Is this problem already solved?
It seems to be a mistake in the sample program.
Although, the variable 'count' in the function i2c_transmit() should be global, it is interpreted as local.
I modified the function 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.