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The slash "/" as used in the documentation


On Mon, Feb 11, 2019 at 12:19 AM Terry Reedy <tjreedy at udel.edu> wrote:
> The pass-by-position limitation is not in CPython, it is the behavior of
> C functions, which is the behavior of function calls in probably every
> assembly and machine language.  Allowing the flexibility of Python
> function calls take extra code and slows function calls.
>
> math.sin, for instance, is a fast-as-possible wrapper around the C math
> library sin function.  It extracts the C double from the Python float,
> calls C sin, and returns the returned C double wrapped as a Python
> float.  Coredevs decided that being able to call math.sin(x=.33333) is
> not worth the resulting slowdown.  I am rather sure that heavy users of
> the math module would agree.

For math.sin, sure, but what about, say, list.index? Here's the start
of the implementation:

static PyObject *
listindex(PyListObject *self, PyObject *args)
{
    Py_ssize_t i, start=0, stop=Py_SIZE(self);
    PyObject *v;

    if (!PyArg_ParseTuple(args, "O|O&O&:index", &v,
                                _PyEval_SliceIndex, &start,
                                _PyEval_SliceIndex, &stop))


This is already paying the cost of not using C-style positional
function arguments, but then it only takes an argument tuple, so it
can't take keyword arguments anyway. Why? Wouldn't it be nice to be
able to write:

    mylist.index(42, start=10, stop=99)

instead of:

    mylist.index(42, 10, 99)

This would also allow stop to be passed alone:

    mylist.index(42, stop=99)

rather than having to pass the default value for start explicitly just
so we can pass stop:

    mylist.index(42, 0, 99)