Best way to set/get an object property

[Hussein B]

On Aug 25, 4:31 am, Steven D'Aprano
<ste... at REMOVE.THIS.cybersource.com.au> wrote:
> On Sun, 24 Aug 2008 23:56:27 -0700, Hussein B wrote:
> > On Aug 24, 7:12 pm, Steven D'Aprano <st... at REMOVE-THIS-
> > cybersource.com.au> wrote:
> >> >> I noted that Python encourage the usage of: -- obj.prop = data
> >> >> x = obj.prop
> >> >> --
> >> >> to set/get an object's property value. What if I want to run some
> >> >> logic upon setting/getting a property? What is Python preferred
> >> >> method to do so (using the new feature 'property')? I don't think
> >> >> __getattr__ and __setattr__ are practical (I have to code the
> >> >> property name into them).
> ...
> >> I think the actual answer to his question is that properties are the
> >> preferred way to "run some logic upon setting/getting" an attribute,
> >> that is, to implement getters and setters.
>
> >> Hussein, the Java habit of writing setters and getters for everything
> >> isn't considered good practice in Python, but if you need them, that's
> >> exactly what the property() function is for.
>
> >> --
> >> Steven
>
> > Thank you Steven :)
> > --
> > public class JClass {
> >   private int answer; // property
> > }
> > --
> > class PyClass(object):
> >   doc __init__(self):
> >     self.answer = None
> > --
> > AFAIUY (understand you), what it is called a property in Java, it is
> > called an attribute in Python?
> > Why Python encourages direct access to object's attributes?  aren't
> > setters/getters considered vital in OOP (encapsulation)?
> > Thank you all for your time and help.
>
> Hussein, first let me ask you to please stop using "--" as a separator
> around code. Many News clients, including mine, expect -- on a line by
> itself to mean "everything from here on is the writer's signature", and
> consequently that makes it harder to reply correctly to your posts. I had
> to manually copy and paste your text in order to quote it. Perhaps you
> could use === or +++ or ***  as a separator?
>
> Now, back to your actual question...
>
> I'm not a Java coder, so the following should be read as my opinion.
> Python attributes are equivalent to Java _public_ properties, not
> private. If you can write:
>
> public class JClass {
>    public int answer;
>
> }
>
> then that would be more or less equivalent to Python's
>
> class PyClass(object):
>     def __init__(self):
>         self.answer = None
>
> Yes, Python does encourage direct access to an object's attributes. The
> Python philosophy is "we're all adults here". If coders wish to shoot
> themselves in the foot by accessing clearly marked private attributes,
> then the language can't stop them and shouldn't try. It's easy to bypass
> such private/public protection in C++, and harder, but still possible, in
> Java.
>
> The Python development team is certainly aware that such a tactic
> introduces some costs, by reducing encapsulation, but it also has many
> benefits (e.g. less boilerplate getter/setter methods, faster development
> time). It is their belief that such costs are worth paying in order to
> get the benefits. That's the philosophy of the language. Python is not
> trying to be Java, and Java should not try to be Python.
>
> Python does not enforce private attributes. By convention attributes
> starting with a single underscore are considered "private -- don't touch
> unless you know what you're doing". Attributes starting with a double
> underscore are "really private", and Python mangles the name to (almost)
> enforce it.
>
> Example:
>
> def Parrot(object):
>     colour = 'red'  # public, free to use
>     _windspan = 15  # semi-private, use it at your own risk
>     __species = 'Norwegian Blue'  # mangled to _Parrot__species
>
> But it's quite rare to see double-underscore "really private" attributes
> in Python code. It is considered to go against the spirit of the language.
>
> I'm told that in Java it is quite difficult to change a class from using
> public attributes to getters/setters, and therefore many Java developers
> prefer to use getters/setters right from the beginning. But in Python it
> is very easy to change from a bare attribute to a computed property
> without messing up calling code. So there's no advantage to writing
> something like this:
>
> class Foo(object):
>     def __init__(self):
>         self.__x = None  # private attribute
>     def setx(self, x):  # setter
>         self.__x = x
>     def getx(self):  # getter
>         return self.__x
>     x = property(getx, setx)
>
> That is considered a waste of time in Python circles and is strongly
> discouraged.
>
> You should read "Python Is Not Java" and "Java Is Not Python Either":
>
> http://dirtsimple.org/2004/12/python-is-not-java.html
>
> http://dirtsimple.org/2004/12/java-is-not-python-either.html
>
> --
> Steven

Thank you all guys and big thank you Steven, I owe you a beer.
Sorry, I wasn't aware of the two dashes problem as I use Google Group/
Reader.
comp.lang.python really rocks & much more friendly and useful than
comp.lang.ruby