Best way to set/get an object property

[Hussein B]

Hey,
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).
Thanks.

[Peter Otten]

Hussein B 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).

Hussein, I don't think you'll learn much from asking these abstract
questions. At some point you have to get your hands dirty and write actual
code to get a feel for the language.

For example, it will then become obvious for you that property works best
for individual attributes while __getattr__ and friends are more convenient
if you want to treat multiple attributes the same way, attributes whose
names may not even be known until runtime (think delegation).

Peter

[Hussein B]

On Aug 24, 5:28 am, Peter Otten <[e-mail]:

Hussein B 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).

Hussein, I don't think you'll learn much from asking these abstract
questions. At some point you have to get your hands dirty and write actual
code to get a feel for the language.

For example, it will then become obvious for you that property works best
for individual attributes while __getattr__ and friends are more convenient
if you want to treat multiple attributes the same way, attributes whose
names may not even be known until runtime (think delegation).

Peter

Thanks Peter,
You are right, I have to try to touch the Python but the problem is I
don't have much time to do so.
I have a Java developer for more than 4 years and I find it is not so
easy to digest Python concepts, this is why I'm asking a lot of
obvious and clear easy to you (long time Pythonists).
Thank you for your time.

[castironpi]

On Aug 24, 5:07*am, Hussein B <[e-mail]:

Hey,
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).
Thanks.

The answer Hussein is you have both options in Python. If neither one
is clearly better-suited to your new application, pick one and go.

[Steven D'Aprano]

On Sun, 24 Aug 2008 12:28:53 +0200, Peter Otten wrote:

Hussein B 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).

Hussein, I don't think you'll learn much from asking these abstract
questions. At some point you have to get your hands dirty and write
actual code to get a feel for the language.

For example, it will then become obvious for you that property works
best for individual attributes while __getattr__ and friends are more
convenient if you want to treat multiple attributes the same way,
attributes whose names may not even be known until runtime (think
delegation).

I think you are misunderstanding Hussein's question. I believe that he is
using "property" to refer to what we would call an attribute. Naturally I
could be wrong, but this is how I interpret his question.

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

[Hussein B]

On Aug 24, 7:12 pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:

On Sun, 24 Aug 2008 12:28:53 +0200, Peter Otten wrote:

Hussein B 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).

Hussein, I don't think you'll learn much from asking these abstract
questions. At some point you have to get your hands dirty and write
actual code to get a feel for the language.

For example, it will then become obvious for you that property works
best for individual attributes while __getattr__ and friends are more
convenient if you want to treat multiple attributes the same way,
attributes whose names may not even be known until runtime (think
delegation).

I think you are misunderstanding Hussein's question. I believe that he is
using "property" to refer to what we would call an attribute. Naturally I
could be wrong, but this is how I interpret his question.

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.

[alex23]

On Aug 25, 4:56*pm, Hussein B <[e-mail]:

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?

The simplest answer is "Because Python is not Java" :)

Speaking of which, have you read the blog post of the same name? It
might be useful given your Java background: http://dirtsimple.org/2004/12/python-is-not-java.html

>*aren't
setters/getters considered vital in OOP (encapsulation)?

Not at all. They're definitely part of the mechanism that Java
provides for encapsulation, sure. However, because Python provides a
consistent interface for accessing attributes and properties, you
don't need to define a property unless your code requires it. If all
your getters & setters are doing is reading & writing to an attribute,
then why not just r&w directly to the attribute? If you later need to
add more complexity to that process, you can easily create a property
without having to change how any other piece of code refers to that
property, given it shares the same interface with attributes.

[Ken Seehart]

This is probably what you want:

http://www.python.org/download/relea...ntro/#property

Available in Python 2.2 or later.

Enjoy,
Ken Seehart

Hussein B wrote:

Hey,
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).
Thanks.
--
http://mail.python.org/mailman/listinfo/python-list

[Bruno Desthuilliers]

Steven D'Aprano a écrit :
(snip)

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.

Not necessarily "against the spirit" - it's mostly than __name_mangling
is only really useful when you want to protect a really vital
implementation attribute from being *accidentaly* overridden, and mostly
annoying anywhere else.

I'm told that in Java it is quite difficult to change a class from using
public attributes to getters/setters,

That's an understatement. Java has *no* support for computed attributes,
so you just can *not* turn a public attribute into a computed one.

and therefore many Java developers
prefer to use getters/setters right from the beginning.

Truth is that they have no other choice if they want to be able to
decouple implementation from interface.