Best way to control assignment to attribute?

[Peter Otten]

Frank Millman wrote:

> Hi all
> 
> I want to control the assignment of a value to an attribute. Instead
> of allowing it to be changed directly, I want to enforce that a method
> is called, which will perform the assignment subject to various
> checks.
> 
> From reading the manuals, this is one way to do it.
> 
> class frank:
>     def __init__(self,x):
>         self.setval_x(x)
> 
>     def __setattr__(self,name,value):
>         if name == 'x':
>             raise 'cannot change value of x - use setval_x(value)'

I think you shouldn't use string exceptions in new code anymore.

>         else:
>             self.__dict__[name] = value
> 
>     def setval_x(self,value):
>         ok = 1
>         # perform any checks required
>         if ok:
>             self.__dict__['x'] = value
> 
> Is this the best way, or does anyone have any other suggestions?
> 
> I notice that an application can beat this by using the __dict__
> syntax itself. Is there any way to prevent this? Just curious, it is
> not a major concern.
> 
> Any comments will be appreciated.
> 
> Thanks
> 
> Frank Millman

Use new style classes and properties:

>>> class Frank(object):
...     def __init__(self, x):
...             self.x = x
...     def getX(self):
...             return self._x
...     def setX(self, x):
...             if x < 0:
...                     raise ValueError("x must be >= 0")
...             self._x = x
...     x = property(getX, setX)
...
>>> f = Frank(3)
>>> f.x = 2
>>> f.x = -2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 8, in setX
ValueError: x must be >= 0

The main advantage is cleaner code, which will become more obvious as the
number of special attributes increases. Also, checked and normal attribute
access is transparent to the client.

Peter