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how to use property?

Terry Reedy tjreedy at udel.edu
Mon Sep 17 19:34:52 EDT 2012 
On 9/17/2012 6:12 PM, Chris Angelico wrote:
> On Tue, Sep 18, 2012 at 7:55 AM, Fernando Jiménez <the.merck at gmail.com> wrote:
>> Hi guys!
>>
>> I'm noob in python and I would know how to correctly use the property. I
>> have read some things about it but I do not quite understand.
>>
>> But I think it's a bad habit to use _ to change the visibility of the
>> attributes as in JAVA.
>>
>> How to correctly use the property?
>
> The single leading underscore is nothing to do with visibility; it's a
> courteous request that external referents not touch something. In a
> "consenting adults" model, that's usually sufficient.
>
> For the most part, in fact, you don't need @property at all. Just make
> an object's members public and save yourself the trouble! Unlike the
> recommendation in C++ and Java, Python doesn't ask you to hide things
> and write code to make them available. Instead of starting with
> getters and setters, just start with a flat property, and move to
> getters/setters only when you find you need them.

More examples:

A class has a data attribute that really is a simple attribute, no 
property. You define a subclass that needs a calculation for the 
attribute. So you use property in the subclass.

A class has an attribute that is a constant that must be computed. You 
do not want to compute is unless and until needed.

def get_x(self):
   try:
     return self._x
   except AttributeError:
     self._x = calculate_x()
     return self._

For a read-only attribute, don't provide a setter. If you do not like 
"AttributeError: can't set attribute", provide one with a customized error.

But I think most of the data attributes in stdlib classes are straight 
attributes.

-- 
Terry Jan Reedy
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