The final 'arg' argument of the sys.settrace() callback is documented to
be None for the 'c_return' and 'c_exception' events, but it appears to
be the function object itself. Additionally, the 'return' event's
argument may be None if the 'return' event is being generated because of
an exception. These errors also exist in the Py_tracefunc documentation
in the C API. The latter error is not particularly important for the
Python API, but in the C API the argument may be a NULL pointer.

I am happy to make the corrections if someone will confirm that my
analysis is correct.