I have a USB to serial converter on a Debian Linux computer but the following code produces no output (and no errors)

Code:
#!/usr/bin/python


import serial
import sys
import time

# Set Port
ser = serial.Serial('/dev/ttyUSB0', 57600, 8, 'N', 1, timeout=1)

ser.open()   # open the port.

ser.flushInput()
ser.flushOutput()

reset = "Hello World"
ser.write(reset)

time.sleep(.5)

print 'serial closed'
ser.close()
What am I doing wrong?