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override a property

Is there a way to override a data property in the instance? Do I need to create
another class with the property changed?
--
Robin Becker

Oct 17 '05 #1
18 4737
No, you can just do it on the fly. You can even create properties
(attributes) on the fly.

class Dummy:
property = True

d = Dummy()
d.property = False
d.new = True

Stani
--
SPE - Stani's Python Editor http://pythonide.stani.be

Oct 17 '05 #2
Robin Becker a écrit :
Is there a way to override a data property in the instance? Do I need to
create another class with the property changed?


Do you mean attributes or properties ?
Oct 17 '05 #3
On Mon, 17 Oct 2005 18:52:19 +0100, Robin Becker <ro***@reportla b.com> wrote:
Is there a way to override a data property in the instance? Do I need to create
another class with the property changed?

How do you need to "override" it? Care to create a toy example with a
"wish I could <override action> here" comment line? ;-)

Regards,
Bengt Richter
Oct 17 '05 #4
On 17 Oct 2005 11:13:32 -0700, "SPE - Stani's Python Editor" <sp**********@g mail.com> wrote:
No, you can just do it on the fly. You can even create properties
(attributes) on the fly.

class Dummy:
property = True

d = Dummy()
d.property = False
d.new = True

a simple attribute is not a property in the sense Robin meant it,
and a "data property" is even more specific. See

http://docs.python.org/ref/descriptor-invocation.html

also
help(property)

Help on class property in module __builtin__:

class property(object )
| property(fget=N one, fset=None, fdel=None, doc=None) -> property attribute
|
| fget is a function to be used for getting an attribute value, and likewise
| fset is a function for setting, and fdel a function for del'ing, an
| attribute. Typical use is to define a managed attribute x:
| class C(object):
| def getx(self): return self.__x
| def setx(self, value): self.__x = value
| def delx(self): del self.__x
| x = property(getx, setx, delx, "I'm the 'x' property.")
|
Regards,
Bengt Richter
Oct 17 '05 #5
Bruno Desthuilliers wrote:
Robin Becker a écrit :
Is there a way to override a data property in the instance? Do I need
to create another class with the property changed?

Do you mean attributes or properties ?


I mean property here. My aim was to create an ObserverPropert y class
that would allow adding and subtracting of set/get observers. My current
implementation works fine for properties on the class, but when I need
to specialize an instance I find it's quite hard.

--
Robin Becker
Oct 18 '05 #6
Robin Becker wrote:
Bruno Desthuilliers wrote:
Robin Becker a écrit :
Is there a way to override a data property in the instance? Do I need
to create another class with the property changed?
Do you mean attributes or properties ?

I mean property here.


Ok, wasn't sure... And sorry, but I've now answer.
My aim was to create an ObserverPropert y class
that would allow adding and subtracting of set/get observers.
Could you elaborate ? Or at least give an exemple ?
My current
implementation works fine for properties on the class, but when I need
to specialize an instance I find it's quite hard.


--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"
Oct 18 '05 #7
Robin Becker <ro***@SPAMREMO VEjessikat.fsne t.co.uk> wrote:
Bruno Desthuilliers wrote:
Robin Becker a écrit :
Is there a way to override a data property in the instance? Do I need
to create another class with the property changed?


Do you mean attributes or properties ?


I mean property here. My aim was to create an ObserverPropert y class
that would allow adding and subtracting of set/get observers. My current
implementation works fine for properties on the class, but when I need
to specialize an instance I find it's quite hard.


A property is an 'overriding descriptor', AKA 'data descriptor', meaning
it "captures" assignments ('setattr' kinds of operations), as well as
accesses ('getattr' kinds), when used in a newstyle class. If for some
reason you need an _instance_ to bypass the override, you'll need to set
that instance's class to one which has no overriding descriptor for that
attribute name. A better design might be to use, instead of the builtin
type 'property', a different custom descriptor type that is specifically
designed for your purpose -- e.g., one with a method that instances can
call to add or remove themselves from the set of "instances overriding
this ``property''" and a weak-key dictionary (from the weakref module)
mapping such instances to get/set (or get/set/del, if you need to
specialize "attribute deletion" too) tuples of callables.
Alex
Oct 18 '05 #8
bruno modulix wrote:
......

Could you elaborate ? Or at least give an exemple ? ......
in answer to Bengt & Bruno here is what I'm sort of playing with. Alex suggests
class change as an answer, but that looks really clunky to me. I'm not sure what
Alex means by
A better design might be to use, instead of the builtin
type 'property', a different custom descriptor type that is specifically
designed for your purpose -- e.g., one with a method that instances can
call to add or remove themselves from the set of "instances overriding
this ``property''" and a weak-key dictionary (from the weakref module)
mapping such instances to get/set (or get/set/del, if you need to
specialize "attribute deletion" too) tuples of callables.


I see it's clear how to modify the behaviour of the descriptor instance, but is
he saying I need to mess with the descriptor magic methods so they know what
applies to each instance?
## my silly example
class ObserverPropert y(property):
def __init__(self,n ame,observers=N one,validator=N one):
self._name = name
self._observers = observers or []
self._validator = validator or (lambda x: x)
self._pName = '_' + name
property.__init __(self,
fset=lambda inst, value: self.__notify_f set(inst,value) ,
)

def __notify_fset(s elf,inst,value) :
value = self._validator (value)
for obs in self._observers :
obs(inst,self._ pName,value)
inst.__dict__[self._pName] = value

def add(self,obs):
self._observers .append(obs)

def obs0(inst,pName ,value):
print 'obs0', inst, pName, value

def obs1(inst,pName ,value):
print 'obs1', inst, pName, value

class A(object):
x = ObserverPropert y('x')

a=A()
A.x.add(obs0)

a.x = 3

b = A()
b.x = 4

#I wish I could get b to use obs1 instead of obs0
#without doing the following
class B(A):
x = ObserverPropert y('x',observers =[obs1])

b.__class__ = B

b.x = 7
--
Robin Becker

Oct 18 '05 #9
Robin Becker wrote:
Is there a way to override a data property in the instance? Do I need to create
another class with the property changed?
--
Robin Becker


It is possible to decorate a method in a way that it seems like
property() respects overridden methods. The decorator cares
polymorphism and accesses the right method.

def overridable(f):
def __wrap_func(sel f,*args,**kwd):
func = getattr(self.__ class__,f.func_ name)
if func.func_name == "__wrap_fun c":
return f(self,*args,** kwd)
else:
return func(self,*args ,**kwd)
return __wrap_func
class A(object):
def __init__(self, x):
self._x = x

@overridable
def get_x(self):
return self._x

x = property(get_x)

class B(A):

def get_x(self):
return self._x**2

class C(B):pass
a = A(7)
a.x 7 b = B(7)
b.x 49 c = C(7)
c.x

49

Oct 18 '05 #10

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