[Tutor] Mutable Properties

[Steven D'Aprano]

On Thu, 9 Sep 2010 06:59:57 am Chris King wrote:
>   Dear Tutors,
>      I noticed that when you use a property to represent a mutable
> value, I you try to use its methods, it will directly change the
> value returned. I know this happens because I'm not really assigning
> it to something new, but changing whats already there, which won't
> fire off the set method. I was wondering if there was a way around
> this.

You're going to need to give an example of:

(1) what you do;
(2) what you want to happen; and
(3) what actually happens.

The simplest example I can think of is:

class K(object):
    def __init__(self):
        self._private = []
    def _getter(self):
        return self._private
    def _setter(self, value):
        self._private = list(value)
    seq = property(_getter, _setter)


And in use:

>>> k = K()
>>> k.seq
[]
>>> k.seq.append(1)
>>> k.seq
[1]

Works fine.


-- 
Steven D'Aprano