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Date: Mon, 17 Sep 2012 19:46:23 -0400
From: Dave Angel <d@davea.name>
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To: Terry Reedy <tjreedy@udel.edu>
Subject: Re: how to use property?
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On 09/17/2012 07:34 PM, Terry Reedy wrote:
> On 9/17/2012 6:12 PM, Chris Angelico wrote:
>> On Tue, Sep 18, 2012 at 7:55 AM, Fernando Jiménez
>> <the.merck@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?
>>
> <snip>

> 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.
>

An important difference from every other language I've used:  The user
of the attribute does not need to change his code when you decide it
needs reimplementation as a property.

In C++ and java, for example, people will define getter and setter
methods just so they don't have to change them later.  Just "in case"
it's needed later.


-- 

DaveA