Static properties

[Per Erik Stendahl]

Hello everyone,

Is it possible to define "static properties" in a class?

Example:
I have a class, Path, that wraps a lot of os.* and os.path.* functions
(and others), so I can write things like this:

x = Path('/usr/local/some/file')
if x.IsFile:
contents = x.Read()
...

Now, I would like to wrap os.getcwd(). I can do it like this:

class Path:
def CurrentDirector y():
return os.getcwd()
CurrentDirector y = staticmethod(Cu rrentDirectory)

pwd = Path.CurrentDir ectory()

Question: Can I make CurrentDirector y a property? Just adding
CurrentDirector y = property(Curren tDirectory) after the staticmethod
line didn't work, unsurprisingly. :-)

Regards,
Per Erik Stendahl

[Alex Martelli]

Re: Static properties

Per Erik Stendahl <python-spam@berrs.net> wrote:
[color=blue]
> Hello everyone,
>
> Is it possible to define "static properties" in a class?[/color]

Yes, but only by defining a custom metaclass to be the type of that
class. Properties are defined in the type, not in the instance; so, for
a class itself to have properties, the class's type, commonly known as
its metaclass, must be the one defining them.

[color=blue][color=green][color=darkred]
>>> class metaPath(type):[/color][/color][/color]
.... def curdir(cls): return os.getcwd()
.... curdir = property(curdir )
....[color=blue][color=green][color=darkred]
>>> class Path: __metaclass__ = metaPath[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> print Path.curdir[/color][/color][/color]
/Users/alex/cb/dblite/dblite_0.5


_However_...:
[color=blue][color=green][color=darkred]
>>> print Path().curdir[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: 'Path' object has no attribute 'curdir'[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

if you want to be able to get at property curdir on INSTANCES of Path as
you normally would for a staticmethod, yet with property syntax, I think
you need to get a bit more clever than this... descriptors don't get
looked up two metalevels up, only one...


Alex

[Bengt Richter]

Re: Static properties

On Thu, 26 Aug 2004 23:01:49 +0200, aleaxit@yahoo.c om (Alex Martelli) wrote:
[color=blue]
>Per Erik Stendahl <python-spam@berrs.net> wrote:
>[color=green]
>> Hello everyone,
>>
>> Is it possible to define "static properties" in a class?[/color]
>
>Yes, but only by defining a custom metaclass to be the type of that
>class. Properties are defined in the type, not in the instance; so, for
>a class itself to have properties, the class's type, commonly known as
>its metaclass, must be the one defining them.[/color]

Um, UIAM a property is just a peculiar descriptor, which you can define
without resorting to metaclass magic. See below ...[color=blue]
>
>[color=green][color=darkred]
>>>> class metaPath(type):[/color][/color]
>... def curdir(cls): return os.getcwd()
>... curdir = property(curdir )
>...[color=green][color=darkred]
>>>> class Path: __metaclass__ = metaPath[/color][/color]
>...[color=green][color=darkred]
>>>> print Path.curdir[/color][/color]
>/Users/alex/cb/dblite/dblite_0.5
>
>
>_However_... :
>[color=green][color=darkred]
>>>> print Path().curdir[/color][/color]
>Traceback (most recent call last):
> File "<stdin>", line 1, in ?
>AttributeError : 'Path' object has no attribute 'curdir'[color=green][color=darkred]
>>>>[/color][/color]
>
>if you want to be able to get at property curdir on INSTANCES of Path as
>you normally would for a staticmethod, yet with property syntax, I think
>you need to get a bit more clever than this... descriptors don't get
>looked up two metalevels up, only one...
>[/color]
But you only need one:
[color=blue][color=green][color=darkred]
>>> class Path(object):[/color][/color][/color]
... class curdir(object):
... def __get__(self, inst, cls): return os.getcwd()
... curdir = curdir()
...[color=blue][color=green][color=darkred]
>>> import os
>>> Path.curdir[/color][/color][/color]
'C:\\pywk\\clp'[color=blue][color=green][color=darkred]
>>> Path().curdir[/color][/color][/color]
'C:\\pywk\\clp'

I wonder if I'll beat you to this post ;-)

Regards,
Bengt Richter

[Alex Martelli]

Re: Static properties

Bengt Richter <bokr@oz.net> wrote:
...[color=blue][color=green][color=darkred]
> >> Is it possible to define "static properties" in a class?[/color]
> >
> >Yes, but only by defining a custom metaclass to be the type of that
> >class. Properties are defined in the type, not in the instance; so, for
> >a class itself to have properties, the class's type, commonly known as
> >its metaclass, must be the one defining them.[/color]
>
> Um, UIAM a property is just a peculiar descriptor, which you can define
> without resorting to metaclass magic. See below ...[/color]

Hmmm -- I think we're talking somewhat at cross-purposes between
property, the built-in type, and the effect it has (that accessing an
attribute runs some code). I was focusing on the formed, but you're
quite right regarding the latter.
[color=blue][color=green]
> >if you want to be able to get at property curdir on INSTANCES of Path as
> >you normally would for a staticmethod, yet with property syntax, I think
> >you need to get a bit more clever than this... descriptors don't get
> >looked up two metalevels up, only one...
> >[/color]
> But you only need one:[/color]

If you don't use property, but "get a bit more clever" by defining a
custom descriptor as you have, yes:
[color=blue]
>[color=green][color=darkred]
> >>> class Path(object):[/color][/color]
> ... class curdir(object):
> ... def __get__(self, inst, cls): return os.getcwd()
> ... curdir = curdir()[/color]

Nice. Unfortunately it doesn't extend to data properties, those with a
__set__ method:
[color=blue][color=green][color=darkred]
>>> class Path(object):[/color][/color][/color]
.... class foo(object):
.... def __get__(*a): return 'gotten'
.... def __set__(*a): print 'setten'
.... foo = foo()
....[color=blue][color=green][color=darkred]
>>> p=Path()
>>> p.foo[/color][/color][/color]
'gotten'[color=blue][color=green][color=darkred]
>>> p.foo=23[/color][/color][/color]
setten[color=blue][color=green][color=darkred]
>>> p.foo[/color][/color][/color]
'gotten'[color=blue][color=green][color=darkred]
>>> Path.foo[/color][/color][/color]
'gotten'[color=blue][color=green][color=darkred]
>>> Path.foo = 23
>>> Path.foo[/color][/color][/color]
23[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

I think (I hope I'm wrong) that to catch 'Path.foo = something' you do
have to use a custom metaclass (not 'metaclass magic', nothing in the
least magical about it). But if you're simply willing to trust users to
just never assign to Path.foo -- you don't need for such assignments to
cause an error nor for them to cause any specific action -- a simple
custom descriptor may indeed suffice.


Alex

[Michael Hudson]

Re: Static properties

aleaxit@yahoo.c om (Alex Martelli) writes:
[color=blue]
> I think (I hope I'm wrong) that to catch 'Path.foo = something' you
> do have to use a custom metaclass[/color]

You're right.

Cheers,
mwh

--
/* I'd just like to take this moment to point out that C has all
the expressive power of two dixie cups and a string.
*/ -- Jamie Zawinski from the xkeycaps source