import argparse
import datetime
import time

def _positive(s):
    try:
        i = int(s)
        if i > 0:
            return i
    except ValueError:
        pass
    raise argparse.ArgumentError('must be positive integer')

parser = argparse.ArgumentParser('Copies a file slowly')
parser.add_argument('--rate', default=1024, type=int, help='Number of bytes to copy per second')
parser.add_argument('src', metavar='SRC', type=argparse.FileType('r'), help='Source file')
parser.add_argument('dst', metavar='DST', type=argparse.FileType('w'), help='Destination file')

args = parser.parse_args()
_READ_SIZE=8192
start_time = time.time()
bytes_copied = 0
while True:
    bytes = args.src.read(_READ_SIZE)
    if not bytes:
        break
    args.dst.write(bytes)
    bytes_copied += len(bytes)
    target_time = start_time + float(bytes_copied) / args.rate
    now = time.time()
    if target_time > now:
        time.sleep(target_time - now)
args.dst.close()
args.src.close()