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I'm new to Python and trying to figure out how to write my first Processing Script using QGIS 3.6 on Windows 10.

I'm using the default template provided in QGIS 3 (Toolbox > New script from template). I don't fully understand how this works and I can't find any good tutorials on the internet.

This is the default template:

from PyQt5.QtCore import QCoreApplication
from qgis.core import (QgsProcessing,
                   QgsFeatureSink,
                   QgsProcessingException,
                   QgsProcessingAlgorithm,
                   QgsProcessingParameterFeatureSource,
                   QgsProcessingParameterFeatureSink)
import processing


class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):
"""
This is an example algorithm that takes a vector layer and
creates a new identical one.

It is meant to be used as an example of how to create your own
algorithms and explain methods and variables used to do it. An
algorithm like this will be available in all elements, and there
is not need for additional work.

All Processing algorithms should extend the QgsProcessingAlgorithm
class.
"""

# Constants used to refer to parameters and outputs. They will be
# used when calling the algorithm from another algorithm, or when
# calling from the QGIS console.

INPUT = 'INPUT'
OUTPUT = 'OUTPUT'

def tr(self, string):
    """
    Returns a translatable string with the self.tr() function.
    """
    return QCoreApplication.translate('Processing', string)

def createInstance(self):
    return ExampleProcessingAlgorithm()

def name(self):
    """
    Returns the algorithm name, used for identifying the algorithm. This
    string should be fixed for the algorithm, and must not be localised.
    The name should be unique within each provider. Names should contain
    lowercase alphanumeric characters only and no spaces or other
    formatting characters.
    """
    return 'myscript'

def displayName(self):
    """
    Returns the translated algorithm name, which should be used for any
    user-visible display of the algorithm name.
    """
    return self.tr('My Script')

def group(self):
    """
    Returns the name of the group this algorithm belongs to. This string
    should be localised.
    """
    return self.tr('Example scripts')

def groupId(self):
    """
    Returns the unique ID of the group this algorithm belongs to. This
    string should be fixed for the algorithm, and must not be localised.
    The group id should be unique within each provider. Group id should
    contain lowercase alphanumeric characters only and no spaces or other
    formatting characters.
    """
    return 'examplescripts'

def shortHelpString(self):
    """
    Returns a localised short helper string for the algorithm. This string
    should provide a basic description about what the algorithm does and the
    parameters and outputs associated with it..
    """
    return self.tr("Example algorithm short description")

def initAlgorithm(self, config=None):
    """
    Here we define the inputs and output of the algorithm, along
    with some other properties.
    """

    # We add the input vector features source. It can have any kind of
    # geometry.
    self.addParameter(
        QgsProcessingParameterFeatureSource(
            self.INPUT,
            self.tr('Input layer'),
            [QgsProcessing.TypeVectorAnyGeometry]
        )
    )

    # We add a feature sink in which to store our processed features (this
    # usually takes the form of a newly created vector layer when the
    # algorithm is run in QGIS).
    self.addParameter(
        QgsProcessingParameterFeatureSink(
            self.OUTPUT,
            self.tr('Output layer')
        )
    )

def processAlgorithm(self, parameters, context, feedback):
    """
    Here is where the processing itself takes place.
    """

    # Retrieve the feature source and sink. The 'dest_id' variable is used
    # to uniquely identify the feature sink, and must be included in the
    # dictionary returned by the processAlgorithm function.
    source = self.parameterAsSource(
        parameters,
        self.INPUT,
        context
    )

    # If source was not found, throw an exception to indicate that the algorithm
    # encountered a fatal error. The exception text can be any string, but in this
    # case we use the pre-built invalidSourceError method to return a standard
    # helper text for when a source cannot be evaluated
    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()
    )

    # Send some information to the user
    feedback.pushInfo('CRS is {}'.format(source.sourceCrs().authid()))

    # If sink was not created, throw an exception to indicate that the algorithm
    # encountered a fatal error. The exception text can be any string, but in this
    # case we use the pre-built invalidSinkError method to return a standard
    # helper text for when a sink cannot be evaluated
    if sink is None:
        raise QgsProcessingException(self.invalidSinkError(parameters, self.OUTPUT))

    # Compute the number of steps to display within the progress bar and
    # get features from source
    total = 100.0 / source.featureCount() if source.featureCount() else 0
    features = source.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.addFeature(feature, QgsFeatureSink.FastInsert)

        # Update the progress bar
        feedback.setProgress(int(current * total))

    # To run another Processing algorithm as part of this algorithm, you can use
    # processing.run(...). Make sure you pass the current context and feedback
    # to processing.run to ensure that all temporary layer outputs are available
    # to the executed algorithm, and that the executed algorithm can send feedback
    # reports to the user (and correctly handle cancelation and progress reports!)
    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 the results of the algorithm. In this case our only result is
    # the feature sink which contains the processed features, but some
    # algorithms may return multiple feature sinks, calculated numeric
    # statistics, etc. These should all be included in the returned
    # dictionary, with keys matching the feature corresponding parameter
    # or output names.
    return {self.OUTPUT: dest_id}

So in the default template, there is a input vector layer and an output vector layer. How can I refer to the input vector layer? For example, how can I calculate the extent of a boundingbox from the input layer?

For example, in the code below, I refer to the active layer. This is what I understand. I write this code in the processAlgorithm method.

    layer = iface.activeLayer()
    ext = layer.extent()
    print(ext.xMaximum())

My question is simple: How could I perform the above action on the input layer, instead of the active layer?

  • Thanks for your answer. I still don't get it, even with the easier way in QGIS 3.6. I know this code doesn't work: ext = source.extent() and print(ext.xMaximum()). What would be the correct syntax? – Geert Mar 15 at 13:11
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It's the source variable as defined in the processAlgorithm method.

To get the extent, use source.sourceExtent() QgsProcessingFeatureSource.

And as you're using QGIS 3.6, there's an easier way... https://anitagraser.com/2019/03/02/easy-processing-scripts-comeback-in-qgis-3-6/

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