Off topic:New To Python And Putty: Help!

Question

[Yasmine]

Can someone explain to me how to test the program that I am writing in my command line? For example, I have to create a program that takes two or three parameters (an optional first parameter) and I am trying to create error messages for those who will be using the program but when I put in my parameters I don't get any message.

For example..

My program should print an error message and quit if: the inputfile does not end with .genes the outputfile does not end with .fa or .fasta

I have written in wings...

def extractGenes(variable,input,output):
        variable = False
        if '-s' in sys.argv[1]:
                variable = True
        if not input.endswith(".genes"):
                print("Incorrect file type.")
                sys.exit(1)
        if not output.endswith(".fa") or output.endswith(".fasta"):
                print("Incorrect file type.")
                sys.exit(1)

I think this should work? but when I go to the putty command line to test it by writing python extractGenes.py (myinputfile) (myoutputfile), nothing happens.