hasattr + __getattr__: I think this is Python bug

[Bruno Desthuilliers]

Bruno Desthuilliers a écrit :
> Ethan Furman a écrit :
>> Bruno Desthuilliers wrote:
>>> Duncan Booth a écrit :
> (snip)
> 
>>>> Or you could create the default as a class attribute 
>>>
>>> from the OP:
>>> """
>>> I have a class (FuncDesigner oofun) that has no attribute "size", but
>>> it is overloaded in __getattr__, so if someone invokes
>>> "myObject.size", it is generated (as another oofun) and connected to
>>> myObject as attribute.
>>> """
>>>
>>> so this solution won't obviously work in this case !-)
>>>
>>> Also and FWIW, I wouldn't advocate this solution if the "default" 
>>> class attribute is of a mutable type.
>>
>> Well, it is Monday, so I may be missing something obvious, but what is 
>> the effective difference between these two solutions?
> 

If you meant "what is the difference between creating the "whatever" 
attribute with a default value in the initializer and creating it on 
demand in the __getattr__ hook", the main difference is that in the 
first case, the instance is garanteed to have this attribute, so you get 
rid of "hasattr" checks (and the unwanted side effects) or, worse, 
direct check of the instance __dict__. Except for a couple of corner 
case, client code shouldn't have to worry about this kind of things - 
this breaks encapsulation.