Resetting imported Python module variables

Question

I'm trying to understand why instantiated module variables will not reset after they have been modified once after they are imported. I've been working with minimalmodbus , and I am trying to reset the Baud rate if the default does not match the connected device's. Setting my own default, I cannot get the re-initialization of of minimalmodbus to change the baud rate. For example:

import minimalmodbus
minimalmodbus.BAUDRATE=9600
comm=minimalmodbus.Instrument('COM4',1) #baud rate set to 9600 here for comm
minimalmodbus.BAUDRATE=19200
comm=minimalmodbus.Instrument('COM4',1) #attempting to change baud rate
print comm #displays all information, and showing that baudrate=9600, not 19200

I've had this issue using several other modules and I would really like to understand why this is occurring.

Answer 1

The first time you use a given serial port, minimalmodbus creates a serial.Serial instance using the current value of BAUDRATE and saves it :

def __init__(self, port, slaveaddress, mode=MODE_RTU):
    if port not in _SERIALPORTS or not _SERIALPORTS[port]:
        self.serial = _SERIALPORTS[port] = serial.Serial(port=port, baudrate=BAUDRATE, parity=PARITY, bytesize=BYTESIZE, stopbits=STOPBITS, timeout=TIMEOUT)
    else:
        self.serial = _SERIALPORTS[port]
        if self.serial.port is None:
            self.serial.open()
    ...

Even if BAUDRATE changes later, future attempts to use that serial port will use the old serial.SERIAL instance with the old baud rate.

I don't know what the Modbus protocol is like or how you're supposed to use this module, so I can't tell you how you're supposed to do what you're trying to do or whether it's a good idea. In any case, now you know what's happening.