Re: [ggt-devel] Python interface to GMTL

[Allen B. <al...@vr...>]

Patrick Hartling wrote:
> Allen Bierbaum wrote:
> 
>> Patrick Hartling wrote:
>>
>>> Allen Bierbaum wrote:
>>>
>>>> Patrick Hartling wrote:
>>>>
>>>>> I have spent many hours today working on exposing GMTL classes and 
>>>>> functions for use with Python, and while I have made a ton of 
>>>>> progress, I have reached something of an impasse.  While it is 
>>>>> possible to instantiate and manipulate matrices, vectors, etc. in 
>>>>> Python, it is not possible at this moment to read any data from 
>>>>> them.  The GMTL interface is built up around C++ data access 
>>>>> idioms, and some of those (operator[] in particular) do not 
>>>>> translate to Python.  The biggest issue I face is that there is no 
>>>>> way (at this time) to map a C-style array to Python using 
>>>>> Boost.Python.  Even if there were a way to do that, Python doesn't 
>>>>> have a data structure with exactly the same semantics as a C-style 
>>>>> array.  The only GMTL class that is even close to having its data 
>>>>> be readable is gmtl::Matrix<>, and that's only because it overloads 
>>>>> operator(), one of the few class operators that can be overloaded 
>>>>> in Python.  Of course, I can't get the relevant Boost.Python code 
>>>>> to compile, so the point is moot.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Why can't we use python's [] operator?  Is there something about 
>>>> Boost::Python that prevents this overload from taking place or is 
>>>> the problem related to slicing?
>>>
>>>
>>>
>>>
>>> I don't know how to overload that operator.  Pyste didn't do it 
>>> automatically, and I have been having trouble making sense of how 
>>> Boost.Python does operator overloading.  My suspicion is that Python 
>>> itself doesn't allow that operator to be overloaded, but I could be 
>>> wrong.
>>
>>
>>
>> the python methods are:
>>
>> __getitem__ __setitem___ and __delitem__
> 
> 
> What documentation did you read to find out about these?  Methods like 
> this seem like a mystery to me, and I don't know where to look to read 
> more about them.  Searches on Google seem to turn up more about Jython 
> than anything else.  Maybe that's not such a bad start, though ...

I am just looking at standard language references.

In the python.org docs you can find the ones I am talking about at:

http://www.python.org/doc/current/ref/sequence-types.html

Basically there are just specific named methods that are used for 
operator based access in python.  The *item methods are the ones to do 
the container type access similar to op[] in C++.

> 
>> We probably only need to define the first two of them.  There may also 
>> be some issues to take into account to support slicing, but we could 
>> ignore those in the first pass at the interface.
> 
> 
> Are these issues in the documentation you're reading?

A little bit of a mention of it but nothing conclusive.  I think we just 
need to try it and see how it works.

-Allen


>  -Patrick
> 
> 


-- 
-- Allen Bierbaum                         al...@vr...
-- PhD Candidate             txtmsg - 515...@us...
-- VR Juggler Team                              www.vrjuggler.org
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