def getAnswer():
  result = "" # step 1

  while True: 
    result = input("Your answer [yes/no]: ")
    if result == "yes": break 
    if result == "no": break 
    print("Let's try this again.")
  
  return result # step 3 
# getAnswer ends here

# main program starts here  
...do stuff...  
if getAnswer() == "yes":
  ...do stuff...