Getting field index from input vector layer in PyQGIS

Question

I am currently trying to convert a python script from QGIS 2.18 to QGIS 3. Therefore, I am trying to integrate my python script into the template from @underdark: see here.

I am currently stepping from one error into another so I seek help as I was not able to find any other question on Stack Exchange or any documentation.

Here is some part of my script so far:

def processAlgorithm(self, parameters, context, feedback):
    inEdges = self.parameterAsVectorLayer(parameters, self.INPUT_VECTOR_LAYER_EDGES, context)
    inNodes = self.parameterAsVectorLayer(parameters, self.INPUT_VECTOR_LAYER_NODES, context)
    outMerged = self.parameterAsOutputLayer(parameters, self.OUTPUT_VECTOR_LAYER_MERGED, context)

    #DO SOME CALCULATION
    
    # get field indices for inputlayer
    idxEdgeId = inEdges.fieldNameIndex(ID)
    idxPriority = inEdges.fieldNameIndex(PRIORITY)
    idxLength  = inEdges.fieldNameIndex(LENGTH)
    idxTime    = inEdges.fieldNameIndex(TIME)
    idxFrom  = inEdges.fieldNameIndex(FROM)
    idxTo  = inEdges.fieldNameIndex(TO)

    idx_id = inNodes.fieldNameIndex(NODEID)
    idx_x = inNodes.fieldNameIndex(COORD_X)
    idx_y = inNodes.fieldNameIndex(COORD_Y)

    # Set the provider to accept the data source
    outdp = outMerged.dataProvider()

    # define attributes and add them to the vector layer
    outdp.addAttributes([QgsField(EDGEID, QVariant.String),
                        QgsField(NODE1, QVariant.String),
                        QgsField(NODE2, QVariant.String),
                        QgsField(PRIORITY, QVariant.Int),
                        QgsField(VTO, QVariant.Double),
                        QgsField(VFROM, QVariant.Double)])
    outMerged.updateFields()

    # get field indices for outputlayer
    outEdgeId = outdp.fieldNameIndex(EDGEID)
    outNode1 = outdp.fieldNameIndex(NODE1)
    outNode2 = outdp.fieldNameIndex(NODE2)
    outPriority = outdp.fieldNameIndex(PRIORITY)
    outVTo = outdp.fieldNameIndex(VTO)
    outVFrom = outdp.fieldNameIndex(VFROM)

    listOfEdgeIds = []
    count = 0
    flag = 0

    (...)

For your information: I have two input vector layers (one point, and one line layer) and out of that, I want to create one outputlayer (which should be a line-vector layer as well).

But currently I have trouble with the fieldNameIndex(QString) - method in the first section of the code. I have already tried to call the method with the dataProvider() of the input layer, but that hasn't worked out. I get error:

Traceback (most recent call last): File "C:\Users\tlichten\AppData\Roaming\QGIS\QGIS3\profiles\default\processing\scripts\merging.py", line 89, in processAlgorithm idxEdgeId = inEdges.fieldNameIndex(ID) AttributeError: 'NoneType' object has no attribute 'fieldNameIndex'

Although I have to input vector layers, I cannot select any of them if I want to start the script. The execute script window looks like this:

In QGIS2.18, there would be two drop-down menus in which to choose between the input layers.

I have adapted the initAlgorithm() to the following:

    def initAlgorithm(self, config=None):
    self.addParameter(QgsProcessingParameterVectorLayer(
        self.INPUT_VECTOR_LAYER_EDGES, 
        self.tr('Edges'),
        [QgsProcessing.TypeVectorAnyGeometry]))
    self.addParameter(QgsProcessingParameterRasterLayer(
        self.INPUT_VECTOR_LAYER_NODES,
        self.tr('Nodes')
        [QgsProcessing.TypeVectorAnyGeometry]))
    self.addParameter(QgsProcessingParameterRasterDestination(
        self.OUTPUT_VECTOR_LAYER_MERGED,
        self.tr("Output Vector Layer Merged"),
        [QgsProcessing.TypeVectorAnyGeometry]))

But I can still not select any input layers if I execute the script.

Answer 1

That method has been deprecated, use this instead to get the field index:

fields = inEdges.fields()
idxEdgeId = fields.indexFromName('fieldName')

Answer 2

fieldNameIndex function was removed in QGIS3:

Removed fieldNameIndex(), use fields().lookupField() or fields().indexFromName() instead

You can convert your code as follows:

inEdges = self.parameterAsVectorLayer(parameters, self.INPUT_VECTOR_LAYER_EDGES, context)
inEdgesFields = inEdges.fields()

idxEdgeId = inEdgesFields.indexFromName(ID)

For more details see https://qgis.org/api/api_break.html

---

Exception you've got, suggest that you have problem with parameters, make sure you add parameters with proper key in initAlgorithm function (in QGIS2 it was defineCharacteristics):

def initAlgorithm(self, config=None):
    self.addParameter(QgsProcessingParameterVectorLayer(self.INPUT_VECTOR_LAYER_EDGES, self.tr('Edges'), types=[QgsProcessing.TypeVector]))

Answer 3

Alternatively, there is also a possibility of using the indexOf(fieldName: str) method.

from qgis.utils import iface

layer = iface.activeLayer()

fields = layer.fields()

fields_indxs = [fields.indexOf(field.name()) for field in fields]