Using a selected layer with processing in QGIS 3

Question

How do I create a Processing script which asks the user for a layer and then conducts an operation on it?

import processing
from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import (QgsProcessingAlgorithm,
 QgsProcessing,
 QgsProcessingParameterFeatureSource,
 QgsProcessingParameterFeatureSink,
 QgsVectorLayer)
class testAlg(QgsProcessingAlgorithm):
 HEXES = 'HEXES'
 OUTPUT = 'OUTPUT'
 def tr(self, text):
 return QCoreApplication.translate('testalg', text)
 def createInstance(self):
 return type(self)()
 def group(self):
 return self.tr('Test')
 def groupId(self):
 return 'test'
 def __init__(self):
 super().__init__()
 def initAlgorithm(self, config=None):
 self.addParameter(QgsProcessingParameterFeatureSource(self.HEXES,
 self.tr('Input layer'),
 [QgsProcessing.TypeVectorAnyGeometry]))
 self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT,
 self.tr('Output'),
 QgsProcessing.TypeVectorPolygon))
 def name(self):
 return 'testalg'
 def displayName(self):
 return self.tr('Test Algorithm')
 def processAlgorithm(self, parameters, context, feedback):
 hex_source1 = self.parameterAsSource(parameters, self.HEXES, context)
 hex_source2 = QgsVectorLayer('/Users/username/temp/hexes.shp', 'hex_source2', 'ogr')
 feedback.pushConsoleInfo(str(type(hex_source1))) # Returns QgsProcessingFeatureSource
 feedback.pushConsoleInfo(str(type(hex_source2))) # Returns QgsVectorLayer
 processing.run('qgis:fieldcalculator',
 {'INPUT':hex_source1, # Dies here as it can't accept a QgsProcessingFeatureSource
 'FIELD_NAME':'test',
 'FIELD_TYPE':0,
 'FIELD_LENGTH':10,
 'FIELD_PRECISION':6,
 'NEW_FIELD':True,
 'FORMULA':'( "c" / sum( "c" )) * 100',
 'OUTPUT':'/Users/username/temp/hexes2.shp'},
 feedback=feedback,
 context=context)
 return {}

This code fails at the processing.run command as it can't accept a QgsProcessingFeatureSource (hex_source1) as an input (actual error is Incorrect parameter value for INPUT). Using hex_source2 works fine as that returns a QgsVectorLayer which is apparently fine as an input.

I have also tried QgsProcessingParameterVectorLayer which also seems to give a QgsProcessingFeatureSource result.

Should I be converting a QgsProcessingFeatureSource to a QgsVectorLayer? If so, how? Or is there a simpler way to load a layer for processing that I'm missing?

Answer 1

In this case, because the field calculator algorithm requires a vector layer input, you'll need to declare a QgsProcessingParameterVectorLayer parameter as the input for your algorithm (not a feature source).

You'll then need to change

 hex_source1 = self.parameterAsSource(parameters, self.HEXES, context)

To

 hex_source1 = self.parameterAsVectorLayer(parameters, self.HEXES, context)

This will give you a QgsVectorLayer value for hex_source1, ready for handing off to the field calculator algorithm.

• Josh: Doesn't seem to work. Feedback gives <class 'NoneType'> and the processing.run fails due to Incorrect parameter value for INPUT.
• Sickboy: Works fine for me. Thx :-)

Answer 2

Keep

 hex_source1 = self.parameterAsSource(parameters, self.HEXES, context)

But instead of

'INPUT':hex_source1

use

'INPUT':hex_source1.materialize(QgsFeatureRequest())

(be sur to add QgsFeatureRequest in your imports)

Answer 3

This is confusing, but As pointed out here , you can simply lookup the input layer's value from parameters

hex_source1 = parameters[self.HEXES]

Answer 4

As ndawson pointed out, the qgis:fieldcalculator algorithm needs a VectorLayer. I have modified your code accordingly (also had to change the formulae and output path, and removed hex_source2), and it works for me in QGIS 3.4.

import processing
from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import (QgsProcessingAlgorithm,
 QgsProcessing,
 QgsProcessingParameterFeatureSource,
 QgsProcessingParameterFeatureSink,
 QgsVectorLayer)
class testAlg(QgsProcessingAlgorithm):
 HEXES = 'HEXES'
 OUTPUT = 'OUTPUT'
 def tr(self, text):
 return QCoreApplication.translate('testalg', text)
 def createInstance(self):
 return type(self)()
 def group(self):
 return self.tr('Test')
 def groupId(self):
 return 'test'
 def __init__(self):
 super().__init__()
 def initAlgorithm(self, config=None):
 self.addParameter(QgsProcessingParameterFeatureSource(self.HEXES,
 self.tr('Input layer'),
 [QgsProcessing.TypeVectorAnyGeometry]))
 self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT,
 self.tr('Output'),
 QgsProcessing.TypeVectorPolygon))
 def name(self):
 return 'testalg'
 def displayName(self):
 return self.tr('Test Algorithm')
 def processAlgorithm(self, parameters, context, feedback):
 hex_source1 = self.parameterAsVectorLayer(parameters, self.HEXES, context)
 #hex_source1 = self.parameterAsSource(parameters, self.HEXES, context)
 #hex_source2 = QgsVectorLayer('/Users/username/temp/hexes.shp', 'hex_source2', 'ogr')
 feedback.pushConsoleInfo(str(type(hex_source1))) # Returns QgsProcessingFeatureSource
 #feedback.pushConsoleInfo(str(type(hex_source2))) # Returns QgsVectorLayer
 processing.run('qgis:fieldcalculator',
 {'INPUT':hex_source1, # Dies here as it can't accept a QgsProcessingFeatureSource
 'FIELD_NAME':'test',
 'FIELD_TYPE':0,
 'FIELD_LENGTH':10,
 'FIELD_PRECISION':6,
 'NEW_FIELD':True,
 'FORMULA':'( "FID" / sum( "FID" )) * 100',
 'OUTPUT':'/tmp/test.shp'},
 feedback=feedback,
 context=context)
 return {}