supplying constants in an extension module

[Fredrik Lundh]

Torsten Mohr wrote:

> i write an extension module in C at the moment.
>
> I want to define some constants (integer mainly,
> but maybe also some strings).
>
> How do i do that best within this extension module
> in C?  Do i supply them as RO attributes?
>
> What's the best way for it?

reading the source for existing modules will teach you many useful
idioms.  here's how this is currently done:

    PyMODINIT_FUNC
    initmymodule(void)
    {
        PyObject *m;

        m = Py_InitModule(...);

        PyModule_AddIntConstant(m, "int", value);
        PyModule_AddStringConstant(m, "string", "string value");
    }

(both functions set the exception state and return -1 if they fail, but you
can usually ignore this; the importing code will check the state on return
from the init function)

if you want to support older versions of Python, you need to add stuff to
the module dictionary yourself.  an example:

    #if PY_VERSION_HEX < 0x02030000
    DL_EXPORT(void)
    #else
    PyMODINIT_FUNC
    #endif
     initmymodule(void)
    {
        PyObject* m;
        PyObject* d;
        PyObject* x;

        m = Py_InitModule(...);
        d = PyModule_GetDict(m);

        x = PyInt_FromLong(value);
        if (x) {
            PyDict_SetItemString(d, "INT", x);
            Py_DECREF(x);
        }

        x = PyString_FromString("string value");
        if (x) {
            PyDict_SetItemString(d, "STRING", x);
            Py_DECREF(x);
        }
    }

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