cups printing

[Barrowman]

I have a raspberrypi I am using as a print server and it has my HP Officejet on a USB port.
I can print to it from libreoffice, gedit and all other programs I use.
I am trying to use python-cups and am getting an error which puzzles me as I have being using code just copied and pasted
and pointing it to a test file.

import cups
conn = cups.Connection()
printers = conn.getPrinters ()
prin = conn.getDefault()
myfile = "/home/norman/Fred.txt"
conn.printFile (prin, myfile, "Project Report", {})

I am running it from python 3.5.2
The error is

Error:
Traceback (most recent call last):
File "./linprint.py", line 6, in <module>
conn.printFile (prin, myfile, "Project Report", {})
TypeError: unicode or bytes object required

I cannot find out what is wrong because the error doesn't seem specific enough.
Can someone explain what is wrong and how to fix it please?