"Run" method of a QGIS test plugin to export a composer iterating layers inside a group and making them visible one at a time.

IterateLayersAndExportComposer.py

    def run(self): # Run method of a QGIS plugin
        ruta="/tmp/"
        
        # The plugin Maps Printer is required!!!
        mp = qgis.utils.plugins['MapsPrinter']
        composer = self.iface.activeComposers()[0] # We'll only use the first composer
        extension = '.png' 
        
        # Get the group to start iterating layers (making only 1 layer visible at a time)
        group = QgsProject.instance().layerTreeRoot().findGroup( 'DEPTOS' ) 

        for layer in QgsMapLayerRegistry.instance().mapLayers().values():
            group.setVisible(0) # Make all the group (and its children) not visible
            self.iface.legendInterface().setLayerVisible( layer, True )
            title = ruta + layer.name()
            for item in composer.composition().items():
                if isinstance(item, QgsComposerLegend):
                    item.updateLegend() # Needed to always get an updated legend
                    break
            mp.exportCompo( composer, ruta, title, extension )