Insert a variable in a Python > Cellular AT command

[ElectronicsNut]

I am an ElectronicsNut that is venturing into foreign coding territory. I have a Raspberry Pi that I have connected to a FONA 3G cellular module via port ttyUSB0. The attached code is up and running, but I would like to modify the line of code in Python script with a variable that would be used to specify a specific ALERT. The current test message that is sent to my cell phone in the code is "ALERT MESSAGE" (for testing purposes), which I would like to make a variable called "Alert Message". Based on the coding that I write, the Alert Messages could be:
Fridge Low Temperature Alert
Freezer Low Temperature Alert
Low Water Pressure Alert
No AC voltage
.....
I just need to figure out how to insert the variable in that line of code (see below).
I left the rcv and print lines in for testing purposes.
Thanks in advance for any assistance

import serial
import RPi.GPIO as GPIO
import os, time


port = serial.Serial("/dev/ttyUSB0", baudrate=115200, timeout=1)
rcv = port.read(10)
print rcv

port.write('AT'+'\r\n')
rcv = port.read(10)
print rcv

port.write('ATE0'+'\r\n')
rcv = port.read(10)
print rcv 

port.write('AT+CMGF=1'+'\r\n')
rcv = port.read(10)
print rcv

port.write('AT+CMGS="xxxxxxxxxx" '+'\r\n')
rcv = port.read(10)
print rcv

[b]port.write('ALERT MESSAGE'+'\r\n')[/b]
rcv = port.read(10)
print rcv

port.write("\x1A")
rcv = port.read(10)
print rcv

port.close()