How to run your functions when receiving a signal from a comport in python?

Question

Python

Pseudocode:

connectionToPort = createConnection(COM3, 9600)
connectionToPort.setHandler(callbackFunction)

callbackFunction(data){
   if(data==SIGNAL_START){
      doSomethingOnStart()
   }else if(data == SIGNAL_ANOTHER){
         doSomethingOnAnother()
   }else{
       doSomethingOnUnKnown()
   }
}