Reading a file in python 3.x as Bytes/Strings for PySerial/Xmodem

Question

I'm trying to read a binary file into a buffer and then transmit it using pyserial/xmodem.

my test code:

send_buf = open('test.py', 'rb') 
xmInst = XMODEM(self.getc, self.putc)
xmInst.send (send_buf)  

xmodem send code:

...
data = stream.read(packet_size)
if not data:
    break/
total_packets += 1
data = data.ljust(packet_size, self.pad)  

However, when it executes that last line it says 'must be a byte string of length 1, not str'. Presumably a byte / string / unicode issue?

The xmodem package was written for python 2.7, so how do I read/pass the file in Python 3.4 such that xmodem can work with it?