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I have looked at what's happening in a bit more detail and
now I agree that the current behaviour seems wrong.

Apparently, when one passes a nonstring to bdb's run(), bdb
sets tracing first and only then throws an exception...
Leaving tracing on seems like a bad thing to do 

Would calling settrace() a couple lines later be the
simplest solution then?

so relevant fragment of  bdb.py's run() becomes:
        if not isinstance(cmd, types.CodeType):
            cmd = cmd+'\n'
        sys.settrace(self.trace_dispatch)

This should prevent bdb from turning tracing too early.
What do you think?