Clean way to not get object back from instantiation attempt gone bad

[Ben Finney]

tobiah <st at tobiah.org> writes:

> myfoo = Foo('grapes', 'oranges')
> 
> And in the __init__() of Foo, there is a real problem with the
> consumption of fruit.  Is there a clean way to ensure that myfoo
> will be None after the call?  Would the __init__() just do
> del(self), or is there a better way to think about this?

I'm not sure I understand your goal, but it seems that you want to
handle two cases: one where Foo can be properly initialised, and one
where it can't -- an error condition.

That sounds like a job for exceptions.

    >>> class ConsumptionError(ValueError):
    ...     """ Exception thrown from bad consumption """
    ...
    >>> class Foo(object):
    ...     def __init__(self, left_thing, right_thing):
    ...         if left_thing == 'grapes':
    ...             raise ConsumptionError("Problem with consumption of fruit")
    ...

    >>> myfoo = Foo('zucchini', 'spam')
    >>> print myfoo
    <__main__.Foo object at 0x401e446c>

    >>> myfoo = Foo('grapes', 'oranges')
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 4, in __init__
    __main__.ConsumptionError: Problem with consumption of fruit

    >>> try:
    ...     myfoo = Foo('grapes', 'oranges')
    ... except ConsumptionError, e:
    ...     myfoo = None
    ...
    >>> print myfoo
    None

-- 
 \     Rommel: "Don't move, or I'll turn the key on this can of Spam!" |
  `\                             -- The Goon Show, _Rommel's Treasure_ |
_o__)                                                                  |
Ben Finney