can't set attributes of built-in/extension type

[Steve Holden]

Neal Becker wrote:
> Steve Holden wrote:
> 
>> Neal Becker wrote:
>>> 7stud wrote:
>>>
>>>> On Feb 21, 11:19 am, Neal Becker <ndbeck... at gmail.com> wrote:
>>>>> I'm working on a simple extension.  Following the classic 'noddy'
>>>>> example.
>>>>>
>>>>> In [15]: cmplx_int32
>>>>> Out[15]: <type 'numpy.cmplx_int32'>
>>>>>
>>>>> Now I want to add an attribute to this type.  More precisely, I want a
>>>>> class attribute.
>>>>>
>>>>> cmplx_int32.test = 0
>>>>> ---------------------------------------------------------------------------
>>>>> TypeError                                 Traceback (most recent call
>>>>> last)
>>>>>
>>>>> /home/nbecker/numpy/<ipython console> in <module>()
>>>>>
>>>>> TypeError: can't set attributes of built-in/extension
>>>>> type 'numpy.cmplx_int32'
>>>>>
>>>>> What am I missing?
>>>> class Dog(object):
>>>>     def __setattr__(self, attr, val):
>>>>         print "TypeError: can't set attributes of built-in/extension"
>>>>         print "type 'Dog.cmplx_int32'"
>>>>
>>>> d = Dog()
>>>> d.test = 0
>>>>
>>>> --output:--
>>>> TypeError: can't set attributes of built-in/extension
>>>> type 'Dog.cmplx_int32'
>>> Not quite, I'm setting a class attribute, not an attribute on an
>>> instance.
>>>
>> Quite. The problem is that extension types' attributes are determined by
>> the layout of the object's slots and forever fixed in the C code that
>> implements them: the slots can't be extended, so there's no way to add
>> attributes. This is an efficiency feature: it would be *extremely* slow
>> to look up the basic types' attributes using late-binding (it would also
>> change the nature of the language somewhat, making it more like Ruby or
>> Self).
>>
>> So the reason you can't do what you want to is the same reason why you
>> can't add attribute to the built-in types (which are, of course, clearly
>> mentioned in the error message).
>>
>>  >>> object.anyoldname = "You lose!"
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> TypeError: can't set attributes of built-in/extension type 'object'
>>  >>>
>>
>> If you look in typeobject.c you'll find this error message occurs when
>> the object's type isn't a PyHeapTypeObject (in other words, if it's one
>> of the built-in or extension types).
>>
> Thanks, but I'm a bit confused.  After reading in my "Python in a Nutshell",
> I found that if after calling PyReady on my type object, if I use
> PyDict_SetItemString (my_type_obj.tp_dict,)
> 
> That seems to work fine (which isn't exactly what it said in the Nutshell
> book, but close).
> 
> 
I'm having a little difficulty parsing that. Could we try again?

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
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