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

[Steve Holden]

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).

regards
  Steve
-- 
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