[Python-ideas] The AttributeError/__getattr__ mechanism

[Guido van Rossum]

I'm confused about how the proposal would work. How would you prevent
self.x.t from also raising AttributeMissError (thereby defeating its
purpose)?

On Fri, Nov 27, 2015 at 12:18 PM, 王珺 <wjun77 at gmail.com> wrote:

> I think the real world use case, as *property* is so common, is just when
> __getattr__ needed. So anyone using __getattr__ in practice? In fact I'm
> new tweaking these advanced features of python. Any suggestions are welcome
> and appreciated.
>
>
> In my case, I delegate failed attribute lookups to one member of the
> instance, as the given simple example in issue25634. For example, there's a
> class Point,
>
> class Point():
>     def __init__(self, x, y):
>         self.x = x
>         self.y = y
>
> and class Circle
>
> class Circle():
>     def __init__(self, center, radius):
>         self.center = center
>         self.radius = radius
>
> I don't think Circle should inherit Point. But I want to write Circle().x
> instead of Circle().center.x, so I write something like
>     def __getattr__(self, name):
>         try:
>             return getattr(self.center, name)
>         except AttributeError:
>             raise AttributeError("'{}' object has no attribute
> '{}'".format(self.__class__.__name__, name)) from None
>
>
> Another case is when I try to implement the design pattern of state. That
> is, when calling window.rightClick(), the behavior differs according to the
> state of the window, or window.__state. So
>
> class ActiveState(State):
>     @staticmethod
>     def rightClick(self):
>         print('right clicked')
> class InactiveState(State):
>     @staticmethod
>     def rightClick(self):
>         pass
>
> and
>
> class Window():
>     def __init__(self):
>         self.__state = ActiveState
>     def __getattr__(self, name):
>         try:
>             return partial(getattr(self.__state, name), self)
>         except AttributeError:
>             raise AttributeError("'{}' object has no attribute
> '{}'".format(self.__class__.__name__, name)) from None
>
> (The real situation is more complicated. In fact I've written a state
> module, and I think I can publish it in a week if anyone is interested.)
>
>
> > Is that really an imperfection? It says right there in the docs for
> __getattribute__ that you can delegate to __getattr__ by raising
> AttributeError
> Just like raising *StopIteration* in generator, as described in PEP479.
> Although I'm not sure *StopIteration* has ever been documented as one way
> to exit generator. And I admit that an unintentional AttributeError in
> __getattribute__ is a rare case if ever.
>
> 2015-11-28 2:34 GMT+08:00 Andrew Barnert <abarnert at yahoo.com>:
>
>> It seems like you can get some of the benefits of this proposal without
>> backward compat issues.
>>
>> Instead of changing things so AttributeError from __getattribute__ or a
>> descriptor no longer calls __getattr__, just add a new subclass that
>> doesn't, and change the Descriptor HOWTO to suggest using that subclass
>> (with a bit of discussion that says you can use AttributeError if you want
>> to trigger __getattr__, but usually you won't).
>>
>> That wouldn't fix any current code, but it also wouldn't break any code
>> that intentionally uses the features as documented. And it would make it
>> easy to write correct new code.
>>
>> One more thing:
>>
>> Without descriptor, unexpected AttributeError could only come from overriding __getattribute__, which is a rare case, although still an imperfection.
>>
>> Is that really an imperfection? It says right there in the docs for
>> __getattribute__ that you can delegate to __getattr__ by raising
>> AttributeError, so if someone does that, presumably it's intentional. It's
>> not like the case with descriptors, where you have to think through the
>> interaction of multiple features to figure out that raising an
>> AttributeError will call __getattr__, and therefore many such uses are
>> probably bugs.
>>
>> Sent from my iPhone
>>
>> On Nov 27, 2015, at 09:49, 王珺 <wjun77 at gmail.com> wrote:
>>
>> Hello everyone:
>>
>> I'm suggesting a modification to the AttributeError/__getattr__
>> mechanism, see issue25634:
>> http://bugs.python.org/issue25634
>>
>> I used __getattr__ sometimes, and descriptor especially property is so
>> widely used. I wonder whether someone had encountered the same problem with
>> me.
>>
>> However, this is a complicated problem including performance issues, and
>> backward compatibility.
>>
>> Thanks for your attention,
>> Jun Wang
>>
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>>
>
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-- 
--Guido van Rossum (python.org/~guido)
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