Many functions use char* where const char* should be used. I've changed this in a few function prototypes which I use while embedding python due to numerous warnings about deprecated casting from string constants to char*.

I was about to submit the very same patch :-)
I missed PyErr_NewException, but you missed the PyMapping_* methods ;)

Please look into this matter.  GCC 4.2 has arrived and it has a new
warning.  The code:

     char* kwlist[] = { "target", "encoding", NULL };

now gives a warning like:

"warning: deprecated conversion from string constant to ‘char*’"

at least when compiling in C++ mode...

This means that people have to declare it as "const char* kwlist", which
will then trigger another warning when calling
PyArg_ParseTupleAndKeywords with such a variable, because the prototype is:

PyAPI_FUNC(int) PyArg_ParseTupleAndKeywords(PyObject *, PyObject *,
                                                  const char *, char **,
...);

The "char **" should be "const char **".  ext-, any chance you could fix
that too?

There was once a rather long discussion about "const" in
PyArg_ParseTupleAndKeywords:
http://mail.python.org/pipermail/python-dev/2006-February/060689.html

If you don't read it to the end, here is the conclusion:
"""
It sounds like the right answer for Python is to change the signature
of PyArg_ParseTupleAndKeywords() back.  We'll fix it when C fixes its
const rules <wink>.
"""

"const char*" was accepted without opposition, though.

Douglas Greiman submitted many similar changes with his issue2135 patch.  
His patch also amends documentation, which is missing here.

I am adding dgreiman to the nosy list here to avoid duplication of 
effort.

I am -0 on the idea.

For external APIs visible to user code, this can cause some compatibility problems and users may need to at the very least re-compile this code. So -1 here.

For new external APIs, having const wherever appropriate should be considered on a case by case basis.

For internal code this is a good idea, but I wouldn't go mass-slashing all the code base now replacing char* by const char*. Presenting some examples where this makes sense in particular would be interesting though. +0

> For external APIs visible to user code, this can cause some
> compatibility problems and users may need to at the very least
> re-compile this code.

Can you explain exactly which compatibility problems this would cause?

Actually, the point is precisely to make life easier for third-party code, since "const char *" is more accepting than "char *".

Antoine, I was referring to the discussion linked earlier (http://mail.python.org/pipermail/python-dev/2006-February/060689.html). Users of the C API needed to recompile their code and also add preprocessor hacks to make things compatible with C and C++, AFAIU.

> Antoine, I was referring to the discussion linked earlier
> (http://mail.python.org/pipermail/python-dev/2006-February/060689.html).

Ok, I've read it through. The problem is specifically with
pointers-to-pointers:
http://mail.python.org/pipermail/python-dev/2006-February/060849.html

And indeed, PyArg_ParseTupleAndKeywords() got its "const char **"
argument changed back to "char **". But the other consts have stayed
(such as the "const char *" argument to the same
PyArg_ParseTupleAndKeywords()).

So it looks like the patch, in essence, is legit.
(of course, in practice, it probably won't apply cleanly, since it's
very old; and perhaps it's incomplete too)

In C++ we may overload functions for backward compatibility:

PyAPI_FUNC(int) PyArg_ParseTupleAndKeywords(PyObject *, PyObject *,
                        const char *, const char * const *, ...);
PyAPI_FUNC(int) PyArg_VaParseTupleAndKeywords(PyObject *, PyObject *,
                        const char *, const char * const *, va_list va);
...
#ifdef __cplusplus
inline int
PyArg_ParseTupleAndKeywords(PyObject *args, PyObject *keywords,
                        const char *format, char * const *kwlist, ...)
{
    int retval;
    va_list va;
    va_start(va, kwlist);
    retval = PyArg_ParseTupleAndKeywords(args, keywords, format,
                        (const char * const *)kwlist, va_start);
    va_end(va);
    return retval;
}
#endif