Class instantiation question

[David C. Fox]

Todd Johnson wrote:

> Ok, say I have a class MyClass and an __init__(self,
> a, b)  Say that a and b are required to be integers
> for example. So my init looks like:
> 
> __init__(self, a, b):
>     try:
>         self.one = int(a)
>         self.two = int(b)
>     except ValueError:
>         #nice error message here
>         return None
> 
> I have even tried a similar example with if-else
> instead of try-except, but no matter what if I call
> 
> thisInstance = MyClass(3, "somestring")
> 
> it will set self.one to 3 and self.two will be
> uninitialised. The behavior I am hoping for, is that
> thisInstance is not created instead(or is None). How
> do I get the behavior I am looking for?

As far as I know, the only way to prevent __init__ from creating an 
instance is for __init__ to raise an exception which is NOT caught and 
handled within __init__.  In your example, I think simply doing

class MyClass(self, a, b):
     def __init__(self, a, b):
         self.one = int(a)
         self.two = int(b)

without the try...except block should do it.  Then the calling code can 
handle the ValueError appropriately.

David