17- Writing your processing script

In QGIS Menu Toolbar click on Processing then click on Toolbox. Next, in Script Editor, choose "Create new script from template". You can edit it and save it as your script like this example:

from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import (QgsProcessing,
                       QgsFeatureSink,
                       QgsProcessingException,
                       QgsProcessingAlgorithm,
                       QgsProcessingParameterFeatureSource,
                       QgsProcessingParameterFeatureSink,
                       QgsMarkerSymbolLayer,
                       QgsMarkerSymbol)
from qgis import processing


class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):

    INPUT = 'INPUT'
    INPUT_PT = 'INPUT_PT'
    OUTPUT = 'OUTPUT'

    def tr(self, string):

        return QCoreApplication.translate('Processing', string)

    def createInstance(self):
        return ExampleProcessingAlgorithm()

    def name(self):
        return 'myscript'

    def displayName(self):
        return self.tr('My Script')

    def group(self):
        return self.tr('Example scripts')

    def groupId(self):
        return 'examplescripts'

    def shortHelpString(self):
        return self.tr("Example algorithm short description")

    def initAlgorithm(self, config=None):


        self.addParameter(
            QgsProcessingParameterFeatureSource(
                self.INPUT,
                self.tr('Input layer'),
                [QgsProcessing.TypeVectorAnyGeometry]
            )
        )
        self.addParameter(
            QgsProcessingParameterFeatureSource(
                self.INPUT_PT,
                self.tr('Point layer'),
                [QgsProcessing.TypeVectorPoint]
            )
        )


        self.addParameter(
            QgsProcessingParameterFeatureSink(
                self.OUTPUT,
                self.tr('Output layer')
            )
        )

    def processAlgorithm(self, parameters, context, feedback):


        source = self.parameterAsSource(
            parameters,
            self.INPUT,
            context
        )


        if source is None:
            raise QgsProcessingException(self.invalidSourceError(parameters, self.INPUT))

        (sink, dest_id) = self.parameterAsSink(
            parameters,
            self.OUTPUT,
            context,
            source.fields(),
            source.wkbType(),
            source.sourceCrs()
        )

        feedback.pushInfo('CRS is {}'.format(source.sourceCrs().authid()))
        pt_lyr = self.parameterAsVectorLayer(
            parameters,
            self.INPUT_PT,
            context
            )

        pt_symbol = QgsMarkerSymbol.createSimple({"color": "0,0,255","name": "rectangle"})
        pt_lyr.renderer().setSymbol(pt_symbol)
        pt_lyr.triggerRepaint()

        if sink is None:
            raise QgsProcessingException(self.invalidSinkError(parameters, self.OUTPUT))


        total = 100.0 / source.featureCount() if source.featureCount() else 0
        features = source.getFeatures()

        for current, feature in enumerate(features):

            if feedback.isCanceled():
                break



            sink.addFeature(feature, QgsFeatureSink.FastInsert)



            feedback.setProgress(int(current * total))

        if False:
            buffered_layer = processing.run("native:buffer", {
                'INPUT': dest_id,
                'DISTANCE': 1.5,
                'SEGMENTS': 5,
                'END_CAP_STYLE': 0,
                'JOIN_STYLE': 0,
                'MITER_LIMIT': 2,
                'DISSOLVE': False,
                'OUTPUT': 'memory:'
            }, context=context, feedback=feedback)['OUTPUT']


        return {self.OUTPUT: dest_id}