using the C API to access objects derived from builtin classes

[Stefan Seefeld]

Thomas Heller wrote:

>>But back to the other issue: isn't there any text that explains
>>when I should be using PyDict_GetItem and when PyObject_GetItem
>>and what they are doing byhind the scene ?

>   7. Concrete Objects Layer 
> 
>   The functions in this chapter are specific to certain Python object
>   types. Passing them an object of the wrong type is not a good idea; if
>   you receive an object from a Python program and you are not sure that
>   it has the right type, you must perform a type check first; for
>   example, to check that an object is a dictionary, use
>   PyDict_Check(). The chapter is structured like the ``family tree'' of
>   Python object types.

yeah, I did read that. The inconsistency (or even error, I would argue) is
in the interpretation of 'is a'. PyDict_Check reports 'true' when called
on my 'Dictionary' object, yet the PyDict_* API only applies to the
slice representing the base class. I don't think that is correct, it is
at least very surprising.

The API only supports two extremes:

* the object's type is an *exact* match for a built-in type, thus a highly
   optimized API can be used to access its methods.

* the object's type is unknown, it may or may not support a certain 'protocol',
   i.e. for all method invocations a full method lookup has to be done.

That sounds good for the pre 2.2 world, where built-in types and user-defined
types were distinct sets.

Regards,
		Stefan