[EMAIL PROTECTED] a écrit :
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...
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.
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.
The hard part I'm guessing is getting the current
instance passed into the getter.
Not really.
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.
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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