On 8 May, 13:56, Peter Otten <__pete...@web.de> wrote:
> Mark Summerfield wrote:
> > On 8 May, 08:19, Peter Otten <__pete...@web.de> wrote:
> >> MarkSummerfieldwrote:
> >> > I had a quick search & didn't find anything _nice_ that produced
> >> > attributes with really private data, so I came up with a possible
> >> > solution---for Python 3.
>
> >> Do really you think what you suggest below is "nice"?
>
> > Well the code isn't ugly and doesn't mess with the call stack etc.
>
> >> By the way, your Attribute descriptor stores the value for all instances
> >> of A in the same variable...
>
> > It seems like it does, but it doesn't. The hidden_value is an instance
> > variable that is created every time an Attribute object is created.
>
> >>>> from Attribute import *
> >>>> class A:
> > a = Attribute("a", 5, lambda *a: True)
> > b = Attribute("b", 5, lambda *a: True)
> >>>> class B:
> > a = Attribute("a", 5, lambda *a: True)
> > b = Attribute("b", 5, lambda *a: True)
> >>>> a = A()
> >>>> b = B()
> >>>> a.a,a.b,b.a,b.b
> > (5, 5, 5, 5)
> >>>> a.a=1;a.b=2;b.a=3;b.b=4
> >>>> a.a,a.b,b.a,b.b
> > (1, 2, 3, 4)
>
> But attribute values are shared between all instances of the same class:
>
> >>> class A:
>
> ... x = Attribute("x", 42, lambda *a: True)
> ...>>> a = A()
> >>> b = A()
> >>> a.x, b.x
> (42, 42)
> >>> a.x = "yadda"
> >>> a.x, b.x
>
> ('yadda', 'yadda')
>
> Peter
OK, I couldn't quite give it up. But the solution isn't nice or good.
It does work, but at the cost of an extra dictionary lookup on every
get or set, plus a dictionary to hold all the getters & setters. I
_don't recommend it_, but here it is anyway. I've done with it now:-)
class Attribute:
__accessors = {}
def __init__(self, name, first_value=None, validator=None):
self.name = name
self.first_value = first_value
self.validator = validator
def __get__(self, instance, owner=None):
if instance is None:
return self
if (id(instance), self.name) not in self.__accessors:
self.__makeAccessors(instance)
return self.__accessors[id(instance), self.name][0](instance)
def __set__(self, instance, value):
if (id(instance), self.name) not in self.__accessors:
self.__makeAccessors(instance)
setter = self.__accessors[id(instance), self.name][1]
if setter is None:
raise AttributeError("'{0}' is read-only".format(
self.__name__))
return setter(instance, value)
def __makeAccessors(self, instance):
hidden_value = self.first_value
getter = lambda self: hidden_value
if self.validator is not None:
def set(instance, value):
if self.validator(instance, value):
nonlocal hidden_value
hidden_value = value
else:
raise ValueError("'{0}' is not valid for {1}"
.format(value, name))
setter = set
else:
setter = None
self.__accessors[id(instance), self.name] = (getter, setter)
--
Mark Summerfield, Qtrac Ltd, www.qtrac.eu
C++, Python, Qt, PyQt - training and consultancy
"Programming in Python 3" - ISBN 0137129297
--
http://mail.python.org/mailman/listinfo/python-list
