[pypy-dev] question about read only attributes of my file object

Laura Creighton lac at strakt.com
Sat Dec 4 18:07:33 CET 2004

In a message of Sat, 04 Dec 2004 16:48:23 GMT, Armin Rigo writes:
>Hi Laura,

Hi Armin:

>On Sat, Dec 04, 2004 at 03:48:14PM +0100, Laura Creighton wrote:
>> Ok, this property takes effect even in __init__, rats.
>Indeed.  But __init__ can still write to the read-only attribute by bypas
>the property and writing to self.__dict__ directly.
>For reading: the name you use, getval(), is misleading.  It should be called
>getmode(), because it is specific to the 'mode' attribute.  Indeed, it only
>receives one argument:
>class C(object):
>    def __init__(self):
>        self.__dict__['mode'] = 'rU'
>    def getmode(self):
>        return self.__dict__['mode']
>    mode = property(getmode)

Yes.  This was what I didn't want, a separate, identical get function for
each readonly attribute.  Code duplication ... yuck!

>Another note.  It's possible to limit the set of attributes that the 'file'
>instances can take (to enforce the failure of user code like 'f.silly = 42').  
>This is done with __slots__, but using it requires some rewriting because
>then the instances don't have a __dict__ any more.  In this case it's better
>to use underscored names:
>class file(object):
>    __slots__ = ['_mode']
>    def __init__(self):
>        self._mode = 'rU'
>    def _getmode(self):
>        return self._mode
>    mode = property(_getmode)

Hmm.  I have been associating with people who think that 'people who abuse
the __slots__ feature to limit attributes, instead of to save memory should
all be shot at dawn'.  I take it you are not in that camp...

>The drawback here is that 'self._mode' is a public and writeable attribute,
>but we probably don't care because of the leading underscore.  If we do care,
>it's probably possible to have a slot called exactly 'mode' which is hidden by
>the 'mode' property, but I can't figure out what kind of obcure hacks are
>needed to do that...  mwh? :-)

Hmm, I think we need to back to consider 'what is it that we really want to
do'  rather than 'can somebody find the next glorious hack to keep me doing
what I thought I wanted to do'.  The latter seems a bit Perlish to me. :-)

So -- do you really want me to use the properties?   I now think they are icky.


>A bientot,

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