Class instantiation question

[Matt Brubeck]

Todd Johnson <overdrive_ss at yahoo.com> 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?

Here's one option:

>>> class C:
...     def __init__(self, a):
...             self.a = int(a)
...
>>> c = C("hello")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in __init__
ValueError: invalid literal for int(): hello

This also allows the following:

>>> try: c = C("hello")
... except ValueError: c = None
...
>>> print c
None

If you really want initialization to return None instead of raising an
exception, you can override the __new__ method of a new-style class:

>>> class C(object):
...     def __new__(cls, a):
...             try: a = int(a)
...             except ValueError: return None
...             return object.__new__(cls, a)
...     def __init__(self, a):
...             self.a = a
...
>>> c = C(1)
>>> c.a
1
>>> c = C("hello")
>>> print c
None