I encountered the same problem and found a solution.
I think this is a mistake in the sample program.

Although the variable `count' in the function i2c_on_transmit must be global it is interpreted as a local variable.

I have modified the i2c_on_transmit() as follows.

def i2c_on_transmit()
  global count
  count = count + 1
  print ("on_transmit, send:", count)
  return count

Then, it worked well.