Properties fun with 2.2

[Mike C. Fletcher]

Hmm, don't really understand what the contract of the result is supposed 
to be:

	A sequence object, where a "controller sequence" declares property 
objects.  These property objects control the data type, allow for bounds 
checking, and do the actual work of storing/retreiving data.

or

	A sequence object, where the sequence object has "numerically indexed 
attributes".  That is, a "sparse sequence of attributes" where each 
attribute is controlled by a particular Property object (that is, data 
type, bounds checking, storage/retrieval are all handled by the 
delegated object).

or

	Something else?



class ControlledSequence:
	# there's no 2.2-specific content here...
	def __getitem__( self, index ):
		return self.__properties[index].__get__( self, index )
	# same for set and del, put in some slice logic with iteration

class ControlledSequenceProperty:
	def __get__( self, client, index ):
		#this isn't the same API as standard property type
		#see the property example for what you'd do here,
		#basically just get/set/del the value from the client

Where, depending on the contract, __properties is a list or a dictionary.


[2.2 content here]
One thing I really don't get in the property design is why the property 
__get__ __set__ __del__ methods don't normally have an API like that 
above (i.e. pointer to property object, pointer to client object, and 
pointer to the key/index/attributename used to call the property).  With 
that, you could re-use the same property object if the property 
attributes were identical.

Enjoy,
Mike


Robin Becker wrote:

...
> very nice. I wondered if this stuff would be easy and it seems it can
> be. One of the things I need is a collection of properties ie a property
> 
> P
> 
> with its own attributes (probably properties themselves) which can be
> accessed as
> 
> P[0]
> 
> ie a thing similar to P, but allowed individual values where these
> differ from the aggregate. I wonder whether this is more easily done in
> 2.2. It's quite hard in 2.1.
...