Add Properties to Instances?

[Bengt Richter]

On 12 Mar 2005 09:48:42 -0800, "Martin Miller" <ggrp1.20.martineau at dfgh.net> wrote:

>I'm trying to create some read-only instance specific properties, but
>the following attempt didn't work:
>
>> class Foobar(object):
>>     pass
>>
>> foobar = Foobar()
>> foobar.x = property(fget=lambda: 42)
>>
>> print "foobar.x:", foobar.x
>
>Which results in the following ouput instead of '42':
>  foobar.x: <property object at 0x00AE57B0>
>
>I also tried this:
>> foobar.__dict__['x'] = property(fget=lambda: 42)
>but get the same behavior.
>
>Can anyone tell me what's wrong with this approach (and perhaps the
>correct way to do it, if there is one)?
>
One way is to doctor the instance attribute access to do what
happens with a property when it is implemented normally as an
attribute of the class. E.g., (using InstProp in place of your Foobar)

 >>> class InstProp(object):
 ...     def __getattribute__(self, attr):
 ...         p = object.__getattribute__(self, attr)
 ...         if isinstance(p, property): return p.__get__(self)
 ...         return p
 ...
 >>> import time
 >>> inst1 = InstProp()
 >>> inst2 = InstProp()
 >>> inst1.t = property(lambda self: time.ctime())
 >>> inst2.p2 = property(lambda self: 'prop2')
 >>> inst1.t
 'Sat Mar 12 19:10:40 2005'
 >>> inst2.p2
 'prop2'

If you want robust writeable and/or deleteable properties, you will have
to do a little more work, but it can be done.

Regards,
Bengt Richter