Adding properties to objects

[Matthew Barnes]

Shalabh Chaturvedi <shalabh at cafepy.com> wrote...

> You can add properties dynamically, but to the type (i.e. 'class') and 
> not the instance (i.e. 'object'). For your example:
> 
>  >>> x = Foo()
>  >>> def f(a,b=None): # a bogus getter, setter, deleter
> ...     print a,b
> ...
>  >>> Foo.property = property(f,f,f)
>  >>> x.property
>  <__main__.Foo object at 0x008F3CB0> None
>  >>> x.property = 1
>  <__main__.Foo object at 0x008F3CB0> 1
>  >>> del x.property
>  <__main__.Foo object at 0x008F3CB0> None
>  >>>
> 
> This property will work for *all* instances of Foo.

Right.  I may not have been clear, but I was trying to add different
properties to different *instances* of Foo.

> What is the problem you are trying to solve?

Here's basically what I was trying to do:

class Foo(object):

    def __init__(self, sourcedict):
        # sourcedict is a dictionary of objects having get() and set()
        # methods.  Nevermind where sourcedict comes from.  The point
        # is that different instances get different sourcedict's.
        for name, object in sourcedict.items():
            # This doesn't do what I had hoped for.
            # My thinking was flawed on several levels.
            self.__dict__['name'] = property(object.get, object.set)

So suppose "sourcedict" has some object named "bar".  The idea was to
create properties on-the-fly based on the contents of "sourcedict",
such that:

>>> x = Foo(sourcedict)  # Given this...
>>> x.bar                # would invoke sourcedict['bar'].get()
>>> x.bar = 1            # would invoke sourcedict['bar'].set(1)

I've since realized that properties are not the right approach for
something like this, and that I would be better off with a more
straight-forward approach using custom Foo.__getattr__ and
Foo.__setattr__ methods (seems obvious now that I've simplified the
problem description :-).

Matthew Barnes