[Zope-dev] Transparent folders?

[Martijn Faassen]

Michel Pelletier wrote:
> > you need to override the entire subfolder and copy over *all* layout
> > elements.
> 
> Ya lost me right there, where is there a copy involved?  Why can't you
> put your Foo layout components in a FooLayout folder in the root folder
> and say:
> 
> <dtml-with FooLayout></dtml-with>
> 
> When you want to use them?

You can't exploit acquisition so easily then. You can use a FooLayout folder.
Now you suddenly (for some bizarre reason) want to support a Dutch version of
the site in some seperate subtree, with different texts, images, and so on.
But not *everything* is replaced. You want to change some of the stuff in
FooLayout, for instance 'header_html', but not all of it. I can't simply 
reate my own FooLayout in /dutch, and put my own header_html there.
I'd need to copy over *everything* from FooLayout to make that work.

Of course there are some ways around that, but they all seem to be a hassle
compared to what's possible with acquisition.

>  The only downside of this from your approach
> is that it's not 'transparent'.

Does this mean you understood what I meant already? That was the downside
I was trying to solve. :)

> But otherwise what you are proposing does make sense, I'm getting the
> impression of 'overlaying' various folder namespaces while keeping their
> organziational structure.

Right. That's another way to look at it. It's quite analogous to
inheritance, except that the base classes are defined 'inline'.

>  Ok, some questions.
> 
> What happens when a transparent folder 'A' contains a tranparent folder
> 'B'?  
> 
> So 'B' contains the object 'Foo' which you try to access from some
> completly different location.  It is not found in the root folder, how
> is 'A' searched?  Are all transparent folders in a container searched in
> a linear fashion?  Does 'B' get descended into when A doesn't contain
> Foo?  This begins to sound like an expensive recursive lookup

True. I don't know the fine details of the acquisition machinery, but
presumably there's somekind of 'expensive recursive lookup' involved at
some point too. Would this increase the expense that much compared to
the current situation? (Ignoring the fact that much of acquisition
happens in C)

> unless a
> transparent folder plays with the namespace of its container, but then I
> think you run into a whole other slew of problems,

Probably. :)

> like why Zope
> complains to newbie eyes that the id 'Foo' exists when it clearly
> doesn't in the root folder (oops, gotta look in A/B!, but if there's
> lots of these transparent folders, you're lost).

Right. You could work around that with error reporting, perhaps, but
that'd be a pain. Better behavior would be that 'Foo' in the root
folder simply overrides any other Foo's in A or B. I think you can 
make the playing with the container's namespace smart enough to do that. 
Not that it still wouldn't be tricky. We haven't even thought about
any security implications yet, for instance.

> What if both A and B contain a 'Foo'?  Is B's Foo ever visible to anyone
> other than explicitly?

Yes; A's Foo would be found. Another question that I *can't* answer is
what if I have transfolders C and D, and they both contain Foo. Which one
gets found? This is analogous to what can happen in multiple inheritance
in Python, but it's less clear what the order of depth first search in
Zope is.

Thanks for your comments. Very useful.

Regards,

Martijn