QGIS python processing.runalg Clip returns None

Question

I'm writing a script in pyQGIS, which would emulate measurement of a line by "walking" along it with a compass with constant length. I have a problem with the Clip algorythm from processing module, namely it returns None, regardless of whether I add layers as inputs or selected features. What do I do wrong? I believe, that should the clip result in an empty layer, it would still be there, just empty? Below is the piece of code with problems,

while d.measureLine(QgsPoint(coorFin[0],coorFin[1]),QgsPoint(coor[0],coor[1]))>step: # If the distance between current and last point is more than chosen step
    # Create buffer polygon around a point
    bufCirc = point.geometry().buffer(step,10) # do a buffer from existing point 
    buforek = QgsFeature()
    buforek.setGeometry(bufCirc)

# Create layer and put the polygon inside
    bufferLayer = QgsVectorLayer("Polygon", "temporary_polygons", "memory")
    pz = bufferLayer.dataProvider()
    bufferLayer.setCrs(my_crs)
    pz.addFeatures([buforek])
    bufferLayer.commitChanges()

# select the buffer
    buforSel = bufferLayer.setSelectedFeatures([buforek]) 

# Clip coastline with the buffer
    liniaWarstwa = processing.runandload("qgis:clip", selectedCoastLine, buforSel, "memory:liniaWarstwa")
    print liniaWarstwa
    layer = QgsMapLayerRegistry.instance().mapLayersByName("memory:liniaWarstwa")[0]

The print prints "none", QgsMapLayerRegistry says "index out of range".

Answer 1

Aside of what Joseph mentioned, there is a number of other things you need to keep in mind when clipping from/to memory, so a little check-list for all of you using processing algorithms and memory layers:

• check, that your version of processing plugin is the newest (2.12.2 behaves in a way described by Joseph), update if not.
• Make sure all layers are in the same crs, as is your canvas. Have it explicitly set in code.
• Run your desired algorithm from GUI and check what name does an output have. For example Polish version of QGIS has the output names translated, so I had to put "Przycięte" instead of "Clipped"... Alternatively you can change language to English, which may be safer, to avoid specific characters of your language.
• Have both clip layers loaded onto canvas (use QgsMapLayerRegistry.instance().addMapLayer(layerIHoldInMemory) )
• But the most important part of my solution was: Don't use QGIS versions newer than 2.4. It seems, that there is some sort of problem with processing polygons/lines, it works nicely, but on QGIS 2.2 Valmiera.

Answer 2

In the newer versions of the Processing plugin, it would seem that you can no longer define the name of the output memory layer. The name of the algorithm is used instead (see similar post).

Therefore, you should replace the last section of your code with something similar:

# Clip coastline with the buffer
    processing.runandload("qgis:clip", selectedCoastLine, buforSel, None)
    layer = QgsMapLayerRegistry.instance().mapLayersByName("Clipped")[0]