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Adding new field and calculating in Pyqgis

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0

I'm trying to write a plugin using QGIS 3.4. What I want to achieve is an excel-style calculation where I can create a new field (field2) and calculate the difference between current and previous row of an existing field (field1).

However, I'm stuck at the first step which is creating the new field. Attached my sample code below. The first part is how I generated my layer from an interpolation process.

    # Interpolate

        mem_layer = QgsVectorLayer("Point?crs="+vlayer.sourceCrs().authid(), 'point', 'memory')

        mem_layer.startEditing()

        fields = [field for field in csvlayer.fields()]

        prov = mem_layer.dataProvider()

        prov.addAttributes(fields)

        mem_layer.updateFields()



        chainageIdx = csvlayer.fields().indexFromName(chainage_field)



        for line in vlayer.getFeatures():

            for feat in csvlayer.getFeatures():

                new_feat = QgsFeature()

                attrs = feat.attributes()

                new_feat.setGeometry(line.geometry().interpolate(float(attrs[chainageIdx])))

                new_feat.setAttributes(attrs)

                mem_layer.addFeatures([new_feat])

        mem_layer.commitChanges()



    # Calc 1

        mem_layer.startEditing()

        datetimeField = QgsField('DateTime', QVariant.Double)

        prov = mem_layer.dataProvider()

        prov.addAttributes([datetimeField])

        mem_layer.updateFields()

        fields.append(datetimeField)



        newdatetimeIdx = mem_layer.fields().indexFromName('DateTime')



        for feat in mem_layer.getFeatures():

            #mem_layer.changeAttributeValue(feat.id(),newdatetimeIdx, 1)

            feat.setAttribute(newdatetimeIdx, 1.0)

            mem_layer.updateFeature(feat)

        mem_layer.commitChanges()



    # Output

        (sink_points, dest_id_points) = self.parameterAsSink(

            parameters,

            self.OUTPOINTS,

            context,

            fields,

            QgsWkbTypes.Point,

            vlayer.sourceCrs()

        )

        features = mem_layer.getFeatures()

        for current, feature in enumerate(features):

            # Stop the algorithm if cancel button has been clicked

            if feedback.isCanceled():

                break

            # Add a feature in the sink

            sink_points.addFeature(feature, QgsFeatureSink.FastInsert)

            # Update the progress bar

            feedback.setProgress(int(current * total))

When I try to run this, I get a KeyError: '2' message which suggests that the field was not created hence it couldn't write the test value.

python pyqgis qgis-3 plugins

share

asked 7 mins ago

strythestrythe

587

add a comment | 

0

I'm trying to write a plugin using QGIS 3.4. What I want to achieve is an excel-style calculation where I can create a new field (field2) and calculate the difference between current and previous row of an existing field (field1).

However, I'm stuck at the first step which is creating the new field. Attached my sample code below. The first part is how I generated my layer from an interpolation process.

    # Interpolate

        mem_layer = QgsVectorLayer("Point?crs="+vlayer.sourceCrs().authid(), 'point', 'memory')

        mem_layer.startEditing()

        fields = [field for field in csvlayer.fields()]

        prov = mem_layer.dataProvider()

        prov.addAttributes(fields)

        mem_layer.updateFields()



        chainageIdx = csvlayer.fields().indexFromName(chainage_field)



        for line in vlayer.getFeatures():

            for feat in csvlayer.getFeatures():

                new_feat = QgsFeature()

                attrs = feat.attributes()

                new_feat.setGeometry(line.geometry().interpolate(float(attrs[chainageIdx])))

                new_feat.setAttributes(attrs)

                mem_layer.addFeatures([new_feat])

        mem_layer.commitChanges()



    # Calc 1

        mem_layer.startEditing()

        datetimeField = QgsField('DateTime', QVariant.Double)

        prov = mem_layer.dataProvider()

        prov.addAttributes([datetimeField])

        mem_layer.updateFields()

        fields.append(datetimeField)



        newdatetimeIdx = mem_layer.fields().indexFromName('DateTime')



        for feat in mem_layer.getFeatures():

            #mem_layer.changeAttributeValue(feat.id(),newdatetimeIdx, 1)

            feat.setAttribute(newdatetimeIdx, 1.0)

            mem_layer.updateFeature(feat)

        mem_layer.commitChanges()



    # Output

        (sink_points, dest_id_points) = self.parameterAsSink(

            parameters,

            self.OUTPOINTS,

            context,

            fields,

            QgsWkbTypes.Point,

            vlayer.sourceCrs()

        )

        features = mem_layer.getFeatures()

        for current, feature in enumerate(features):

            # Stop the algorithm if cancel button has been clicked

            if feedback.isCanceled():

                break

            # Add a feature in the sink

            sink_points.addFeature(feature, QgsFeatureSink.FastInsert)

            # Update the progress bar

            feedback.setProgress(int(current * total))

When I try to run this, I get a KeyError: '2' message which suggests that the field was not created hence it couldn't write the test value.

python pyqgis qgis-3 plugins

share

asked 7 mins ago

strythestrythe

587

add a comment | 

I'm trying to write a plugin using QGIS 3.4. What I want to achieve is an excel-style calculation where I can create a new field (field2) and calculate the difference between current and previous row of an existing field (field1).

However, I'm stuck at the first step which is creating the new field. Attached my sample code below. The first part is how I generated my layer from an interpolation process.

    # Interpolate

        mem_layer = QgsVectorLayer("Point?crs="+vlayer.sourceCrs().authid(), 'point', 'memory')

        mem_layer.startEditing()

        fields = [field for field in csvlayer.fields()]

        prov = mem_layer.dataProvider()

        prov.addAttributes(fields)

        mem_layer.updateFields()



        chainageIdx = csvlayer.fields().indexFromName(chainage_field)



        for line in vlayer.getFeatures():

            for feat in csvlayer.getFeatures():

                new_feat = QgsFeature()

                attrs = feat.attributes()

                new_feat.setGeometry(line.geometry().interpolate(float(attrs[chainageIdx])))

                new_feat.setAttributes(attrs)

                mem_layer.addFeatures([new_feat])

        mem_layer.commitChanges()



    # Calc 1

        mem_layer.startEditing()

        datetimeField = QgsField('DateTime', QVariant.Double)

        prov = mem_layer.dataProvider()

        prov.addAttributes([datetimeField])

        mem_layer.updateFields()

        fields.append(datetimeField)



        newdatetimeIdx = mem_layer.fields().indexFromName('DateTime')



        for feat in mem_layer.getFeatures():

            #mem_layer.changeAttributeValue(feat.id(),newdatetimeIdx, 1)

            feat.setAttribute(newdatetimeIdx, 1.0)

            mem_layer.updateFeature(feat)

        mem_layer.commitChanges()



    # Output

        (sink_points, dest_id_points) = self.parameterAsSink(

            parameters,

            self.OUTPOINTS,

            context,

            fields,

            QgsWkbTypes.Point,

            vlayer.sourceCrs()

        )

        features = mem_layer.getFeatures()

        for current, feature in enumerate(features):

            # Stop the algorithm if cancel button has been clicked

            if feedback.isCanceled():

                break

            # Add a feature in the sink

            sink_points.addFeature(feature, QgsFeatureSink.FastInsert)

            # Update the progress bar

            feedback.setProgress(int(current * total))

When I try to run this, I get a KeyError: '2' message which suggests that the field was not created hence it couldn't write the test value.

python pyqgis qgis-3 plugins

share

asked 7 mins ago

strythestrythe

587

    # Interpolate

        mem_layer = QgsVectorLayer("Point?crs="+vlayer.sourceCrs().authid(), 'point', 'memory')

        mem_layer.startEditing()

        fields = [field for field in csvlayer.fields()]

        prov = mem_layer.dataProvider()

        prov.addAttributes(fields)

        mem_layer.updateFields()



        chainageIdx = csvlayer.fields().indexFromName(chainage_field)



        for line in vlayer.getFeatures():

            for feat in csvlayer.getFeatures():

                new_feat = QgsFeature()

                attrs = feat.attributes()

                new_feat.setGeometry(line.geometry().interpolate(float(attrs[chainageIdx])))

                new_feat.setAttributes(attrs)

                mem_layer.addFeatures([new_feat])

        mem_layer.commitChanges()



    # Calc 1

        mem_layer.startEditing()

        datetimeField = QgsField('DateTime', QVariant.Double)

        prov = mem_layer.dataProvider()

        prov.addAttributes([datetimeField])

        mem_layer.updateFields()

        fields.append(datetimeField)



        newdatetimeIdx = mem_layer.fields().indexFromName('DateTime')



        for feat in mem_layer.getFeatures():

            #mem_layer.changeAttributeValue(feat.id(),newdatetimeIdx, 1)

            feat.setAttribute(newdatetimeIdx, 1.0)

            mem_layer.updateFeature(feat)

        mem_layer.commitChanges()



    # Output

        (sink_points, dest_id_points) = self.parameterAsSink(

            parameters,

            self.OUTPOINTS,

            context,

            fields,

            QgsWkbTypes.Point,

            vlayer.sourceCrs()

        )

        features = mem_layer.getFeatures()

        for current, feature in enumerate(features):

            # Stop the algorithm if cancel button has been clicked

            if feedback.isCanceled():

                break

            # Add a feature in the sink

            sink_points.addFeature(feature, QgsFeatureSink.FastInsert)

            # Update the progress bar

            feedback.setProgress(int(current * total))

share

asked 7 mins ago

strythestrythe

587

share

asked 7 mins ago

strythestrythe

587

asked 7 mins ago

strythestrythe

587

asked 7 mins ago

strythestrythe

587