On Thu, 13 Nov 2008, Faheem Mitha wrote:
> On Wed, 12 Nov 2008, Michael Bayer wrote:
>
>> On Nov 12, 2008, at 3:31 PM, Faheem Mitha wrote:
>>> 4) A more general question:
>>>
>>> I'm having difficulty parsing the last three lines in the itemized list in
>>> Section "associationproxy".
>>>
>>> "The relation is inspected ..." through "A creator funcion can be used..."
>>>
>>> In particular, I'm having difficulty understanding the point of the
>>> creator function in association_proxy. I see where creator functions are
>>> defined (two places), but don't see them being used.
>>
>> the association proxy manages an association relation across three objects,
>> such as:
>>
>> A->B->C
>>
>> when you say something along the lines of:
>>
>> A.someelement = C
>>
>> the "B" is created implicitly by the association proxy. The creator
>> function is used to customize the process of creating the B instance.
>
> Hmm. Adding something like this would not be a bad idea, except with more
> elaboration.
>
> Just to repeat, I didn't really follow the point of the last three bullet
> points. No doubt I'm being obtuse, but I think a little explicitness would
> not hurt here.
>
> * The relation is inspected to determind the type of the related objects.
>
> What relation are we talking about, and what types? What is the point of
> determining the types? I guess in this case there are three objects and two
> relations as described in your diagram above, namely
>
> A->B->C
>
> * To construct new instances, the type is called with the value being
> assigned, or key and value for dicts.
>
> I have no idea what this means. Are the instances referred to instances of
> the association object B as above?
>
> * A creator function can be used to create instances intead.
>
> I suppose we are talking about instances of B again.
>
> I'm looking at the simplest example that uses a creator function, namely the
> User/Keyword example starting with
>
> def get_current_uid():
>
> Here the creator function is
>
> def _create_uk_by_keyword(keyword):
> """A creator function."""
> return UserKeyword(keyword=keyword)
>
> Now, can you point me a line in this example
>
> where there is the equivalent of
>
> A.someelement = C
>
> ? In this case it would be something like
>
> user.keywords = Keyword('foo')
>
> Hmm, I suppose
>
> for kw in (Keyword('its_big'), Keyword('its_heavy'), Keyword('its_wood')):
> user.keywords.append(kw)
>
> is also creating association objects, since there is one association object
> (here, UserKeyword) created for each new Keyword.
>
> In any case, I think it would be helpful to emphasize this point, since it is
> not obvious, at least to me. Also, how does creator know where to get the
> arguments from in general? Does it come via the "C" object (here Keywords)?
> What happens if it isn't specified? Does it fall back to the constructor
> instead, or something else?
>
> Incidentally, in the Broker/Stock example, the creator function, namely
>
> def _create_holding(stock, shares):
> """A creator function, constructs Holdings from Stock and share
> quantity."""
> return Holding(stock=stock, shares=shares)
>
> isn't used anywhere that I can see, even implicitly, since no stocks are
> added to holdings in the example.
On further consideration,
for stock in (Stock('JEK'), Stock('STPZ')):
broker.holdings[stock] = 123
*is* actually creating Holding objects. Also, it is using stock and shares
arguements, so I suppose
def _create_holding(stock, shares):
"""A creator function, constructs Holdings from Stock and share
quantity."""
return Holding(stock=stock, shares=shares)
is coming into play somehow. However, I'm still unclear about the precise
mechanism by which this works. Regardless, I think it would be helpful to
flag these examples explicitly, as in "look, look, we're invoking the
creator function and implicitly creating Holding objects here!" (or
whatever).
Take care, Faheem.
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