题目描述：

ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 4 digits or exactly 6 digits.

If the function is passed a valid PIN string, return true, else return false.

eg:

validate_pin("1234") == True
validate_pin("12345") == False validate_pin("a234") == False


我的解答：

def invalid_pin(pin):
    if str(pin).startswith("-") or str(pin).find(".") >= 1:
        return False
    elif str(pin).isdigit() and len(str(pin)) == 4 or len(str(pin)) == 6:
        return True
    else:
        return False