Philipp von Weitershausen wrote:
Jean-Marc Orliaguet wrote:
Lennart Regebro wrote:
On 11/9/06, Chris Withers <[EMAIL PROTECTED]> wrote:
Why do you say "extra" ZCML registration? You need that ZCML
registration whether or not you have to write the marker interface...
Sure, but with the marker interface you need only one. You need one
for each class, in your example, thats two. So the second one is
I think it is a mistake to use interfaces to specify what object
_are_ as opposed to what they can _do_. It is better to use base
classes for that. I agree with Chris that making classes adaptable
would be simpler.
Classes *are* adaptable as has been said many times already. Let's
please not make it sound like it's not possible.
Indeed technically yes. Practically not. I couldn't find many examples
in the zope3 codebase that adapts classes.
The #1 pattern is to adapt from interfaces, it appears as though there
is a reason for it.
So the best way to not make it sound so, is to use it so :-)
Interfaces were designed to specify what an object can do, e.g. a
media player will do: play(), stop(), rewind(), without specifying
the actual implementation ... whereas classes tell what objects are
(an mp3 player, an LP player, a cassette player, ...), they are more
specific to the object.
More generally, interfaces were designed to specify the *behaviour* of
an object. Three basic types of behaviour have been identified in Zope:
* perform an action (method API)
* storing data (schema)
* assumed behaviour that can't be expressed in an API or schema
(persistable, attribute annotatable, etc.)
The last use case is the one marker interfaces serve. A marker
interface isn't any less of an interface. It just describes a
different category of behaviour, one that can't be expressed through
method or attribute APIs. It's still behaviour that we would like to
express formally, and using interfaces for that seems the most logical
Interface docstrings are as much part of the formal interface as
method or attribute specifications! (Otherwise an interface could
never mandate the type of an argument passed to a method, e.g.
IContainer.__setitem__ only takes unicode or string names.)
yes, I agree. A perfect use case for interfaces without methods is when
the specification cannot be expressed in a programmatic way. For
instance persistable, etc. in that case only a natural language is
FYI, the same discussion has occurred in Java (since there are marker
interfaces there too and since there are interfaces too and the language
does not prohibit using interfaces without methods). Are they an
anti-pattern? An answer which I find perfectly valid is given at:
under "Appropriate Use of Marker Interfaces"
however my reaction was against using interfaces without specification
or interfaces with a very implicit and blurry specification, as in the
cases described by Martin, in which he defends the use of marker
interfaces basically as a way to tag classes "externally" (on/off
switch, ...). A class annotation API would be better in that case, maybe
based on the same internals as the interface API.
"marker interfaces" have empty specifications, that's the problem:
they can't grow into "real" interfaces that have a specification. If
you add methods to the marker interface you will have to track down
all classes that implement the interface (unless you don't really
care whether classes do implement their interface).
That's always a problem with interfaces. Whether or not an interface
had specifications before or not, when you change an interface by
adding a spec, you will *always* have to track down all
implementations. That's why changing an existing, public interface
isn't such a good idea. This argument has nothing to do with marker
interfaces, though. It's a general problem of public interfaces.
yes, with the difference that marker interfaces have so little
specification that they are almost meant to see their specification
increase. That's what happened with all the magic flags and meta-tags in
Zope2/CMF (isPrincipiaFolderish, meta_type, portal_type, ...). Using
interfaces in that case does not solve the problem, and only shifts the
problem to another area of the system.
as said in the article mentioned above, the same applies here:
[QUOTE] If the object you pass in has this marker interface, I will
treat it in the following way. But I think that down that path lies
darkness. Nobody knows the behavior of objects when they get their 17th
marker interface. What if the object implements this marker interface
and not this one, or this one and that one but not that other one? .
All that said, I don't think we should overdo marker interfaces (like
any kind of interface). I'm just saying they're not evil and it's not
like they violate the "pure truth" of interfaces. The third category
of behaviour may be unique to Zope interfaces, but that doesn't
necessarily mean it's a perverted use of interfaces.
they're not evil as long as you know what they are used for, and you
don't abuse them.
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