Duncan Booth wrote:
> Alex Martelli wrote:
>
>
>>>What I think I'm trying to get at is that I believe that most
>>>situations where someone actually tries to do something in the base
>>>initialiser which requires calling a virtual method are probably also
>>>cases where the initialiser is doing too much: i.e. separating the
>>>construction/initialisation from actually doing something is usually
>>>a good idea.
>>
>>But why should that be? Template Method is perhaps the MOST generally
>>useful design pattern -- why would it be any less useful in
>>initialization than elsewhere?!
>>
>
> Because it is error prone?
Programming *is* error prone.
> Any method which is called from the constructor/initialiser has to operate
> correctly
any method has to operate correctly anyway !-)
> on an object which at that point is not fully
> constructed/initialised.
In Python, when the __init__ method is called, the object is at least
fully constructed.
> So instead of having to write a method on a Foo
> object, your template method has to operate on a partial Foo. The danger is
> that you haven't clearly defined the partial Foo interface sufficiently and
> the method tries to use other parts of the object which haven't yet been
> set up.
If so, the worse thing that can happen is an exception - and you'll
surely spot the problem really soon.
> That situation gets worse when you have a class hierarchy as the
> subclass needs to know that it has to do complete its own initialisation
> before constructing the base class instead of afterwards, and if you are
> going to document that requirement, why not do it properly and split the
> construction in two?
It's *already* split : __new__ construct the object, __init__ initialize it.
> That's why I would go for the 2-phase construction:
But that's already what you have.
> after the first phase
> you have an object which is fully initialised, just not yet
> used/connected/running. For example httplib.HTTPConnection does this: you
> construct the object with a host and port, but the actual connection is
> triggered by a separate object.
If you look at file objects, they do try and open the file at init time.
Is a net or db connection that different ?
(snip)
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in '[EMAIL PROTECTED]'.split('@')])"
--
http://mail.python.org/mailman/listinfo/python-list
