7.5: Debugging

A Time object is well-formed if the values of minute and second are between 0 and 60 (including 0 but not 60) and if hour is positive. hour and 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, then something has gone wrong.

Writing code to check your invariants can help you 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


Then 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.