Looks up the default for a parameter name.  This by default
        looks into the :attr:`default_map` if available.

def lookup_default(self, name):
        """Looks up the default for a parameter name.  This by default
        looks into the :attr:`default_map` if available.
        """
        if self.default_map is not None:
            rv = self.default_map.get(name)
            if callable(rv):
                rv = rv()
            return rv