Python 3的C / C ++扩展中的关键字解析

Question

I am currently trying to implement a Python module using C++. Official Python 3 docs provide an example here , but to my surprise the code provided there throws errors in my Visual Studio 2017!

If you take a look at Custom_init function implementation, you will see a function call PyArg_ParseTupleAndKeywords that takes several arguments, including a keyword array kwlist declared 3 lines above (Example copy-pasted below):

static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
    static char *kwlist[] = {"first", "last", "number", NULL};
    PyObject *first = NULL, *last = NULL, *tmp;

    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist,
                                     &first, &last,
                                     &self->number))
        return -1;

    if (first) {
        tmp = self->first;
        Py_INCREF(first);
        self->first = first;
        Py_DECREF(tmp);
    }
    if (last) {
        tmp = self->last;
        Py_INCREF(last);
        self->last = last;
        Py_DECREF(tmp);
    }
    return 0;
}

However upon trying to declare a kwlist of my own like in example above, VS2017 throws the following error:

a value of type "const char *" cannot be used to initialize an entity of type "char *"

The simple thing would be to declare my own kwlist as const , but PyArg_ParseTupleAndKeywords expects kwlist as char * . (Documentation here )

Is there a way to deal with this without resorting to extremes like const_cast ?

Thank you.

EDIT:

One of the comments below suggested to simply copy from const char* to char* and I believe that will be the simplest approach to resolve this.

Answer 1

I cannot think of anything less horrible than

static char first[] = "first";
static char last[] = "last";
static char number[] = "number";
static char *kwlist[] = {first, last, number, NULL};

Answer 2

You can store the strings as std::string s, and then use std::string::data to get your char * s

static int
Custom_init(CustomObject *self, PyObject *args, PyObject *kwds)
{
    static std::array<std::string, 4> keywords { "first", "last", "number", "" };
    std::array<char *, 4> kwlist;
    std::transform(keywords.begin(), keywords.end(), kwlist.begin(), [](auto & str){ return str.data(); });

    PyObject *first = nullptr;
    PyObject *last = nullptr;

    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|UUi", kwlist.data(),
                                     &first, &last,
                                     &self->number))
        return -1;

    if (first) {
        auto tmp = self->first;
        Py_INCREF(first);
        self->first = first;
        Py_DECREF(tmp);
    }
    if (last) {
        auto tmp = self->last;
        Py_INCREF(last);
        self->last = last;
        Py_DECREF(tmp);
    }
    return 0;
}

Alternatively, you can use offsets into a single string literal

static char keywords[] = "first\0last\0number\0\0";
static char * kwlist = { keywords, keywords + 6, keywords + 11, keywords + 18  };