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:

enter image description here

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.

• applebrown: I have shortened the code segment. The error message I receive when trying to execute the script is as follows: 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'
• applebrown: I have edited my question. Please see above. How do I select the layer in the input field. If I run the script from the toolbox I cannot select any inputlayer as it was with QGIS 2.18. I can just press start...
• artwork21: Your code snippet is different than the sourced example you reference. That one uses a class which gets the parameters.
• artwork21: in initAlgorithm() function
• barteksch: You can't mix "Raster" and "Vector" in one parameter definition (your INPUT_VECTOR_LAYER_NODES name suggest it should be a vector not a raster) . Do you need Raster input or output? For output you should call self.addOutput.

Answer 1

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

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

• applebrown: thank you for your answer, I have tried your solution, but it looks like the fields() method is not known. This is my error now: Traceback (most recent call last): File "C:\Users\tlichten\AppData\Roaming\QGIS\QGIS3\profiles\default\processing\scripts\merging.py", line 89, in processAlgorithm fields = inEdges.fields() AttributeError: 'NoneType' object has no attribute 'fields'
• artwork21: How/where is the processAlgorithm() being called? The vector layer "inEdges" is not valid.
• applebrown: I have edited the question. Please have a look! Thank you so much
• applebrown: is the newFeature.setAttribute() method deprecated as well?? How can I set an attribute of a new feature for the output layer?
• artwork21: That method should still work. The referenced blog you noted in your question shows how to do that. Also, that question is different from this question so you should post a new question about that.

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]))

• applebrown: thank you for your answer, I have tried your solution, but it looks like the fields() method is not known. This is my error now: Traceback (most recent call last): File "C:\Users\tlichten\AppData\Roaming\QGIS\QGIS3\profiles\default\processing\scripts\merging.py", line 90, in processAlgorithm inEdgesFields = inEdges.fields() AttributeError: 'NoneType' object has no attribute 'fields'
• barteksch: Make sure the parameters are properly added in initAlgorithm(self, config=None) function and you select valid layer in the input field
• applebrown: the initAlgorithm method should work properly. But how do I select the layer in the input field. If I run the script from the toolbox I cannot select any inputlayer as it was with QGIS 2.18. I can just press start...
• applebrown: I have edited my question. Please have a look for better understanding :)
• barteksch: See my edited answer

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]