[Zope3-dev] calling objects when evaluating path expressions

[Steve Alexander]

Shane Hathaway wrote:
> Steve Alexander wrote:
> 
>> However, the following gives an unexpected result.
>>
>>   <span tal:replace="foo/bar/spoo" />
>>
>> result:
>>
>>   <bound method baz of <Foo object at 0x92c668c>>
> 
> 
> This result is surprising because it should have thrown an error.  It 
> should not have attempted to call foo.bar, and attempting to find "spoo" 
> should have raised an AttributeError.

Ok. I think my example was wrong. I think it probably does raise an 
AttributeError.
I still say that, from the page-template-user's point of view, an 
AttributeError is an unexpected result.


> I agree with your proposal and Guido's enhancement--we should rely on 
> properties now and not attempt to call anything in path expressions.

I can't find Guido's enhancement just now. Perhaps he posted it via his 
time-machine.


> I once considered suggesting a minor enhancement to path expressions to 
> deal with this: use an exclamation point to call things.  For example::
> 
>   <span tal:replace="foo/bar!/spoo" />
> 
> As exciting as this makes page templates look <wink>, it could be 
> redundant.  The class author could just create a property instead.

I'd rather not see any more odd symbols in path expressions.

But, you're right: it does make them more exciting :-)


One of the goals of page templates in Zope 3 is to allow them to present 
content classes that were not written with such use in mind. So, in Zope 
3 we cannot assume that properties are always provided where appropriate.

We can address this in Zope 3 by using view classes, and by making the 
default traversable component call simple things we want to be called, 
and by providing traversable components for other components.

--
Steve Alexander