__getattribute__ and __slots__

[pascal.parent@free.fr]

Hi,
I try to define a (new-style) class who:
- have a __slots__ defined to be strict attributes,
- return None if the attribute is 'ok' but not set, or raise a 'normal'
error if the attribute isn't in __slots__.

This code runs, but is it the good way?

Thanks.

class test(object):
__slots__ = ['id']
def __getattr__(sel f, attr):
if not attr in self.__slots__: raise AttributeError
try:
return self.attr
except:
return None

[Peter Otten]

Re: __getattribute_ _ and __slots__

pascal.parent@f ree.fr wrote:

Python has both __getattribute_ _() and __getattr__(). While
__getattribute_ _() will always be called if you ask for an attribute
__getattr__() serves only as fallback if the attribute if not found by
other means.
[color=blue]
> I try to define a (new-style) class who:
> - have a __slots__ defined to be strict attributes,[/color]

__slots__ is a performance/memory optimization. Using __slots__ to introduce
bondage & discipline through the backdoor is against Python's spirit.
[color=blue]
> - return None if the attribute is 'ok' but not set, or raise a 'normal'
> error if the attribute isn't in __slots__.[/color]

The approach you have chosen is a very ineffective one. Why don't you just
set the attribute to a default value in the initializer?
[color=blue]
> This code runs, but is it the good way?[/color]

I don't think so...
[color=blue]
> class test(object):
> __slots__ = ['id']
> def __getattr__(sel f, attr):
> if not attr in self.__slots__: raise AttributeError
> try:
> return self.attr[/color]

The line above does not do what you think it does. It just calls
test_instance._ _getattr__("att r"), and since "attr" is not in __slots__ it
raises an AttributeError.
[color=blue]
> except:
> return None[/color]

You should get the same effect with
[color=blue][color=green][color=darkred]
>>> class T(object):[/color][/color][/color]
.... __slots__ = ["id"]
.... def __getattr__(sel f, name):
.... if name not in self.__slots__:
.... raise AttributeError
.... return 42 # for better visibility
....[color=blue][color=green][color=darkred]
>>> t = T()
>>> t.id[/color][/color][/color]
42[color=blue][color=green][color=darkred]
>>> t.whatever[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 5, in __getattr__
AttributeError[color=blue][color=green][color=darkred]
>>> t.id = "something else"
>>> t.id[/color][/color][/color]
'something else'[color=blue][color=green][color=darkred]
>>> t.whatever = "something else"[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: 'T' object has no attribute 'whatever'

Peter