[Python-Dev] properties and block statement

Wed Oct 19 11:11:16 CEST 2005

Stefan Rank <stefan.rank at ofai.at> wrote in news:4355EF41.4010803 at ofai.at:
> I think there is no need for a special @syntax for this to work.
>> I suppose it would be possible to allow a trailing block after any 
> function invocation, with the effect of creating a new namespace that 
> gets treated as containing keyword arguments.
>
I suspect that without any syntax changes at all it will be possible (for 
some stack crawling implementation of 'propertycontext', and assuming 
nobody makes property objects immutable) to do:
 class C(object):
 with propertycontext() as x:
 doc = """ Yay for property x! """
 def fget(self):
 return self._x
 def fset(self, value):
 self._x = value
for inheritance you would have to specify the base property:
 class D(C):
 with propertycontext(C.x) as x:
 def fset(self, value):
 self._x = value+1
propertycontext could look something like:
import sys
@contextmanager
def propertycontext(parent=None):
 classframe = sys._getframe(2)
 cvars = classframe.f_locals
 marker = object()
 keys = ('fget', 'fset', 'fdel', 'doc')
 old = [cvars.get(key, marker) for key in keys]
 if parent:
 pvars = [getattr(parent, key) for key in
 ('fget', 'fset', 'fdel', '__doc__')]
 else:
 pvars = [None]*4
 args = dict(zip(keys, pvars))
 prop = property()
 try:
 yield prop
 for key, orig in zip(keys, old):
 v = cvars.get(key, marker)
 if v is not orig:
 args[key] = v
 prop.__init__(**args)
 finally:
 for k,v in zip(keys,old):
 if v is marker:
 if k in cvars:
 del cvars[k]
 else:
 cvars[k] = v