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Wed Jul 14 10:23:45 CEST 2004 · https://mail.python.org/pipermail/python-dev/2004-July/046157.html

> Documented the new Py_VISIT macro to simplify implementation of
> tp_traverse handlers. (Tim made me do it. ;)

That Tim has been tyrant lately ;-)

> + /* Utility macro to help write tp_traverse functions */
> + #define Py_VISIT(op)					\
> +         do { 						\
> +                 if (op) {				\
> +                         int vret = visit((op), arg);	\
> +                         if (vret)			\
> +                                 return vret;		\
> +                 }					\
> +         } while (0)

I tried out this macro with the itertools module and found that it did a
nice job of compacting boilerplate code.

One thought jumped out though.  The macro assumes that the enclosing
function has consistent with the use of the variable names "visit" and
"arg": 

  static int
  islice_traverse(isliceobject *lz, visitproc visit, void *arg)
  {
      Py_VISIT(lz->it);
      return 0;
  }

If the parameters are named something else ("vfunc" for example),
MSVC++6.0 complains:

C:\py24\Modules\itertoolsmodule.c(1118) : warning C4013: 'visit'
undefined; assuming extern returning int

Don't know if it is worth it, but you could safeguard the macro by
including visit and arg in the parameter list.

Raymond Hettinger

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[Python-checkins] python/dist/src/Include objimpl.h, 2.58, 2.59