Accessors in Python (getters and setters)

[Steve Holden]

mystilleef wrote, making me somewhat tired of his/her repeated inability 
to get what's being said [sigh]:
> Bruno Desthuilliers wrote:
>>mystilleef wrote:
>>>Bruno Desthuilliers wrote:
>>>>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.
>>>>
This makes it somewhat obvious that you don't appear to fully understand 
the concept of coupling as applied to software systems.

>>>>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 ?
>>>>
>>>Are you serious?
>>
>>Deadly serious. But I'm afraid you're still missing the point.
>>
>>>Well, you design an object that serves as a mediator.
>>>All objects can then only interact with themselves and the mediator
>>>only. Via signals, objects learn to automatically adjust their states
>>>and respond to events.
>>
>>signal -> message -> method call -> change state.
>>
>>Spell it how you like, add as many indirection levels you want, it still
>>boils down to the fact that *something* triggers the state change.
>>
> Riiiiight!
> 
If you implement an accessor to change a class's instances' states, 
surely something has to call that accessor. You seem to be implying that 
such calls can only be made from within other methods of the same 
object, which (if true, which it isn't) would tend to leave each class 
in a vacuum where nothing else can affect its instances.

Of *course* objects are subject to external influences: since you like 
the concept of coupling, how else could different components be coupled 
at all?
> 
>>>This is just one of several methods you can
>>>dramatically reduce coupling.
>>
>>It's just one of several methods that dramatically increases complexity,
>>without changing anything to the fact that in the end, *practically*,
>>some object ObjA changes its state as a response to a message sent by ObjB.
>>
> Say that to game/simulation developers.
> 
This is a complete non-sequitur as you don't say why game developers 
specifically benefit from the reduced coupling that you allege the 
provision of accessor methods introduces.

Tight coupling would be (for example) where you provided the argument to 
a method by storing it in a global variable rather than passing it as an 
argument. From a coupling point of view it makes no difference whether 
you call an accessor method or (in Python) read or write a referenced 
object's attributes directly. You still have to know the required API: 
whether you call a method (in which case you have to know its name) or 
read/write an attribute (in which case you have to know its name ...) 
makes no essential difference.

It appears you have seen the term "content coupling" as defined, for 
example, in

   http://en.wikipedia.org/wiki/Coupling_%28computer_science%29

and taken that to mean that any knowledge at all of another object's 
internals will lead to over-tight coupling and hence low cohesion. The 
Python point of view is somewhat different, and says that since both 
methods and data items are attributes of instances there is little 
difference (except in efficiency) between accessing data via a method 
(inefficient) and accessing data directly through the attribute 
containing that data (efficient).

It has already been pointed out to you several times that once you have 
written your code to access attributes you can introduce properties 
without changing the client (accessing) code should further isolation or 
additional computation be required.
> 
>>>I'm sure glad I didn't burn my book.
>>
>>No comment.
>>
>>>>>>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* ?
>>>
>>>That's language dependent.
>>>
>>>>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 ?
>>>>
>>>I'm not advocating anything.
>>
>>cf below on this.
>>
>>>I'm just stating the use case for
>>>accessors and the wisdom behind them. My qualm with implicit accessors
>>>remains the name issue.
>>
>>The "name issue" is *your* problem. And AFAICT, it's a "problem" because
>>you refuse to free your mind from a "what's in the book" mindset.
>>
> What book are we talking about?
> 
Well you should know, you're the one who wants to hang on to it. So 
please enlighten us, what is this source of knowledge that appears to 
contradict sound computer science?

>>>>>>>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.

Whereas you appear to feel that there can be no possibility if a crash 
because someone calls thing.is_active(True), and yet you repeatedly fail 
to demonstrate the difference. Which is why this thread has been so 
mind-numbingly long. As is pointed out AGAIN here:
>>>>>>
>>>>>>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)
>>>
>>>I never advocated anything.
>>
>>You advocated
>>"""
>>1). Make all attributes of a class private/protected .
>>2). If a "non-callable" attribute is going to be used outside a class,
>>think about making it a property and name the property well, because
>>you never know...
>>"""
>>
> You use accessors when you need to control access to a data attribute.
> That's not advocacy, that's common sense.
> 
Perhaps so, but you still refuse to explain why it's better, when all 
you need to do is read or write the value of an instance's attribute, to 
define accessor methods to do it, introducing unnecessary (in Python) 
overhead in the process.
> 
>>>>>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.
>>>>
Am I the only one here who has lost track of this "'tis/'tisn't" stuff?

>>>>>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.
>>>>
>>>I agree. And I already told you I think in terms of state and behavior
>>>and not language dependent semantics.
>>
>>Then why do you advise "(making) all attributes of a class
>>private/protected" and systematically using properties ?
>>
> Because you don't want third parties illegimately tampering with an
> object's internal data and thus crashing your system?
> 
In which case you write correct software and provide it with a thorough 
set of tests to allow you to modify it without worrying too much about 
breakage. Attribute access is part of an object's API just like method 
calls are.

You seem to think that every ill in software design can be legislated 
away by strangling design freedom with specific rules. I am afraid that 
experience will teach you this is far from the case: go look at the way 
that the "bondage" languages that use static typing (like Java and C++) 
still fail to impose the required typing discipline on a determinedly 
incompetent user.

As has already been said, the Python philosophy is to provide the 
features that "consenting adults" (i.e. programmers who know what they 
are doing) can use most effectively to produce complex software systems 
to reasonable deadlines. Unfortunately not all the knowledge required to 
do this is readily available in books; the rest must come from experience.

regards
  Steve
-- 
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