Add Properties to Instances?

[Steven Bethard]

Martin Miller 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
> 
[snip]
> 
> Can anyone tell me what's wrong with this approach (and perhaps the
> correct way to do it, if there is one)?

Properties cannot be defined on a per-instance basis.  Properties must 
be defined at the class level.  So you need to do something like:

py> class Foobar(object):
...     x = property(fget=lambda self: 42)
...
py> Foobar().x
42

or

py> class Foobar(object):
...     pass
...
py> Foobar.x = property(fget=lambda self: 42)
py> Foobar().x
42

If you want to have different properties on different instances, you'll 
need to make each of the different instances a different subtype of 
Foobar, e.g.:

py> class Foobar(object):
...     pass
...
py> foobar = type('FoobarSub', (Foobar,),
...               dict(x=property(fget=lambda self: 42)))()
py> foobar.x
42
py> Foobar().x
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
AttributeError: 'Foobar' object has no attribute 'x'

What's the situation in which you think you want different properties 
for different instances of the same class?

STeVe