how do i create such a thing?

[Lowell Kirsh]

I want to create a class called DefaultAttr which returns a default
value for all attributes which haven't been given values yet. It will
work like this:
[color=blue][color=green]
>> x = DefaultAttr(99)
>> print x.foo[/color][/color]
99[color=blue][color=green]
>> print x.bar[/color][/color]
99[color=blue][color=green]
>> x.foo = 7
>> print x.foo[/color][/color]
7

I already have a similar class called DefaultDict which works similarly
which I assume would be a good thing to use.

Lowell

[Pedro Werneck]

Re: how do i create such a thing?

Hi,

If you need direct access to some atribute, use object.__getatt ribute__.

[color=blue][color=green][color=darkred]
>>> class DefaultAttr(obj ect):[/color][/color][/color]
.... def __init__(self, default):
.... self.default = default
.... def __getattribute_ _(self, name):
.... try:
.... value = object.__getatt ribute__(self, name)
.... except AttributeError:
.... value = self.default
.... return value
....[color=blue][color=green][color=darkred]
>>> x = DefaultAttr(99)
>>>
>>> print x.a[/color][/color][/color]
99[color=blue][color=green][color=darkred]
>>> x.a = 10
>>> print x.a[/color][/color][/color]
10[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]



On Sun, 30 Jan 2005 13:54:38 -0800
Lowell Kirsh <lkirsh@cs.ubc. ca> wrote:
[color=blue]
> I want to create a class called DefaultAttr which returns a default
> value for all attributes which haven't been given values yet. It will
> work like this:
>[color=green][color=darkred]
> >> x = DefaultAttr(99)
> >> print x.foo[/color][/color]
> 99[color=green][color=darkred]
> >> print x.bar[/color][/color]
> 99[color=green][color=darkred]
> >> x.foo = 7
> >> print x.foo[/color][/color]
> 7
>
> I already have a similar class called DefaultDict which works
> similarly which I assume would be a good thing to use.
>
> Lowell
> --
> http://mail.python.org/mailman/listinfo/python-list[/color]

[Steven Bethard]

Re: how do i create such a thing?

Pedro Werneck wrote:[color=blue]
> If you need direct access to some atribute, use object.__getatt ribute__.
>[color=green][color=darkred]
>>>>class DefaultAttr(obj ect):[/color][/color]
>
> ... def __init__(self, default):
> ... self.default = default
> ... def __getattribute_ _(self, name):
> ... try:
> ... value = object.__getatt ribute__(self, name)
> ... except AttributeError:
> ... value = self.default
> ... return value
> ...
>[color=green][color=darkred]
>>>>x = DefaultAttr(99)
>>>>print x.a[/color][/color]
> 99
>[color=green][color=darkred]
>>>>x.a = 10
>>>>print x.a[/color][/color]
> 10[/color]

Of if you only want to deal with the case where the attribute doesn't
exist, you can use getattr, which gets called when the attribute can't
be found anywhere else:

py> class DefaultAttr(obj ect):
.... def __init__(self, default):
.... self.default = default
.... def __getattr__(sel f, name):
.... return self.default
....
py> x = DefaultAttr(99)
py> x.a
99
py> x.a = 10
py> x.a
10

Steve

[Alex Martelli]

Re: how do i create such a thing?

Steven Bethard <steven.bethard @gmail.com> wrote:
[color=blue]
> Of if you only want to deal with the case where the attribute doesn't
> exist, you can use getattr, which gets called when the attribute can't
> be found anywhere else:
>
> py> class DefaultAttr(obj ect):
> ... def __init__(self, default):
> ... self.default = default
> ... def __getattr__(sel f, name):
> ... return self.default
> ...
> py> x = DefaultAttr(99)
> py> x.a
> 99
> py> x.a = 10
> py> x.a
> 10[/color]

This is a good approach, but it's fragile regarding specialnames.
[color=blue][color=green][color=darkred]
>>> class DefaultAttr(obj ect):[/color][/color][/color]
.... def __init__(self, default): self.default = default
.... def __getattr__(sel f, name): return self.default
....[color=blue][color=green][color=darkred]
>>> import copy
>>> copy.copy(Defau ltAttr(99))[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/usr/local/lib/python2.4/copy.py", line 87, in copy
rv = reductor(2)
TypeError: 'int' object is not callable

There are many more, worse example for classic classes, but even in
newstyle classes you must consider that once in a while a system routine
will try a getattr(someins t, '__aspecialname __', None) or the like --
and your class is making its instances claim to have ANY special name
that may be introspected for in this way. This can be a pernicious lie,
since your class in fact has no idea whatsoever what that specialname
and the corresponding value might be for.

It's HIGHLY advisable to have your __getattr__ methods raise
AttributeError for any requested name that starts and ends with double
underscores, possibly with some specific and specifically designed
exceptions.


Alex

[Lowell Kirsh]

Re: how do i create such a thing?

What might these exceptions be?
[color=blue]
> It's HIGHLY advisable to have your __getattr__ methods raise
> AttributeError for any requested name that starts and ends with double
> underscores, possibly with some specific and specifically designed
> exceptions.
>
>
> Alex[/color]

[Alex Martelli]

Re: how do i create such a thing?

Lowell Kirsh <lkirsh@cs.ubc. ca> wrote:
[color=blue]
> What might these exceptions be?
>[color=green]
> > It's HIGHLY advisable to have your __getattr__ methods raise
> > AttributeError for any requested name that starts and ends with double
> > underscores, possibly with some specific and specifically designed
> > exceptions.[/color][/color]

For example, delegation of such requests to some other object:
def __getattr__(sel f, name):
return getattr(self._d elegate, name)

In such cases you may decide you do not need to block __specials__,
because you're OK with having self._delegate supply them or be
responsible to raise AttributeError if necessary.


Alex

[Lowell Kirsh]

Re: how do i create such a thing?

I'm not sure I get it. What's the purpose of using a delegate rather
than having the object itself supply the return value?

Alex Martelli wrote:[color=blue]
> Lowell Kirsh <lkirsh@cs.ubc. ca> wrote:
>
>[color=green]
>>What might these exceptions be?
>>
>>[color=darkred]
>>>It's HIGHLY advisable to have your __getattr__ methods raise
>>>AttributeErr or for any requested name that starts and ends with double
>>>underscore s, possibly with some specific and specifically designed
>>>exceptions .[/color][/color]
>
>
> For example, delegation of such requests to some other object:
> def __getattr__(sel f, name):
> return getattr(self._d elegate, name)
>
> In such cases you may decide you do not need to block __specials__,
> because you're OK with having self._delegate supply them or be
> responsible to raise AttributeError if necessary.
>
>
> Alex[/color]

[Steve Holden]

Re: how do i create such a thing?

Lowell Kirsh wrote:
[color=blue]
> I'm not sure I get it. What's the purpose of using a delegate rather
> than having the object itself supply the return value?
>
> Alex Martelli wrote:
>[color=green]
>> Lowell Kirsh <lkirsh@cs.ubc. ca> wrote:
>>
>>[color=darkred]
>>> What might these exceptions be?
>>>
>>>
>>>> It's HIGHLY advisable to have your __getattr__ methods raise
>>>> AttributeError for any requested name that starts and ends with double
>>>> underscores, possibly with some specific and specifically designed
>>>> exceptions.[/color]
>>
>>
>>
>> For example, delegation of such requests to some other object:
>> def __getattr__(sel f, name):
>> return getattr(self._d elegate, name)
>>
>> In such cases you may decide you do not need to block __specials__,
>> because you're OK with having self._delegate supply them or be
>> responsible to raise AttributeError if necessary.
>>
>>
>> Alex[/color][/color]

The point is that you can keep a reference to some object, and it's the
next best thing to having subclassed the object's class but with closer
control.

regards
Steve

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