[Python-Dev] property syntax

[john coppola]

Hey Fred.  It might not be a good idea to nest the
"property class" like an inner class.  It may be
plausible that property objects are reusable between
classes.  As implied by this syntax, it wouldn't be
reuseable.  Another point, is that they may be very
large.  Which would be messy.

I did i bit of brainstorming.

One purpose of the type objects is a means to coerce
one object to another.  So here is the pattern.

Just like str(MyObject) requires __str__, or
len(MyObject) requires __len__ or any of the factory
functions for that matter, the property factory
function would require that your object support both
__get__ , __set__ , and __del__.  Thats it.  So
instead of, property(fset,fget,fdel)you would instead
have, property(AnyObjectSupportingAboveInterface).

How the property factory function differs from the
others is that it will only check for the existence of
these methods, and will not execute the code within
them.  It instead sets a flag on the object indicating
that it is active.  Will be necessary to do checking
on every object for every set, or every get.  Not too
bad though.  How time consuming is two if statements?

Is this making sense?

John Coppola

--- "Fred L. Drake, Jr." <fdrake@acm.org> wrote:
> 
> Fred L. Drake, Jr. writes:
> [describing a suggested property syntax]
>  > class Foo(object):
>  >     property myprop:
>  >         """A computed property on Foo objects."""
>  > 
>  >         def __get__(self):
>  >             return ...
> 
> Perhaps it was obvious to everyone else, but it just
> occured to me
> that this lends itself to inheriting descriptor
> types:
> 
> 
> class ReadOnly(object):
>     def __get__(self):
>         raise NotImplementedError("sub-class must
> override this!")
> 
>     def __set__(self):
>         raise AttributeError("read-only attribute")
> 
>     def __delete__(self):
>         raise AttributeError("read-only attribute")
> 
> 
> class Foo(object):
>     property myprop(ReadOnly):
>         def __get__(self):
>             return ...
> 
> 
> 
>   -Fred
> 
> -- 
> Fred L. Drake, Jr.  <fdrake at acm.org>
> PythonLabs at Zope Corporation
> 
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