Calling Python from C | Set 1

Last Updated : 27 Mar, 2019

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. The 
arguments and return result of the function must 
be 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); 
} 

Calling Python from C | Set 2

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Calling Python from C | Set 1

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