Understanding the pythonic way: why a.x = 1 is better than a.setX(1) ?

Diez B. Roggisch deets at nospam.web.de
Fri Sep 5 17:23:10 CEST 2008


Ivan Illarionov schrieb:
> On 4 сент, 21:49, Bruno Desthuilliers
> <bdesth.quelquech... at free.quelquepart.fr> wrote:
>> Ivan Illarionov a écrit :
>>
>>
>>
>>> On 4 сент, 22:59, Carl Banks <pavlovevide... at gmail.com> wrote:
>>>> You can write code to guard against this if you want:
>>>> class A:
>>>>     legal = set(["x"])
>>>>     def __setattr__(self,attr,val):
>>>>         if attr not in self.legal:
>>>>             raise AttributeError("A object has no attribute '%s'" %
>>>> attr)
>>>>         self.__dict__[attr] = val
>>>>     def __init__(self,x):
>>>>         self.y = x
>>>> I suspect most people who go into Python doing something like this
>>>> soon abandon it when they see how rarely it actually catches anything.
>>> '__slots__' is better:
>> For which definition of "better" ? __slots__ are a mean to optimize
>> memory usage, not to restrict dynamism. Being able to dynamically add
>> arbitrary attributes is actually a feature, not a bug, and uselessly
>> restricting users from doing so is not pythonic. IOW : don't do that.
> 
> Carl's example is restricting dynamism in the same way as __slots__.
> I've just suggested a better implementation. It is not me who
> suggested dynamism restriction as a way to guard against errors.

This is not correct. While Carl's Example looks similar, you actually
*can* create new attributes using

obj.__dict__['name'] = value

__slots__ OTOH prevents that because it does create instances *without*
an instance-dict.

This being said, I think we all agree that it's nothing to be desired.


Diez



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