Automatically creating themes in QGIS 3.x

Question

I have to make around 1000 maps that represent the same area (it is reclassified satellite image - so raster). Style is the same, only date changes. I figured out how to style all layers in no time and how to print all images in Atlas. I would like to automatize also the process of making Theme (it was called preset in QGIS 2.x) in the layer panel, so I would not need to manually make all the themes I would use later in Atlas. I would like to name them after the name of the layer or raster image (it represents the date of the satellite image).

Answer 1

You can do it by running in the Python console the following, using PyQGIS

root = QgsProject.instance().layerTreeRoot()

mapThemesCollection = QgsProject.instance().mapThemeCollection()
mapThemes = mapThemesCollection.mapThemes()
# Where you need to set your images names
# Could be retrieve if only raster names wanted with
# [layer.name() for layer in QgsProject.instance().mapLayers().values() if isinstance(layer, QgsRasterLayer)]
# If you want all layers names and filter them manually
# [layer.name() for layer in QgsProject.instance().mapLayers().values()]
layersToChanges = ['CartoDB Light', 'OpenStreetMap', 'OpenTransports'] # Replace with your list of raster layers instead


for layer in layersToChanges:
    for child in root.children():
        if isinstance(child, QgsLayerTreeGroup):
            print("- group: " + child.name())
        elif isinstance(child, QgsLayerTreeLayer):
            print("- layer: " + child.name() + "  ID: " + child.layerId())
            # Layer you want to tick
            if (child.name() == layer):
                child.setItemVisibilityChecked(True)
                print("Check only once")
            elif child.name() in layersToChanges:
                child.setItemVisibilityChecked(False)
                print("Check the others you want to hide")
    mapThemeRecord = QgsMapThemeCollection.createThemeFromCurrentState(
        QgsProject.instance().layerTreeRoot(),
        iface.layerTreeView().model()
        # For QGIS 3.18+, instead of above line, use iface.layerTreeView().layerTreeModel()
    )
    mapThemesCollection.insert(layer, mapThemeRecord)