Descriptor: class name precedence over instance name

[Veek. M]

Ian Kelly wrote:

> On Sat, Jul 2, 2016 at 3:34 AM, Veek. M <vek.m1234 at gmail.com> wrote:
>> So essentially from what Ian said:
>> data_descriptor_in_instance -> instance_attribute -> non-
>> data_descriptor_in_instance -->__mro__
>>
>> is how the search takes place. Correct?
> 
> Close, but I would write it as:
> data_descriptor_in_class_including_mro -> instance_attribute ->
> non_data_descriptor_or_class_attribute_including_mro
> 
> In either case the class dict search is in __mro__ order. You can find
> the actual implementation here:
> 
> https://hg.python.org/cpython/file/30099abdb3a4/Objects/object.c#l1326
> 
> Roughly, the algorithm is this:
> 
>     tp = type(obj)
> 
>     # This call searches the MRO.
>     descr = _PyType_Lookup(tp, name)
> 
>     if descr != NULL and is_data_descriptor(descr):
>         return descr.__get__(obj, tp)
> 
>     if name in obj.__dict__:
>         return obj.__dict__[name]
> 
>     if descr != NULL and is_non_data_descriptor(descr):
>         return descr.__get__(obj, tp)
> 
>     if descr != NULL:
>         return descr
> 
>     raise AttributeError(name)
> 
>> ------------------------------
>>
>> Regarding part2 of the Q, :) Ian hasn't explained it, so I'm not sure
>> how to explain it better :) but i'll try given that he has clarified
>> part of the answer.
>>
>> Basically Beazley has a TypedProperty descriptor class, and in class
>> Foo he instantiates:
>>  name = TypedProperty
>> Then he does f = Foo()
>>
>> Thanks to Ian, we now know that any lookup on 'f' eg: f.name would
> 
> Not *any* lookup, only one for which there is a declared descriptor.
> So in this example f.name or f.num would resolve according to the
> descriptor's __get__ method, but any other attribute lookup would just
> return whatever value is set on the instance (if any).
> 
>> cause.. well.. the f.name(TypedProperty-descriptor) to gain
>> precedence thus hiding the f.name attribute! Therefore he needs to
>> name-decorate or in this example append an '_' for whatever stupid
>> reason.
> 
> Right, if the "name" descriptor tried to store the value in the
> unmangled "name" instance attribute, then its getattr call would just
> recur back to the descriptor and you'd have an infinite loop.
> 
> In my opinion the mangling should be a bit heavier than what's done in
> this example, to avoid collisions between the descriptor and the class
> that's using it. I like to follow the pattern of __foo name mangling,
> so I would have instead used:
> 
>     self.name = "_TypedProperty__" + name

Ouch - that's above my skill level (Python types, especially 
PyType_Lookup has no docs but i did find https://docs.python.org/3/c-api/index.html) and I got side tracked into reading the PCIe spec so 
today went down the drain. I'm putting this on hold as totally not 
understood and requiring much more reading. I'll drag it up later if 
everyones okay with it.