Python - string to hexbytes conversion - HELP!

[me]

I'm a newbie to Python and I am trying
to convert a string of decimal numbers - read from raw_input
to a series of hexbytes
eg
if I do a read and get a='1234567'
I need  an equivalent string to be
b=Chr(0x01)+chr(0x23)+chr(x045)+chr(0x67)

all this is to control a device from the serial port
but I cannot seem to recall how to conver the input to my required output
An hints or suggestion most appreciated! I've spend hours trying various
built in's
which I would prefer to use

HELP!