'ensures a clean copy of the extra_vars are used to set the value'

@extra_vars.setter
def extra_vars(self, value):

    assert isinstance(value, MutableMapping), ("the    type    of    'value'    for    extra_vars    should    be    a    MutableMapping,    but    is    a    %s" % type(value))
    self._extra_vars = value.copy()
