def btn_nuf_pressed(self):

    layer = geo_utils.LayerUtils.get_lay_from_id(self.cbo_layer.itemData(self.cbo_layer.currentIndex()))
    selection = layer.selectedFeatures()
    expression = QgsExpression("eval(@BARE)")
    expression.prepareFields(layer.pendingFields())
    layer.startEditing()
    for feature in selection:
        #expression.prepare(layer.pendingFields())
        value = expression.evaluate(feature)
        layer.changeAttributeValue(feature.id(), 5, expression)