def prepare(self):
        """
        Preparatory checks for whether this Executor can go ahead
        and (try to) build its targets.
        """
        for s in self.get_all_sources():
            if s.missing():
                msg = "Source `%s' not found, needed by target `%s'."
                raise SCons.Errors.StopError(msg % (s, self.batches[0].targets[0]))
Preparatory checks for whether this Executor can go ahead
        and (try to) build its targets.