I was just reading the _pickle sources and it appears that AC does not
generate a second arg for METH_NOARGS functions:

#define _PICKLE_PICKLERMEMOPROXY_CLEAR_METHODDEF    \
    {"clear", (PyCFunction)_pickle_PicklerMemoProxy_clear, METH_NOARGS, _pickle_PicklerMemoProxy_clear__doc__},

static PyObject *
_pickle_PicklerMemoProxy_clear(PicklerMemoProxyObject *self)


While this is a common occurrence in the source tree, the consensus
in #15402 was that the unused second arg should be present, e.g.:

msg166250
msg166405

I prefer to omit the unused parameter, even if NULL *is* passed to the function.

If you prefer to add the unused parameter, what do you propose to avoid compiler warnings if unused parameters are checked?

Stefan is right.  I'll fix Clinic.

STINNER Victor <report@bugs.python.org> wrote:
> If you prefer to add the unused parameter, what do you propose to avoid compiler warnings if unused parameters are checked?

This works quite portably in _decimal (I don't get warnings from gcc, icc,
suncc, Visual Studio, aCC, clang):

#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
  #define UNUSED __attribute__((unused))
#else
  #define UNUSED
#endif

static PyObject *
signaldict_copy(PyObject *self, PyObject *args UNUSED)
{
    return flags_as_dict(SdFlags(self));
}

We could call the macro PY_UNUSED or something.

A quick google suggests:

http://sourcefrog.net/weblog/software/languages/C/unused.html

To do it properly with Clang requires a pragma:

http://stackoverflow.com/questions/3417837/what-is-the-best-way-to-supress-unused-variable-x-warning/18724213#18724213

What a mess.

> We could call the macro PY_UNUSED or something.

I would prefer Py_UNUSED name. This sounds like a nice addition to Include/pymacros.h.

In C++, you can omit the parameter name, so the macro should take the parameter name: Py_UNUSED(name). Example:

   int foo(int Py_UNUSED(bar)) { return 1 }

In Visual Studio, you can use:

#define Py_UNUSED(NAME) __pragma(warning(suppress:4100)) NAME

For Clang, you can try:

#define Py_UNUSED(NAME) _Pragma(diagnostic ignored "-Wunused") NAME

Larry Hastings <report@bugs.python.org> wrote:
> To do it properly with Clang requires a pragma:

Hmm. I just tested and clang warns with -Wall -W, but does not warn if
__attribute__((unused)) is present.

The macro I posted really works on all obscure buildbot platforms.

Stefan Krah <report@bugs.python.org> wrote:
> The macro I posted really works on all obscure buildbot platforms.

N.B. clang also defines __GNUC__, as does the intel compiler.

STINNER Victor <report@bugs.python.org> wrote:
> I would prefer Py_UNUSED name. This sounds like a nice addition to Include/pymacros.h.

Yes, Py_UNUSED is nicer.

Here's a first attempt at a patch.

The Visual Studio pragma disables for the rest of the file, which is undesirable.  Maybe we could turn it on and off inline, but it's not clear to me that that would have the desired effect of turning off the warning for explicitly that parameter declaration.

Also, a little googling confirms that clang supports the GCC __attribute((unused)) extension, so I just went with that.

> The Visual Studio pragma disables for the rest of the file, which is undesirable.  Maybe we could turn it on and off inline, but it's not clear to me that that would have the desired effect of turning off the warning for explicitly that parameter declaration.

Oh, I didn't know that it is file-wide. There are
__pragma(warning(push)) and __pragma(warning(pop)) commands to disable
a pragma. I don't know it is can be used using Py_UNUSED(name) macro
(is it possible to "pop" the pragma before the function body?).

If a compiler does not provide a syntax to disable the warning just in
one function, the warning should be disabled for the compilation of
the whole project.

New changeset c4ababa110a2 by Larry Hastings in branch 'default':
Issue #19976: Argument Clinic METH_NOARGS functions now always
http://hg.python.org/cpython/rev/c4ababa110a2

New changeset 2138163621196a186975796afb2b0a6aa335231d by Victor Stinner (Petr Viktorin) in branch 'master':
bpo-29179: Document the Py_UNUSED macro (#4341)
https://github.com/python/cpython/commit/2138163621196a186975796afb2b0a6aa335231d