Re: [ctypes-users] Accessing C part of PyObjects?

[Thomas H. <th...@ct...>]

Lenard Lindstrom schrieb:
> On 16 Nov 2006 at 20:01, Thomas Heller wrote:
> 
>> Tim Ansell schrieb:
>> > Hello,
>> > 
>> > I'm currently trying to access the C parts of some python objects. (Such
>> > as the SDL Surface of a Pygame Surface.)
>> 
>> I also want this because it would allow to define a byref_at(cobj, offset) function
>> that would behave like byref(cobj) but would accept an additional offset argument;
>> the request for something like this has been made several times on this list.
>> This could be accomplished by calling byref() and then changing the pointer
>> contained in the created object.
>> 
>> Since Python2.5 is now out is it not possible to change ctypes for that,
>> changes adding new features are only allowed for Python2.6 and above.
>> 
> Is it possible to write byref_at in Python? There is nothing to stop 
> having a "ctypestools" module.
> 
>> > To do this I need to know the structure of PyObject_HEAD which is a
>> > macro which is specific to the python build. I think it would be nice if
>> > ctypes included a definition of this macro. It could be generated at
>> > build time and would fit well with the pythonapi module. Is there any
>> > chance of something like this?
> 
> Do you want to directly access the PyObject_HEAD fields? All you 
> really need is to find its size in a portable way. Fortunately Python 
> has the tools. By definition a builtin type 'object' instance is a 
> PyObject_HEAD and nothing else. And attribute object.__basicsize__  
> gives its size in bytes.

Nice!  I'll attach a byref_at(obj, offset) implementation (or hack?) that works in
ctypes 1.0.1.
Well, actually it is not byref_at(obj, index) instead it is byref(obj, offset) where
offset is specified in bytes.

For the ctypestools module, I think it would be better to publish the byref_offset()
code in the wiki, so that everyone can paste it somewhere into his code.

For a single function it doesn't imo make much sense to invent a new package or module
that has to be installed separately.  Or should it go into the ctypeslib package?

Thanks,
Thomas