Python C-API 访问字符串常量答案

【问题标题】：Python C-API access String constantsPython C-API 访问字符串常量
【发布时间】：2015-08-16 20:20:00
【问题描述】：

我想使用 python 的 C-API 实现我为 C 语言编写的 python 库。在 python 中，我可以在我的模块中声明“常量”，只需声明：

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返回。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：

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;
}

我已经标记了有问题的部分。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返回为PyObject*？退货时是否需要增加 ref-counter？

【问题讨论】：

标签： python c interface python-c-api

【解决方案1】：

由于PyModule_AddStringConstant(module, name, value) 将常量添加到模块中，它应该可以从模块的字典中获得，可以通过PyModule_GetDict(module) 获取。然后，您可以使用 PyDict_GetItemString(dict, key) 通过其字典访问模块中的任何属性，这就是您可以从模块中访问常量的方式（在它们定义之后）：

// 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");

要将其与您的 solve() 函数结合起来，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;
}

【讨论】：