Declarative properties

[Bruno Desthuilliers]

Dan Stromberg a écrit :
> On Thu, 11 Oct 2007 13:46:12 +0000, Marc 'BlackJack' Rintsch wrote:
> 
>> On Thu, 11 Oct 2007 13:04:53 +0000, Artur Siekielski wrote:
>>
>>> On Oct 11, 2:27 pm, Marc 'BlackJack' Rintsch <bj_... at gmx.net> wrote:
>>>> But why?  Default getters and setters are unnecessary and if you need
>>>> something other than the default you need to write it anyway more
>>>> explicitly.
>>> I see some problems with your approach:
>>>
>>> 1. If I use instance field 'name' which is accessed directly by other
>>> classes,
>>> and later I decide to implement nonstandard getter, I must refactor
>>> 'Person' class
>>> and in some places change 'name' to '_name' (assuming this is now the
>>> field's name).
>>> The problem is that I cannot automatically change 'name' to '_name'
>>> everywhere, because
>>> in some places I want public property value (eg. validated and
>>> formatted), and in other
>>> places raw property value.
>> So what?  Otherwise you carry *always* the baggage of a public property
>> and a private attribute whether you need this or not.  At least for me it
>> would be unnecessary in most cases.
> 
> That "baggage" of carrying around "unneeded" methods is something the
> computer carries for you - IE, no big deal in 99.99% of all cases.

1/ Accessing the value of a property is not free. Accessing a plain 
attribute is much cheaper.

2/ cluttering the class's namespace with useless names and the source 
code with useless code is definitively not a good thing.

> The "baggage" of possibly fixing (AKA "generalizing") how your attributes
> are accessed is something you lug around while your deadline looms.
> 
> Here's some code that defines such methods for you:
> 
> #!/usr/bin/env python
> 
> def gimme_set_get(foo, attribute):
>    lst = [ \
>       'def set_%s(self, value):' % attribute, \
>       '  self._%s = value' % attribute, \
>       'def get_%s(self):' % attribute, \
>       '  return self._%s' % attribute, \
>       'foo.set_%s = set_%s' % (attribute, attribute), \
>       'foo.get_%s = get_%s' % (attribute, attribute) \
>       ]
>    s = '\n'.join(lst)
>    code = compile(s, '<string>', 'exec')
>    eval(code)

Yuck. This is not only a waste of time, but also badly coded. have mercy 
and at least learn how to properly use lexical scoping and closures.

> class foo:
>    def __init__(self, value):
>       self.public_value = value
> gimme_set_get(foo, 'via_accessor_method_only')
> 
> f = foo(1)
> f.set_via_accessor_method_only(1/9.0)
> print f.get_via_accessor_method_only()
> 
> print dir(f)
> 
Correction: this is not only a waste of time and badly coded, this is 
also misleading and braindead.