Python C-API 访问字符串常量

Question

我想使用 python 的 C-API 在 C 中实现我为 python 编写的库。在 python 中，我可以通过声明 "constants" 在我的模块中声明：

RED = "red"   # Not really a constant, I know
BLUE = "blue" # but suitable, nevertheless

def solve(img_h):
    # Awesome computations
    return (RED, BLUE)[some_flag]

这些常量随后由模块提供的函数return编辑。我在 C 中做同样的事情时遇到了一些麻烦。这是我到目前为止得到的结果：

PyMODINIT_FUNC
PyInit_puzzler(void)
{
    PyObject* module = PyModule_Create(&Module);
    (void) PyModule_AddStringConstant(module, "BLUE",   "blue");
    (void) PyModule_AddStringConstant(module, "RED",    "red");
    return module;
}

PyObject* solve(PyObject* module, PyObject* file_handle)
{
    // Do some awesome computations based on the file
    // Involves HUGE amounts of memory management, thus efficient in C
    // PROBLEM: How do I return the StringConstants from here?
    return some_flag ? BLUE : RED;
}

我已经把有问题的部分标出来了。在我使用 PyModule_AddStringConstant(module, "FOO", "foo"); 将字符串常量添加到模块后，我如何才能从我的方法中将它们实际 return 作为 PyObject*？当我 return 它们时，我需要增加 ref-counter 吗？

Answer 1

因为 PyModule_AddStringConstant(module, name, value) adds the constant to the module, it should be available from the module's dictionary which can be acquired with PyModule_GetDict(module). You can then access any attribute from the module via its dictionary using PyDict_GetItemString(dict, key) 这是您可以从模块访问常量的方式（在定义之后）：

// Get module dict. This is a borrowed reference.
PyObject* module_dict = PyModule_GetDict(module);

// Get BLUE constant. This is a borrowed reference.
PyObject* BLUE = PyDict_GetItemString(module_dict, "BLUE");

// Get RED constant. This is a borrowed reference.
PyObject* RED = PyDict_GetItemString(module_dict, "RED");

要将此与您的 solve() 函数放在一起，您需要类似于：

PyObject* solve(PyObject* module, PyObject* file_handle)
{
    // Do some awesome computations based on the file
    // Involves HUGE amounts of memory management, thus efficient in C

    // Return string constant at the end.
    PyObject* module_dict = PyModule_GetDict(module);
    PyObject* constant = NULL;
    if (some_flag) {
        // Return BLUE constant. Since BLUE is a borrowed 
        // reference, increment its reference count before 
        // returning it.
        constant = PyDict_GetItemString(module_dict, "BLUE");
        Py_INCREF(constant);
    } else {
        // Return RED constant. Since RED is a borrowed 
        // reference, increment its reference count before 
        // returning it.
        constant = PyDict_GetItemString(module_dict, "RED");
        Py_INCREF(constant);
    }

    // NOTE: Before you return, make sure to release any owned
    // references that this function acquired. `module_dict` does
    // not need to be released because it is merely "borrowed".

    // Return the constant (either BLUE or RED) as an owned
    // reference. Whatever calls `solve()` must make sure to
    // release the returned reference with `Py_DECREF()`.
    return constant;
}

Python C-API 访问字符串常量

Python C-API access String constants

c

python

interface

python-c-api