WithFunctionWhile

my_input = ""
CowID = []

def validate():
    ID = input("What is the cow's ID code?")
    if len(ID) == 3:
        return ID
    else:
        print("Cow ID has been entered wrongly.")
        return ""

while my_input =="":
   my_input = validate()
CowID.append(my_input)