[capi-sig] Creating type object dynamically in run-time

Mateusz Loskot mateusz at loskot.net
Fri May 11 11:38:35 CEST 2012

On 10 May 2012 18:58, Stefan Behnel <python_capi at behnel.de> wrote:
> Mateusz Loskot, 10.05.2012 18:58:
>> Long story short, I have a software system implemented in C/C++ which
>> provides public API. The API is not fixed, may change, so scripting
>> engines use provided means of reflection to inspect this API and
>> generate bindings on-fly.
>> I embedded Python in this system and now I'm using the mentioned
>> reflection mechanisms to discover API and generate Python extension
>> modules and types dynamically during run-time.
>> I simply use the reflection mechanisms provided to iterate over
>> definitions (structures, functions, constants, etc.) of the public API
>> and I compose Python extension types grouping methods, etc.
> Interesting. So you're reflecting on a C/C++ API to dynamically export it
> to Python space?

Yes, roughly, that's the idea.

> Then I guess that your internal binding code is also
> rather generic, right?

Let's describe it in simplified way:
There are intermediate Python extensions defined which bridge
communication between the C++ world and dynamically generated Python classes.
For instance, there is a car and car engine.
Now, there is 'base car engine' which defines common protocol for
possible behaviours.
The 'base car engine" is specified and known at compile time,
Next, there is variety of kinds of engines where each provide
subset of the well-specified behaviours. Polymorphism, indeed.
So, those specific engines are dynamically generated in run-time
and they communicate with car through the statically defined 'base car
engine' protocol.

> It won't be able to call the application code
> directly, for example, if it doesn't know the API at compile time.

Yes, so there are 'base' extension types defined statically.

>> I can not use Cython, nor I can use tools like SWIG, Boost.Python.
>> The (C)Python is the only tool I can use.
> Also interesting. I wonder who would notice with which tool you have
> written your source code once it's compiled...

I guess nobody. It's just I had not known Cython before I started.
Now, I have most of my implementation ready.
I also wouldn't want to introduce new dependencies
and as long as Cython is not guaranteed to be 100% compatible
drop-in replacement for Python 3.2+, then I'm afraid I can't use it.
The FAQ does not confirm it is

But, I will keep an eye on Cython, it looks & feels a clever and useful idea.
and even for "he's stuck his head in C++" hacker :D

Stefan, I appreciate your help. I wish I found CAPI-SIG list earlier!

Best regards,
Mateusz Loskot, http://mateusz.loskot.net

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