qgis - Are features of new labelling tool available to control via Python API?

I am writing a plugin using Python and currently using methods that set the label similar to those seen under Layer Properties. Some of the features under the new labelling engine available on the Toolbar would be useful, such as (not) "Label every part of a multi-part feature" and avoiding label collisions. Also the look of the labels generated using the latter method are usually better. Is there any way of setting these programatically in Python?

I understand the Toolbar labelling system will supersede the version under Layer Properties, so if they are not available in the API, will they be in the future?

Answer

You can publicly access many of the 'advanced' (or PAL) labeling engine functions via several classes, though the settings classes do not have sip (Python) bindings yet:

QgsLabelingEngineInterface

This is the main PAL engine interface that is really part of QgsMapRenderer, so you will want to gain access to it with the following in QGIS's Python console:

iface = qgis.utils.iface
mc = iface.mapCanvas()
mr = mc.mapRenderer()
le = mr.labelingEngine()

You will generally not need to work with the engine via Python. One aspect of accessing the engine directly that might be useful: gain direct access to the labels that have already be rendered. This is how the PAL label manipulation tools in the toolbar work (from master branch: Pin/Unpin, Show/Hide, Move, Rotate, Change):

le.labelsAtPosition(QgsPoint)  # returns a label of class QgsLabelPosition
le.labelsWithinRect(QgsRectangle)  # returns list of labels
# e.g. le.labelsWithinRect(mc.extent()) would return all visible labels drawn to canvas

see also: QgsLabelPosition

QgsPalLabeling

This is where you can set temporary engine-specific options like search method, show candidates, etc. as you can in the Engine settings dialog. There is currently no Python binding for this class.

Having a sip file created (Python binding) for this class, and the following, is the feature request you will want to submit, as per @lynxlynxlynx's suggestion.

QgsPalLayerSettings

This is where you can access, manipulate and write PAL settings for a given vector layer (which I think is what you are looking to do). Unfortunately, there is currently no Python binding for this class, which is defined in the QgsPalLabeling file, and I don't think there is a means of accessing it via another bound class.

Once a binding is in place, you will probably be able to access PAL layer settings like:

palyr = QgsPalLayerSettings()
palyr.readFromLayer(iface.activeLayer())  # assuming active layer is of vector type

if not pallyr.labelPerPart:  # access "Label every part of a multi-part feature"
    pallyr.labelPerPart = True
palyr.writeToLayer(iface.activeLayer())
# you have just adjusted settings like via the GUI, now redraw the canvas
mc.refresh()

Check the the QgsPalLabeling class's header and source file comments for info on defaults for QgsPalLayerSettings options.