Callbacks from Python to C/C++

Callbacks are used extensively in asynchronous systems. Libraries such as libevent provide a powerful asynchronous core to process events. Let's build an example to set a C function as a callback into a Python backend, which will notify back again into the C code. Firstly, we will declare a public callback function typedef:

cdef public:
    ctypedef void (*callback)(int)

This will output a callback typedef. Next, we can declare a global callback on the stack:

cdef callback GlobalCallback

Once this is set, we can then notify the callback easily. Next, we need a way to set the callback and another to call the callback:

cdef public void SetCallback(callback cb):
    global GlobalCallback
    GlobalCallback = cb

Notice the global keyword from Python through which the compiler knows to use the global keyword and not create a temporary instance from within that suite:

cdef public void Notify(int value):
    global GlobalCallback
    if GlobalCallback != <callback>...