Using memory layer for processing algorithms in QGIS 3

Question

I would like to run a processing algorithms in QGIS3 with a memory vector layer as result.

When I define the memory layer as shown in the following, I get the error

Incorrect parameter value for output

while in the Python Error window output is indicated as QgsVectorLayer and nodesLayer.isValid() returns True:

crs = str(inputLayer.crs().authid())
outputLayer = QgsVectorLayer('Point?crs=' + crs , "points", "memory")
processing.run('grass7:v.net',
 {"input":inputLayer,
 ...
 "output": outputLayer})

If I use a path to the output layer, everything works fine:

outputLayer = r"path_to_file\output.shp"
processing.run('grass7:v.net',
 {"input":inputLayer,
 ...
 "output": outputLayer})

Any idea on how to correctly create a memory layer in QGIS 3?

• Germán Carrillo: Set "output": 'memory:'

Answer 1

As Germán noted, you should use "memory:" as the output string. But you'll also need to store the algorithm results, or the memory layer will be garbage collected by python!

results = processing.run('grass7:v.net',
 {"input":inputLayer,
 ...
 "output": 'memory:'})
result_layer = results['output']

But more generally - it's a bit inefficient to use a memory layer here. Because you're calling a grass algorithm (not a native QGIS one), all the outputs and inputs are being converted to disk based formats for use by grass, and all outputs from grass are also disk-based (behind the scenes these are shapefiles in a temporary directory). So when you ask for a memory layer output, QGIS has to read over the shapefile created by grass and convert that to a QGIS memory layer. So in this case, it's actually more efficient to just give the algorithm a path to save the shapefile and then read that directly.

• Sophie Crommelinck: Thanks for your explanation, it sounds very reasonable. However, when I create the shapefile for use in an interactive QGIS plugin, I thought it would be more common practice to store the intermediate processing files in memory. If I would store the shapefile on the disk, is there a general qgis directory for temporary files craeted within the plugin? If yes, whats the path valid for all plugin users?
• ndawson: You could use QgsProcessingUtils.tempFolder() or QgsProcessingUtils.generateTempFilename
• bugmenot123: Or QgsProcessing.TEMPORARY_OUTPUT as value for the OUTPUT parameter to have QGIS place it in your temp directory automatically.