Python: Is isinstance() necessary in this case?

Question

I defined a class Time that has three int attributes: hrs, min, sec

And I defined methods intToTime() that convert a Time instance to an int, which is the number of seconds in that time, and also a method timeToInt() that do the reverse.

I'd like them to implement __add__, so I can do things like "TimeA + TimeB" or "TimeA + 100", where 100 is the number of seconds add to TimeA.

As I'd like to merge these two (since there's no overloading in Python),

def __add__(self,num):
    return Time.intToTime(self,Time.timeToInt(self)+num)

def __add__(self,other):
    return Time.intToTime(self,Time.timeToInt(self)+Time.timeToInt(other))

"num" is supposed to be an int, "other" is another Time instance. I know one way using isinstance().

But my question is, in this case, how should I implement such an add without using isinstance()?

Answer 1

You really have two choices: EAFP or LYBL. EAFP (easier to ask forgiveness than permission) means use try/except:

def __add__(self, other):
   try:
       return Time.intToTime(self, Time.timeToInt(self)+Time.timeToInt(other))
   except AttributeError as e:
       return Time.intToTime(self, Time.timeToInt(self) + other)

Note that Time.timeToInst(self) is kind of weird; you would normally write that self.timeToInt().

LYBL means look before you leap - i.e. isinstance. You already know that one.

Answer 2

You'd better make intToTime and timeToInt module level functions, the same level as your class Time, and implement your __add__ like this:

def __add__(self, num):
    if isinstance(num, Time):
        num=timeToInt(num)
    elif not isinstance(num, int):
        raise TypeError, 'num should be an integer or Time instance'
    return intToTime(timeToInt(self)+num)