从 C 调用Python |设置 1

在本文中，我们将主要关注从 C 调用Python的安全执行、将结果返回给 C 以及编写需要访问Python函数作为回调的 C 代码。

下面的代码重点介绍了从 C 调用Python所涉及的棘手部分。

代码 #1 : [步骤 1 和 2] 拥有 GIL 并验证该函数是正确的可调用函数

#include 
  
/* 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;
    }

代码 #2：构建参数、调用函数、检查Python异常

创建返回值，恢复之前的 GIL 状态并返回。

// 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(); 
}

需要传入对现有Python可调用对象的引用才能使用此函数。要做到这一点，有很多方法，比如简单地编写 C 代码以从现有模块中提取符号或将可调用对象传递给扩展模块。下面给出的代码显示了从嵌入式Python解释器调用函数。

代码#3：从模块加载符号

#include 
/* 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);
}