[Python-Dev] defmacro (was: Anonymous blocks)

[Robert Brewer]

Shane Hathaway wrote:
> Robert Brewer wrote:
> > So currently, all subclasses just override __set__, which leads to a
> > *lot* of duplication of code. If I could write the base 
> class' __set__
> > to call "macros" like this:
> > 
> >     def __set__(self, unit, value):
> >         self.begin()
> >         if self.coerce:
> >             value = self.coerce(unit, value)
> >         oldvalue = unit._properties[self.key]
> >         if oldvalue != value:
> >             self.pre()
> >             unit._properties[self.key] = value
> >             self.post()
> >         self.end()
> > 
> >     defmacro begin:
> >         pass
> >     
> >     defmacro pre:
> >         pass
> >     
> >     defmacro post:
> >         pass
> >     
> >     defmacro end:
> >         pass
> 
> Here is a way to write that using anonymous blocks:
> 
>     def __set__(self, unit, value):
>         with self.setting(unit, value):
>             if self.coerce:
>                 value = self.coerce(unit, value)
>             oldvalue = unit._properties[self.key]
>             if oldvalue != value:
>                 with self.changing(oldvalue, value):
>                     unit._properties[self.key] = value
> 
>     def setting(self, unit, value):
> 	# begin code goes here
>         yield None
>         # end code goes here
> 
>     def changing(self, oldvalue, newvalue):
>         # pre code goes here
>         yield None
>         # post code goes here
> 
...
> Which do you prefer?  I like fewer methods. ;-)

I still prefer more methods, because my actual use-cases are more
complicated. Your solution would work for the specific case I gave, but
try factoring in:

* A subclass which needs to share locals between begin and post, instead
of pre and post.

or

* A set of 10 subclasses which need the same begin() but different end()
code.

Yielding seems both too restrictive and too inside-out to be readable,
IMO.


Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org