def _expect_vars(vs=None):
        """Verify the input type and return a list of Variables."""
        if vs is None:
            return list()
        elif isinstance(vs, Variable):
            return [vs]
        else:
            checked = list()
            # Will raise TypeError if vs is not iterable
            for v in vs:
                if isinstance(v, Variable):
                    checked.append(v)
                else:
                    fstr = "expected Variable, got {0.__name__}"
                    raise TypeError(fstr.format(type(v)))
            return checked
Verify the input type and return a list of Variables.