
	currentsize = os.lseek(file, 0, os.SEEK_END)
	currenttime = time.perf_counter()
	
	print("{:.1f} kByte/s,\t".format((currentsize-lastsize)/1024/(currenttime-lasttime)), end="")
#	print("written {} Byte,\t".format(currentsize), end="")
#	print("remaining {} Byte,\t".format(targetsize-currentsize), end="")
	print("avg {:.1f} kByte/s,\t".format((currentsize-startsize)/1024/(currenttime-starttime)), end="")
	try:
		seconds = (targetsize-currentsize)/((currentsize-startsize)/(currenttime-starttime))
		if seconds < 0:
			raise Exception()
		eta = datetime.timedelta(seconds=int(seconds))
		print("ETA {}".format(eta))
	except:
		print("ETA inf")
	
	lastsize = currentsize
	lasttime = currenttime
	time.sleep(interval)