Overriding PyArg_ParseTuple arguments

I'm SWIGging a C library into Python that does a lot of string handling.
One of the functions takes a char * argument, which can be NULL (to get
the default behavior).

If I don't do anything special in my SWIG wrapper, when I try to call
this function with a None argument, I get a TypeError saying the
argument must be a string, not None.

I know that Python's PyArg_ParseTuple function supports the "z" format
character, which converts a Python string into a char *, and converts
Python None into NULL.  This functionality has been part of Python since
1.5.2.  Yet I don't see any way to achieve this behavior in SWIG except
by using a typemap to explicitly check for PyNone.

Is there any way to override the automatically generated
PyArg_ParseTuple arguments in order to get this behavior?

Thanks,
--Tessa
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