Declarative properties

Marc 'BlackJack' Rintsch bj_666 at gmx.net
Thu Oct 11 20:42:16 CEST 2007


On Thu, 11 Oct 2007 09:58:48 -0700, Dan Stromberg wrote:

> 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:
>> 
>>> 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.

It shows twice as much attributes if I inspect the objects and I don't know
which are merely useless default getters and setters.  And it is more and
more complex code.  Code can be cut down a bit by some metaclass magic but
this brings in another complexity.

> The "baggage" of possibly fixing (AKA "generalizing") how your attributes
> are accessed is something you lug around while your deadline looms.

Sorry I don't get it.  If I want to customize the access to a "normal"
attribute I simply turn it into a property.

> 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)
> 
> 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)

And the benefit of this evil ``eval`` dance is exactly what!?

Ciao,
	Marc 'BlackJack' Rintsch



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