A Time object is well-formed if the values of
second are between 0 and 60 (including 0 but not 60) and if
hour is positive.
minute should be integral values, but we might allow
second to have a fraction part.
Requirements like these are called invariants because they should always be true. To put it a different way, if they are not true, something has gone wrong.
Writing code to check invariants can help detect errors and find their causes. For example, you might have a function like
valid_time that takes a Time object and returns False if it violates an invariant:
def valid_time(time): if time.hour < 0 or time.minute < 0 or time.second < 0: return False if time.minute >= 60 or time.second >= 60: return False return True
At the beginning of each function you could check the arguments to make sure they are valid:
def add_time(t1, t2): if not valid_time(t1) or not valid_time(t2): raise ValueError('invalid Time object in add_time') seconds = time_to_int(t1) + time_to_int(t2) return int_to_time(seconds)
Or you could use an assert statement, which checks a given invariant and raises an exception if it fails:
def add_time(t1, t2): assert valid_time(t1) and valid_time(t2) seconds = time_to_int(t1) + time_to_int(t2) return int_to_time(seconds)
assert statements are useful because they distinguish code that deals with normal conditions from code that checks for errors.