Daniel Franke, 30.06.2011 12:07:
For a project I implemented a extension module in C. Given the
excellent documentation everything was straightforward and works fine
so far.
Don't miss out taking a look at Cython, just in case it's going to be a
non-trivial project.
Up until now I kept local state in global static variables, but would
like to change this to pass a pointer to a state structure around. The
Py_InitModule4() [1] function seems to allow this by giving the user
the option to specify the 'self' object passed to the module's
functions. In this case, the 'self' argument should be a raw C pointer
malloc'ed elsewhere that should be rolled into a PyObject and be made
accessible later in the extension functions.
There's a PEP for Py3 that enables this through a general framework. In
general, to achieve this, you may want to allocate the module content
(including types etc.) on the heap rather than statically.
http://www.python.org/dev/peps/pep-3121/
Example code:
static PyObject* module_func(PyObject *self, PyObject *args) {
state_t *state = (state_t*) PyRawPointerFromPyObject(self);
/* use state */
Py_RETURN_NONE;
}
static PyMethodDef bms_methods[] = {
{ "func", module_func, METH_VARARGS, "" },
{ NULL, NULL, NULL, NULL }
};
void my_module_init(state_t *state) {
Py_InitModule4("_mymodule", module_methods,
"My Extension Module",
PyObjectFromRawPointer(state),
PYTHON_API_VERSION);
}
Here, of course, the functions PyObjectFromRawPointer(void*) and void*
PyRawPointerFromPyObject(PyObject*) are missing. Is there anything
like this in the Python C-API? If not, how could it be implemented? Or
is this approach a stupid idea to begin with? Pointers would be highly
appreciated.
Spot the pun. ;)
Take a look at PyCapsule, it may (or may not) be enough for your use case.
However, note the comment about Py_InitModule4() in the docs, where it says
that the 'self' passing feature isn't used very often. You may or may not
want to (or need to) use it.
Stefan
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