Home Original page

Class instantiation question

anton muhin antonmuhin.REMOVE.ME.FOR.REAL.MAIL at rambler.ru
Tue Oct 7 15:06:59 EDT 2003 
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?
> 
> Thanks in advance,
> Todd

If I'm correct, __init__ method is somewhat different from what you 
expect: it *doesn't* return values (None is just a fake return value). 
Actually MyClass(...) proceeds more or less in the following way:

1) create an object
2) call __init__ with parameters passed
3) return the object's reference.

Therefore, you cannot prevent object creation in __init__. You can only 
throw an exception.

Actually, I see no reason to use if/then instead try/except or maybe 
even simple self.one = int(a) (that will throw if anythins goes wrong), 
but if you want, for example, to create another object depending on 
parameters passed, take a look at __new__ method.

regards,
anton.
More information about the Python-list mailing list