Python C api - function overloading

问题

I have a number of C functions that accept different arguments, e.g.

foo_i(int a)

foo_c(char c)

Is it possible to overload these functions in python C api?

I tried to use the following methods table:

static PyMethodDef test_methods[] = {
    {"foo", (PyCFunction)foo_i, METH_VARARGS, "int"},
    {"foo", (PyCFunction)foo_c, METH_VARARGS, "char"},
    {NULL, NULL, 0, NULL}
};

But when I invoke foo from python I always end up using the function at the bottom of the table.

Any ideas on how to invoke both foo_i() and foo_c() using foo() in python C-api?

Thanks!

回答1:

Either give them different Python level names, or write a single wrapper function that type checks the argument provided and dispatches to the correct "real" function. Python itself has no direct support for overloading functions based on argument types.

If you want the wrapper written for you, you might take a look at pybind11, which does allow overloading in the sense you're attempting (it does so via a type checking wrapper under the hood, so it's just syntactic sugar, not a change in behavior).

Untested example code:

static PyObject*
foo_wrapper(PyObject *self, PyObject *arg)
{
    Py_buffer view;
    Py_ssize_t ival;

    // Check for/handle length 1 bytes-like object (bytes, bytearray, small mmap, etc.)
    if (PyObject_GetBuffer(arg, &view, PyBUF_SIMPLE) == 0) {
        if (view.len != 1) {
             PyErr_Format(PyExc_ValueError, "Must receive exactly one byte, got %zd", view.len);
             PyBuffer_Release(&view);
             return NULL;
        }
        foo_c(((char*)view.buf)[0]);
        Py_RETURN_NONE; // Or convert return from foo_c if it exists
    }

    // Check for/handle integer-like object that fits in C int
    PyErr_Clear(); // Ignore error for objects not supporting buffer protocol
    ival = PyNumber_AsSsize_t(arg, PyExc_ValueError);
    if (PyErr_Occurred()) {
        if (PyErr_ExceptionMatches(PyExc_TypeError)) {
            // Replace with general error message about both accepted argument types,
            // since only reporting error from int conversion might confuse folks
            PyErr_Format(PyExc_TypeError, "Argument must be length 1 bytes-like object or integer; received %R", Py_TYPE(arg));
        }
        return NULL;
    }

    // Check valid range (Py_ssize_t often larger than int)
    if (ival < INT_MIN or ival > INT_MAX) {
        return PyErr_Format(PyExc_ValueError, "Integer must be in range [%d-%d]; received %zd", INT_MIN, INT_MAX, ival);
    }

    foo_i((int)ival);
    Py_RETURN_NONE; // Or convert return from foo_i if it exists
}

static PyMethodDef test_methods[] = {
    {"foo", (PyCFunction)foo_wrapper, METH_O, "Wrapper for foo_c and foo_i"},
    {NULL, NULL, 0, NULL}
};

来源：https://stackoverflow.com/questions/56042357/python-c-api-function-overloading

标签

python

overloading

python-c-api