Dewfy Dewfy - 1 month ago 14
Python Question

Inheritance in Python C++ extension

I have c++ library that need communicate with Python plugged in modules. Communication supposes implementing by Python some callback c++ interface.

I have read already about writing extensions, but no idea how to develop inheritance.

So something about:
C++:

class Broadcast
{
void set(Listener *){...
}

class Listener
{
void notify(Broadcast* owner) = 0;
}


I need something like in Python:

class ListenerImpl(Listener):
...
def notify(self, owner):
...


Note, I don't want use Boost.

Answer

Writing Python types in C that are inheritable is explained in PEP 253. It's not all that different from writing a normal builtin type as explained in the Extending/Embedding guide but you have to do certain things, like attribute access, through the Python API instead of accessing anything directly.

Exposing the Python subclasses back to C++ code is a little more tedious. The Python classes won't be C++ subclasses, so you need a C++ wrapper class (that does inherit from Listener) that contains a PyObject* for the Python subclass instance, and that has a notify method that translates the arguments to Python objects, calls the notify method of the PyObject* (using, e.g., PyObject_CallMethod), translates the result back to C++ types, and returns.