Setting a theme in the QGIS Print Composer via python?

Question

I'm using python to command the print composer and export images/pdf (https://opensourceoptions.com/blog/pyqgis-create-and-print-a-map-layout-with-python/)

I'm looping through layers by name to export each layer. It works quite well for all but it's messy if layers are next to each other as I want to only show one. I need and want to use the themes that I've set up but I can't find a way to bring them to Print Composer.

I can change the theme using iface.mapCanvas().setTheme('green'), but this doesn't affect the layout in the Print Composer.

How can I use a different theme for each layouts via Python? Can it be done through QgsMapThemeCollection and applyTheme()?

from qgis.PyQt import QtGui

layernames = [layer.name() for layer in QgsProject.instance().mapLayers().values()]

for each in layernames: 

    layers = QgsProject.instance().mapLayersByName(each)
    print(layers)
    layer = layers[0]
    project = QgsProject.instance()
    manager = project.layoutManager()
    layoutName = each + "_layout"
    iface.mapCanvas().setTheme(each)

    layouts_list = manager.printLayouts()
    #remove any duplicate layouts
    for layout in layouts_list:
        if layout.name() == layoutName:
            manager.removeLayout(layout)

    layout = QgsPrintLayout(project)
    layout.initializeDefaults()
    layout.setName(layoutName)
    manager.addLayout(layout)

    #create map item in the layout
    map = QgsLayoutItemMap(layout)
    map.setRect(20, 20, 20, 20)

    #set the map extent
    ms = QgsMapSettings()
    ms.setLayers([layer])  #set layers to be mapped
    rect = QgsRectangle(ms.fullExtent())
    rect.scale(1.6)
    ms.setExtent(rect)

    map.setExtent(rect)
    map.setBackgroundColor(QColor(255, 255, 255, 0))

    layout.addLayoutItem(map)

    map.attemptMove(QgsLayoutPoint(5, 20, QgsUnitTypes.LayoutMillimeters))
    map.attemptResize(QgsLayoutSize(200, 180, QgsUnitTypes.LayoutMillimeters))

    legend = QgsLayoutItemLegend(layout)
    legend.setTitle("Legend")
    layerTree = QgsLayerTree()
    layerTree.addLayer(layer)
    legend.model().setRootGroup(layerTree)
    layout.addLayoutItem(legend)
    legend.attemptMove(QgsLayoutPoint(245, 5, QgsUnitTypes.LayoutMillimeters))

    scalebar = QgsLayoutItemScaleBar(layout)
    scalebar.setStyle('Line Ticks Up')
    scalebar.setUnits(QgsUnitTypes.DistanceKilometers)
    scalebar.setNumberOfSegments(4)
    scalebar.setNumberOfSegmentsLeft(0)
    scalebar.setUnitsPerSegment(0.5)
    scalebar.setLinkedMap(map)
    scalebar.setUnitLabel('km')
    scalebar.setFont(QFont('Arial', 14))
    scalebar.update()
    layout.addLayoutItem(scalebar)
    scalebar.attemptMove(QgsLayoutPoint(200, 190, QgsUnitTypes.LayoutMillimeters))

    title = QgsLayoutItemLabel(layout)
    title.setText(each)
    title.setFont(QFont('Arial', 24))
    title.adjustSizeToText()
    layout.addLayoutItem(title)
    title.attemptMove(QgsLayoutPoint(10, 5, QgsUnitTypes.LayoutMillimeters))

    #checking theme names can be accessed:
    themes = QgsProject.instance().mapThemeCollection().mapThemes()
    print(themes)

    layout = manager.layoutByName(layoutName)
    exporter = QgsLayoutExporter(layout)

    fn = 'C:/export/' + each + '.png'
    exporter.exportToImage(fn, QgsLayoutExporter.ImageExportSettings())
    #exporter.exportToPdf(fn, QgsLayoutExporter.PdfExportSettings())

Answer 1

try inserting the following 2 lines of code after:

ms.setLayers([layer])  #set layers to be mapped

insert these:

map.setFollowVisibilityPreset(True)
map.setFollowVisibilityPresetName(each)

It's the same as ticking the 'Follow map theme' in the item properties of your layout map

Answer 2

This also does the trick instead of relying on themes:

QgsProject.instance().layerTreeRoot().findLayer(name_of_the_layer_in_project.id()).setItemVisibilityChecked(False)