[Zope3-dev] keeping attributes abstract

[Benji York]

Sam Stainsby wrote:
> So I'm inclines to use the IFooB approach by default to achieve more
> abstract attributes. Am I missing something here in the IFooA approach
> that allows me to do this anyway? 

You are :)  Python has properties.  They give you a good migration path 
from a simple attribute to what you would otherwise have to use 
getter/setters for.  An example would help.  Today you do this:

class Foo(Persistent):
     implements(IFooB)

     bar = None

And then you decide, oops I need to calculate bar every time it's 
accessed.  Then you can do this:

class Foo(Persistent):
     implements(IFooB)

     @property
     def bar(self):
         return time.time() * 2 # crazy thing, but just an example

But that bar is read-only, what if you also want to be able to set bar, 
you can do this:

class Foo(Persistent):
     implements(IFooB)

     @apply
     def bar(self):
         doc = """The bar attribute"""

         def fset(self, bar):
             if bar is None:
                 self._bar = 47
             else:
                 self._bar = bar

         def fget(self):
             return self._bar * 44

         return property(**locals)

Some people like a more explicit version of the last line, if so you can 
use "return property(fset, fget, None, doc)".

(WARNING: I didn't actually run any of the code in this message, typos 
may have crept in)
-- 
Benji York
Senior Software Engineer
Zope Corporation