In this article, we will mainly focus on safe execution of a Python callable from C, returning a result back to C and writing C code that needs to access a Python function as a callback.
The code below focuses on the tricky parts that are involved in calling Python from C.
Code #1 : [Step 1 and 2] Own the GIL and Verify that function is a proper callable
#include <Python.h> /* Execute func(x, y) in the Python interpreter. Thearguments and return result of the function mustbe Python floats */ double call_func(PyObject *func, double x, double y){ PyObject *args; PyObject *kwargs; PyObject *result = 0; double retval; // Make sure we own the GIL PyGILState_STATE state = PyGILState_Ensure(); // Verify that func is a proper callable if (!PyCallable_Check(func)) { fprintf(stderr, "call_func: expected a callable\n"); goto fail; } |
Code #2 : Building Arguments, calling function, Check for Python exceptions
Create the return value, Restore previous GIL state and return.
// Step3 args = Py_BuildValue("(dd)", x, y); kwargs = NULL; // Step 4 result = PyObject_Call(func, args, kwargs); Py_DECREF(args); Py_XDECREF(kwargs); // Step 5 if (PyErr_Occurred()) { PyErr_Print(); goto fail; } // Verify the result is a float object if (!PyFloat_Check(result)) { fprintf(stderr, "call_func: callable didn't return a float\n"); goto fail; } // Step 6 retval = PyFloat_AsDouble(result); Py_DECREF(result); // Step 7 PyGILState_Release(state); return retval; fail: Py_XDECREF(result); PyGILState_Release(state); abort(); } |
A reference to an existing Python callable needs to be passed in, to use this function. To do that there are many ways like – simply writing C code to extract a symbol from an existing module or having a callable object passed into an extension module.
The code given below shows calling a function from an embedded Python interpreter.
Code #3 : Loading a symbol from a module
#include <Python.h>/* Definition of call_func() same as above */ /* Load a symbol from a module */PyObject *import_name(const char *modname, const char *symbol){ PyObject *u_name, *module; u_name = PyUnicode_FromString(modname); module = PyImport_Import(u_name); Py_DECREF(u_name); return PyObject_GetAttrString(module, symbol);} |
