'Lx-levels to expose as dropdowns'

@property
def levels(self):

    levels = self._levels
    if self._initlx:
        lx = []
        if (not levels):
            levels = current.gis.get_relevant_hierarchy_levels()
        if (levels is None):
            levels = []
        if (not isinstance(levels, (tuple, list))):
            levels = [levels]
        for level in levels:
            if (level not in lx):
                lx.append(level)
        for level in self.required_levels:
            if (level not in lx):
                lx.append(level)
        levels = self._levels = lx
        self._initlx = False
    return levels
