def get_valid_command1():
    '''Prompt for and return a string that is a valid command: one of "xxx",
    "yyy", and "zzz".'''
    
    command = ""
    
    while True:
        command = raw_input("Please enter a command: ")
        if command == "xxx" or command == "yyy" or command == "zzz":
            return command
        else:
            print "Invalid command"

def get_valid_command2():
    '''Prompt for and return a string that is a valid command: one of "xxx",
    "yyy", and "zzz".'''
    
    command = raw_input("Please enter a command: ")
    
    # While we don't have a valid command, go get another one.
    while command != "xxx" and command != "yyy" and command != "zzz":
        print "Invalid command"
        command = raw_input("Please enter a command: ")
            
    # We have a valid command!  Return it.
    return command

#def get_valid_command3(valid_commands):
#    '''Prompt for and return a string that is in the list of valid commands.'''


if __name__ == "__main__":
    command = get_valid_command1()
    print command

    #command = get_valid_command2()
    #print command