Build a PyObject* from a C function?

Question

I am embedding Python in a C++ library which I am making. I would like users to be able to pass C functions in the form of function pointers PyObject* (fpFunc*)(PyObject*,PyObject*); so that I can use those functions in the embedded Python.

So I have a function pointer and I know that it is possible to put this function as a module's method using a PyMethodDef struct and passing it to Py_InitModule("module", ModMethods); and thus obtaining a PyObject* module which I can easily grab functions from.

But, what I would really appreciate is to be able to create this function on the fly without necessarily recreating another module each time.

I have looked into the Python docs, and some of the Python headers to think about a hacky way of doing this without real success... but I'm wondering if's there's a more conventional way of doing this.

From what I understood, every function belongs to a module, even __builtin__, so I guess I would require at least on module.

Any idea on how to achieve this?

It's not clear to me what you mean. Why are you using a function pointer? Who's setting it? — Fred Foo
– Fred Foo, Commented Jul 8, 2011 at 14:59
I'm making a C++ library which embeds Python. I'm using a function pointer because it's handy to pass around. (edited my post) — Manux
– Manux, Commented Jul 8, 2011 at 15:16

Manux · Accepted Answer · 2011-07-08 15:35:06Z

27

Found it. Though it's not in the docs and it's hardly explicit in the source.

PyObject* (*fpFunc)(PyObject*,PyObject*) = someFunction;
PyMethodDef methd = {"methd",fpFunc,METH_VARARGS,"A new function"};
PyObject* name = PyString_FromString(methd.ml_name);
PyObject* pyfoo = PyCFunction_NewEx(&methd,NULL,name);
Py_DECREF(name);

It works. I can call the function like I normally would call a Python function object.

If you're curious, all I found in the doc was about Py_InitModule4 and loading modules, so I went to check Python's source, and found out about PyCFunction, then I looked around in the doc but I couldn't find anything about PyCFunction_NewEx, so I had to check the source to make sure it was as I thought.

answered Jul 8, 2011 at 15:35

Manux

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1 Comment

Slava Over a year ago

Thanks, not sure though you really need name as PyCFunction_New macro just calls PyCFunction_NewEx with NULL as third parameter

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