Description of problem:
See the attachment in bug 1286733 (URL: https://bugzilla.redhat.com/attachment.cgi?id=1100522), for a code like:

        try:
            self.context.pmRegisterDerived("foo", "rat(disk.dev.read)")
        except:
            print self.context.pmDerivedErrStr()

It currently prints:

$ python test.py                                          
REGISTERED 1
EXCEPTION 2
None

But libpcp gets it right:

http://oss.sgi.com/pipermail/pcp/2016-January/009333.html

So this looks like a Python PMAPI issue.