[Python-Dev] C++ for CPython 3? (Re: str.count is slow) (original) (raw)

martin at v.loewis.de martin at v.loewis.de
Thu Mar 2 04:37:40 CET 2006

Zitat von Michael Urman <murman at gmail.com>:

On 3/1/06, martin at v.loewis.de <martin at v.loewis.de> wrote: How would this be accomplished - by a function with a ton of optional templated arguments?

That's how I would do it. Actually, I would have PyArg_ParseTuple overloaded with different numbers of arguments, say 0..64. Each of them would be a template.

Only true when the rules are consistent with what smart pointers or the like do. When there's more than a single rule, this goes out the window because you have to use the correct smart class...

Sure. You have to use PyObject* now, so changing to usage of PyObject_ptr wouldn't be that bad. Remember, we are talking about extension modules to python here.

...and exceptions make it impossible to not use smart classes. Since there isn't a nested level of C for each function call in Python, I don't see how exceptions in the implementation language would help exceptions in Python. Do I misunderstand your point, or is there some really cool trick I'm missing?

Instead of checking for a NULL return value on all functions, a Python exception could be expressed (on the C++ stack) as a C++ exception.

So instead of writing

            n = PyString_FromString(name);
            if (n == NULL)
                    return NULL;
            for (ml = methods; ml->ml_name != NULL; ml++) {
                    if ((ml->ml_flags & METH_CLASS) ||
                        (ml->ml_flags & METH_STATIC)) {
                            PyErr_SetString(PyExc_ValueError,
                                            "module functions cannot set"
                                            " METH_CLASS or METH_STATIC");
                            Py_DECREF(n);
                            return NULL;
                    }
                    v = PyCFunction_NewEx(ml, passthrough, n);
                    if (v == NULL) {
                            Py_DECREF(n);
                            return NULL;
                    }
                    if (PyDict_SetItemString(d, ml->ml_name, v) != 0) {
                            Py_DECREF(v);
                            Py_DECREF(n);
                            return NULL;
                    }
                    Py_DECREF(v);
            }

            Py_DECREF(n);

you would write

            n = PyString_FromString(name);
            for (ml = methods; ml->ml_name != NULL; ml++) {
                    if ((ml->ml_flags & METH_CLASS) ||
                        (ml->ml_flags & METH_STATIC))
                            raise new PyExc_ValueError(PyExc_ValueError,
                                            "module functions cannot set"
                                            " METH_CLASS or METH_STATIC");
                    v = PyCFunction_NewEx(ml, passthrough, n);
                    PyDict_SetItemString(d, ml->ml_name, v);
            }

(To explain my bias, I'm against the idea of the C++ rewrite as I also fail to see the advantages outweighing the disadvantages, especially in light of the amount of rewriting necessary to see the "advantages" cited so far.)

That's why I'm explaining the advantages to you.

I'm not saying Python 3 should be written in C++, I'm only saying that doing so would have not just disadvantages.

Regards, Martin

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