synthetic properties

[rowland@river2sea.org]

I'm trying to come up with solution for adding synthetic properties to
python, similar to synthetic properties in Objective-C.

I'm playing around with doing this in a MetaClass. I can dynamically
create the attributes that will back the property, but I'm having
trouble figuring out how to dynamically generate get/set methods to
pass to the built-in property() function.

Is it possible to define a lambda or a callable object that will act
as a getter method (or setter, that takes a value argument) during
MetaClass.__ini t__? The hard part I'm guessing is getting the current
instance passed into the getter. This is my first foray into
MetaClasses and dynamic functions/methods so any pointers are greatly
appreciated.


class ObjectivePython Object( type ) :

def __new__( cls, name, bases, dct ) :
#print "Allocating memory for class", name
return type.__new__(cl s, name, bases, dct )

def __init__( cls, name, bases, dct ) :
#print "Initializi ng class", name
for propertyInfo in cls.synthesized :
property = propertyInfo[ 0 ]
defaultValue = propertyInfo[ 1 ]
print property
setattr( cls, '_' + property, defaultValue )
# Create property with get/set methods...

class Person( object ) :

__metaclass__ = ObjectivePython Object


synthesized = [ ( 'name', 'BobC' ),
( 'age', '48' ) ]

def __init__( self ) :
print self._name
print self._age


Thanks,
Rowland

[Diez B. Roggisch]

Re: synthetic properties

rowland@river2s ea.org schrieb:

I'm trying to come up with solution for adding synthetic properties to
python, similar to synthetic properties in Objective-C.
>
I'm playing around with doing this in a MetaClass. I can dynamically
create the attributes that will back the property, but I'm having
trouble figuring out how to dynamically generate get/set methods to
pass to the built-in property() function.

Why on earth do you want to do that? The reason synthesized properties
exist in ObjC is simply that to expose properties for key-value-coding,
one needs the getter/setters. But mostly these are boilerplate, so apple
introduced the @synthesized-annotation.

But in python, you don't need that. You use simple attributes. In the
very moment you need logic attached, use the builtin property to do so.




Diez