[Zope-dev] Re: zcml entry points

[Martin Aspeli]

Fred Drake wrote:
> On 10/17/07, Martin Aspeli <optilude at gmx.net> wrote:
>> The main win, IMHO, is to avoid the requirement for people to install
>> slugs for third party products. Slugs suck - they are confusing to
>> explain and people forget them all the time. Buildout makes it a bit
>> easier, but it's still not a terribly good solution.
> 
> Slugs are evil; agreed.  We never use them.  They were an accident of
> the "instance" tree inherited from older versions of Zope, and how
> that tree was re-interpreted for Zope 3.  They never worked well, and
> did no one any favors.

Glad we all agree.

>> For example, say you want to install oi.plum. You need to add the line
>> 'oi.plum' twice - once under 'eggs' and once under 'zcml' in your
>> buildout.cfg. Forget the latter, and the package doesn't work properly
>> (or at all).
> 
> I actually really like this; I don't get the configuration for a
> package unless I ask for it.  It's not unusual to want only the code
> and not the default configuration for a package.

Right - but you're building an application, and you're pretty 
experienced with Zope. A lot of Plone users just want to install a 
plug-in (a product), basically. Before, they just dropped it into a 
directory. Now, they declare it twice in a file (presuming there's a 
cheese shop release). That's a (small) step backwards (duplication). 
Declaring it once would be a step forwards (no manual download)

>> If we had entry points, we could just load the egg. Internally, oi.plum
>> may use <include /> as much as necessary to load *its* dependencies, but
>> that's not something the user of the package needs to worry about.
> 
> Requiring a package doesn't say anything about how it's going to be
> used; making an assumption about that is really bad.

Maybe. I agree there should be an option not to do this, and many eggs 
may choose *not* to use an entry point. For things like Plone add-on 
products, though, it makes much more sense to have it be automatic. To 
generalise further - if what you're installing is a high level, cohesive 
product, with a UI and an install method and all the rest of it, having 
installation be as simple as possible is a big win. For the components 
further down the stack (e.g. the things that these high level products 
are depending on!), having the processing be explicit (via <include /> 
directives) makes more sense.

>> If Zope loaded entry-point ZCML automatically (maybe with an on/off
>> switch in zope.conf for those who need more fine grained control) that'd
>> be a pretty nice solution.
> 
> I suspect I'd always want it off.  Picking up configuration
> willy-nilly is too dangerous.

Maybe it's off by default, then, but I *know* this would make the add-on 
products story in Plone easier: I've seen people struggle with slugs, 
editing site.zcml is far tto scary, and you don't always have a package 
that you own to include other things from.

I suspect entry points would make life simpler for other systems that 
used a similar mechanism for plug-ins/add-ons. However, I agree that 
they're not appropriate for more fundamental libraries.

Martin

-- 
Author of `Professional Plone Development`, a book for developers who
want to work with Plone. See http://martinaspeli.net/plone-book