|  | synthetic properties | | |
| | | Guest |  |
|  Posted: Tue Sep 02, 2008 7:48 pm Post subject: synthetic properties |  |
| |  | |
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.__init__? 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 ObjectivePythonObject( type ) :
def __new__( cls, name, bases, dct ) :
#print "Allocating memory for class", name
return type.__new__(cls, name, bases, dct )
def __init__( cls, name, bases, dct ) :
#print "Initializing 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__ = ObjectivePythonObject
synthesized = [ ( 'name', 'BobC' ),
( 'age', '48' ) ]
def __init__( self ) :
print self._name
print self._age
Thanks,
Rowland |
| |
| | | Diez B. Roggisch | |
| | Posted: Tue Sep 02, 2008 7:48 pm Post subject: Re: synthetic properties | |
rowland@river2sea.org schrieb:
| Quote: | 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 |
| |
| | | Bruno Desthuilliers | |
| | Posted: Tue Sep 02, 2008 7:48 pm Post subject: Re: synthetic properties | |
| | | |
rowland@river2sea.org a écrit :
| Quote: | I'm trying to come up with solution for adding synthetic properties to
python, similar to synthetic properties in Objective-C.
|
Please explain what Objective-C "synthetic properties" are - not
everyone here has a strong Objective-C background...
| Quote: | 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.
|
FWIW, the property type is just one possible application of the
descriptor protocol. You can easily define your own custom descriptor
classes - and you can even do this dynamically.
| Quote: | 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.__init__?
|
Yes.
| Quote: | The hard part I'm guessing is getting the current
instance passed into the getter.
|
Not really.
| Quote: | This is my first foray into
MetaClasses and dynamic functions/methods so any pointers are greatly
appreciated.
class ObjectivePythonObject( type ) :
def __new__( cls, name, bases, dct ) :
#print "Allocating memory for class", name
return type.__new__(cls, name, bases, dct )
|
This is what type.__new__ do, so you just don't need it.
| Quote: | def __init__( cls, name, bases, dct ) :
#print "Initializing 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__ = ObjectivePythonObject
synthesized = [ ( 'name', 'BobC' ),
( 'age', '48' ) ]
def __init__( self ) :
print self._name
print self._age
|
<side-note>
May I suggest you read about Python's coding conventions ? Your code
formatting is utterly unpythonic...
</side-note>
Ok, here's a working (and slightly more pythonic - at least wrt/ coding
style) version of your code:
def _make_prop(attrname, default):
def fget(self):
return getattr(self, attrname, default)
def fset(self, value):
setattr(self, attrname, value)
return property(fget=fget, fset=fset)
class ObjectivePythonType(type) :
def __init__(cls, name, bases, dct):
# we don't want to redefine properties already
# defined in a base class
synthesized = dct.get('synthesized', ())
for propname, default in synthesized:
attrname = "_%s" % propname
setattr(cls, propname, _make_prop(attrname, default))
class ObjectivePythonObject(object):
__metaclass__ = ObjectivePythonType
class Person(ObjectivePythonObject):
synthesized = [
('name', 'BobC'),
('age', '48'),
]
def __init__(self) :
print self.name
print self.age
Now... While all this is certainly usefull as a learning exercice on
metaclasses etc, it's also an awfully overcomplexificated way to do
something quite simple. Here's some code that do exactly the same thing:
class PythonPerson(object):
name = "BobC"
age = "48"
def __init__(self):
print self.name
print self.age
HTH |
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