Accessors in Python (getters and setters)

[Bruno Desthuilliers]

mystilleef wrote:
> Gerhard Fiedler wrote:
> 
>>On 2006-07-15 06:55:14, mystilleef wrote:
>>
>>
>>>In very well designed systems, the state of an object should only be
>>>changed by the object.
>>
>>IMO that's not quite true. Ultimately, the state always gets changed by
>>something else (user interaction, physical events); very few objects are
>>completely self-contained in their behavior.
>>
> Then in those cases the system becomes a victim of high coupling.

Time to burn your book and face reality. ObjA sends message Msg1 to
ObjB. Part of the associated behaviour is that in responce to Msg1, objB
changes it's own state. Practical result : ObjB's state has been changed
by ObjA. Practical question : how do you hope to avoid this "hi
coupling" (lol), apart from making all your objects totally autistic ?

>>In most systems (and you possibly have written some of them) are objects
>>whose state gets changed by other objects -- possibly through the
>>intermediation of setter methods that do nothing else but set the state.
>>There's no conceptual difference between directly setting the state or
>>calling a setter function that does nothing else but directly setting the
>>state -- except for one unnecessary level of indirection in the latter.
>>
> 
> 
> It depends. If certain conditions need to be met before changing the
> state of an object, then arbitrarily changing it can be dangerous.

Does this imply a 'method call' *syntax* ? Given the existence of
"computed attributes" (ie: support for 'attribute access' *syntax* with
hidden accessors) and the possibility to redefine implementation (from
default attribute r/w access to computed/controlled) without touching
the interface, why advocate the *systematic* use of computed attributes
when it's just duplicating the default behaviour ?


> 
>>>For example, a third party randomly changing is_active, (which Python
>>>lets you do freely and easily) from False to True may crash your GUI.
>>>And I'm not making this up. Things like this do really happen depending
>>>on the whackyness of your toolkit.
>>
>>That's quite true, but a setter that does nothing but change is_active
>>doesn't prevent this. If there is logic necessary to prevent state changes
>>in certain situations, this should be implemented. But whether you then
>>call this a part of the "behavior" (looking at the implementation as being
>>a setter method) or a part of the "state" (looking at the implementation as
>>being an added feature of the attribute) doesn't really make an objective
>>difference.
>>
> 
> 
> Of course using setters for the sake of just using them is pointless.

Indeed.

> The reason to use them is if pre-conditions or post-conditions need to
> be met. Or to control access to an objects states.

Then why advocate *systematic* use of them ?

(snip)
> 
> State - behavior is not something I made up, so it isn't subjective.

The words (and the concept they describe) are not. Interpretation of
what is state and what is behaviour is subjective.

> It
> is a common term used in OO literature. In fact, the only reason I used
> it is because I thought is was common knowledge.

It is.

> And behaviors are not
> just necessarily getters/setters, they are methods of objects.

Behaviour is how a given object reacts to a given message. *Nothing* in
this implies the notions of attributes or methods. Attributes and
methods are implementation details of the concepts of state and
behaviour, and - while this is a common implementation of OO concepts -
  the choice to use non-callable attributes as representing the state
and callable ones as representing behaviour is totally
implementation-dependant.

-- 
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb at xiludom.gro'.split('@')])"