26.9. Escrevendo novos algoritmos de Processamento como scripts Python

Existem duas opções para escrever algoritmos de Processamento usando Python.

Within QGIS, you can use Create new script in the Scripts menu at the top of the Processing Toolbox to open the Processing Script Editor where you can write your code. To simplify the task, you can start with a script template by using Create new script from template from the same menu. This opens a template that extends QgsProcessingAlgorithm.

If you save the script in the scripts folder (the default location) with a .py extension, the algorithm will become available in the Processing Toolbox.

26.9.1. Extending QgsProcessingAlgorithm 

O seguinte código

recebe uma camada vetorial como entrada
conta o número de feições
faz uma operação de buffer
cria uma camada raster a partir do resultado da operação do buffer
retorna a camada de buffer, a camada raster e o número de feições

from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import (QgsProcessing,
                       QgsProcessingAlgorithm,
                       QgsProcessingException,
                       QgsProcessingOutputNumber,
                       QgsProcessingParameterDistance,
                       QgsProcessingParameterFeatureSource,
                       QgsProcessingParameterVectorDestination,
                       QgsProcessingParameterRasterDestination)
from qgis import processing


class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):
    """
    This is an example algorithm that takes a vector layer,
    creates some new layers and returns some results.
    """

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

    def createInstance(self):
        # Must return a new copy of your algorithm.
        return ExampleProcessingAlgorithm()

    def name(self):
        """
        Returns the unique algorithm name.
        """
        return 'bufferrasterextend'

    def displayName(self):
        """
        Returns the translated algorithm name.
        """
        return self.tr('Buffer and export to raster (extend)')

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

    def groupId(self):
        """
        Returns the unique ID of the group this algorithm belongs
        to.
        """
        return 'examplescripts'

    def shortHelpString(self):
        """
        Returns a localised short help string for the algorithm.
        """
        return self.tr('Example algorithm short description')

    def initAlgorithm(self, config=None):
        """
        Here we define the inputs and outputs of the algorithm.
        """
        # 'INPUT' is the recommended name for the main input
        # parameter.
        self.addParameter(
            QgsProcessingParameterFeatureSource(
                'INPUT',
                self.tr('Input vector layer'),
                types=[QgsProcessing.TypeVectorAnyGeometry]
            )
        )
        self.addParameter(
            QgsProcessingParameterVectorDestination(
                'BUFFER_OUTPUT',
                self.tr('Buffer output'),
            )
        )
        # 'OUTPUT' is the recommended name for the main output
        # parameter.
        self.addParameter(
            QgsProcessingParameterRasterDestination(
                'OUTPUT',
                self.tr('Raster output')
            )
        )
        self.addParameter(
            QgsProcessingParameterDistance(
                'BUFFERDIST',
                self.tr('BUFFERDIST'),
                defaultValue = 1.0,
                # Make distance units match the INPUT layer units:
                parentParameterName='INPUT'
            )
        )
        self.addParameter(
            QgsProcessingParameterDistance(
                'CELLSIZE',
                self.tr('CELLSIZE'),
                defaultValue = 10.0,
                parentParameterName='INPUT'
            )
        )
        self.addOutput(
            QgsProcessingOutputNumber(
                'NUMBEROFFEATURES',
                self.tr('Number of features processed')
            )
        )

    def processAlgorithm(self, parameters, context, feedback):
        """
        Here is where the processing itself takes place.
        """
        # First, we get the count of features from the INPUT layer.
        # This layer is defined as a QgsProcessingParameterFeatureSource
        # parameter, so it is retrieved by calling
        # self.parameterAsSource.
        input_featuresource = self.parameterAsSource(parameters,
                                                     'INPUT',
                                                     context)
        numfeatures = input_featuresource.featureCount()

        # Retrieve the buffer distance and raster cell size numeric
        # values. Since these are numeric values, they are retrieved
        # using self.parameterAsDouble.
        bufferdist = self.parameterAsDouble(parameters, 'BUFFERDIST',
                                            context)
        rastercellsize = self.parameterAsDouble(parameters, 'CELLSIZE',
                                                context)
        if feedback.isCanceled():
            return {}
        buffer_result = processing.run(
            'native:buffer',
            {
                # Here we pass on the original parameter values of INPUT
                # and BUFFER_OUTPUT to the buffer algorithm.
                'INPUT': parameters['INPUT'],
                'OUTPUT': parameters['BUFFER_OUTPUT'],
                'DISTANCE': bufferdist,
                'SEGMENTS': 10,
                'DISSOLVE': True,
                'END_CAP_STYLE': 0,
                'JOIN_STYLE': 0,
                'MITER_LIMIT': 10
            },
            # Because the buffer algorithm is being run as a step in
            # another larger algorithm, the is_child_algorithm option
            # should be set to True
            is_child_algorithm=True,
            #
            # It's important to pass on the context and feedback objects to
            # child algorithms, so that they can properly give feedback to
            # users and handle cancelation requests.
            context=context,
            feedback=feedback)

        # Check for cancelation
        if feedback.isCanceled():
            return {}

        # Run the separate rasterization algorithm using the buffer result
        # as an input.
        rasterized_result = processing.run(
            'qgis:rasterize',
            {
                # Here we pass the 'OUTPUT' value from the buffer's result
                # dictionary off to the rasterize child algorithm.
                'LAYER': buffer_result['OUTPUT'],
                'EXTENT': buffer_result['OUTPUT'],
                'MAP_UNITS_PER_PIXEL': rastercellsize,
                # Use the original parameter value.
                'OUTPUT': parameters['OUTPUT']
            },
            is_child_algorithm=True,
            context=context,
            feedback=feedback)

        if feedback.isCanceled():
            return {}

        # Return the results
        return {'OUTPUT': rasterized_result['OUTPUT'],
                'BUFFER_OUTPUT': buffer_result['OUTPUT'],
                'NUMBEROFFEATURES': numfeatures}

Funções padrão do algoritmo de processamento:

createInstance (mandatory)
Deve retornar uma nova cópia do seu algoritmo. Se você alterar o nome da classe, certifique-se de atualizar também o valor retornado aqui para corresponder!
nome (obrigatório)
Retorna o nome exclusivo do algoritmo, usado para identificar o algoritmo.
displayName (mandatory)
Retorna o nome do algoritmo traduzido.
grupo
Retorna o nome do grupo ao qual este algoritmo pertence.
groupId
Retorna o ID único do grupo ao qual este algoritmo pertence.
shortHelpString
Returns a localised short help string for the algorithm.
initAlgorithm (mandatory)
Aqui definimos as entradas e saídas do algoritmo.

INPUT and OUTPUT are recommended names for the main input and main output parameters, respectively.

If a parameter depends on another parameter, parentParameterName is used to specify this relationship (could be the field / band of a layer or the distance units of a layer).
processAlgorithm (mandatory)
É onde ocorre o processamento.

Parameters are retrieved using special purpose functions, for instance parameterAsSource and parameterAsDouble.

processing.run can be used to run other processing algorithms from a processing algorithm. The first parameter is the name of the algorithm, the second is a dictionary of the parameters to the algorithm. is_child_algorithm is normally set to True when running an algorithm from within another algorithm. context and feedback inform the algorithm about the environment to run in and the channel for communicating with the user (catching cancel request, reporting progress, providing textual feedback). When using the (parent) algorithm’s parameters as parameters to “child” algorithms, the original parameter values should be used (e.g. parameters['OUTPUT']).

It is good practice to check the feedback object for cancelation as much as is sensibly possible! Doing so allows for responsive cancelation, instead of forcing users to wait for unwanted processing to occur.

The algorithm should return values for all the output parameters it has defined as a dictionary. In this case, that’s the buffer and rasterized output layers, and the count of features processed. The dictionary keys must match the original parameter/output names.

26.9.2. The @alg decorator 

Using the @alg decorator, you can create your own algorithms by writing the Python code and adding a few extra lines to supply additional information needed to make it a proper Processing algorithm. This simplifies the creation of algorithms and the specification of inputs and outputs.

One important limitation with the decorator approach is that algorithms created in this way will always be added to a user’s Processing Scripts provider – it is not possible to add these algorithms to a custom provider, e.g. for use in plugins.

The following code uses the @alg decorator to

usar uma camada vetorial como entrada
contar o número de feições
fazer uma operação de buffer
criar uma camada raster a partir do resultado da operação do buffer
retorna a camada de buffer, a camada raster e o número de feições

from qgis import processing
from qgis.processing import alg
from qgis.core import QgsProject

@alg(name='bufferrasteralg', label='Buffer and export to raster (alg)',
     group='examplescripts', group_label='Example scripts')
# 'INPUT' is the recommended name for the main input parameter
@alg.input(type=alg.SOURCE, name='INPUT', label='Input vector layer')
# 'OUTPUT' is the recommended name for the main output parameter
@alg.input(type=alg.RASTER_LAYER_DEST, name='OUTPUT',
           label='Raster output')
@alg.input(type=alg.VECTOR_LAYER_DEST, name='BUFFER_OUTPUT',
           label='Buffer output')
@alg.input(type=alg.DISTANCE, name='BUFFERDIST', label='BUFFER DISTANCE',
           default=1.0)
@alg.input(type=alg.DISTANCE, name='CELLSIZE', label='RASTER CELL SIZE',
           default=10.0)
@alg.output(type=alg.NUMBER, name='NUMBEROFFEATURES',
            label='Number of features processed')

def bufferrasteralg(instance, parameters, context, feedback, inputs):
    """
    Description of the algorithm.
    (If there is no comment here, you will get an error)
    """
    input_featuresource = instance.parameterAsSource(parameters,
                                                     'INPUT', context)
    numfeatures = input_featuresource.featureCount()
    bufferdist = instance.parameterAsDouble(parameters, 'BUFFERDIST',
                                            context)
    rastercellsize = instance.parameterAsDouble(parameters, 'CELLSIZE',
                                                context)
    if feedback.isCanceled():
        return {}
    buffer_result = processing.run('native:buffer',
                               {'INPUT': parameters['INPUT'],
                                'OUTPUT': parameters['BUFFER_OUTPUT'],
                                'DISTANCE': bufferdist,
                                'SEGMENTS': 10,
                                'DISSOLVE': True,
                                'END_CAP_STYLE': 0,
                                'JOIN_STYLE': 0,
                                'MITER_LIMIT': 10
                                },
                               is_child_algorithm=True,
                               context=context,
                               feedback=feedback)
    if feedback.isCanceled():
        return {}
    rasterized_result = processing.run('qgis:rasterize',
                               {'LAYER': buffer_result['OUTPUT'],
                                'EXTENT': buffer_result['OUTPUT'],
                                'MAP_UNITS_PER_PIXEL': rastercellsize,
                                'OUTPUT': parameters['OUTPUT']
                               },
                               is_child_algorithm=True, context=context,
                               feedback=feedback)
    if feedback.isCanceled():
        return {}
    return {'OUTPUT': rasterized_result['OUTPUT'],
            'BUFFER_OUTPUT': buffer_result['OUTPUT'],
            'NUMBEROFFEATURES': numfeatures}

As you can see, it involves two algorithms (‘native:buffer’ and ‘qgis:rasterize’). The last one (‘qgis:rasterize’) creates a raster layer from the buffer layer that was generated by the first one (‘native:buffer’).

The part of the code where this processing takes place is not difficult to understand if you have read the previous chapter. The first lines, however, need some additional explanation. They provide the information that is needed to turn your code into an algorithm that can be run from any of the GUI components, like the toolbox or the model designer.

These lines are all calls to the @alg decorator functions that help simplify the coding of the algorithm.

The @alg decorator is used to define the name and location of the algorithm in the Toolbox.
The @alg.input decorator is used to define the inputs of the algorithm.
The @alg.output decorator is used to define the outputs of the algorithm.

26.9.3. Tipos de entrada e saída para Algoritmos de Processamento 

Aqui está a lista dos tipos de entrada e saída que são suportados no processamento com suas constantes de decoração de algoritmos (o arquivo :fonte:`algfactory.py <python/processing/algfactory.py>` contém a lista completa de constantes de algoritmos). Classificado no nome da classe.

26.9.3.1. Tipos de entrada 

Classes	Alg constant	Descrição
`QgsProcessingParameterAnnotationLayer`	`alg.ANNOTATION_LAYER`	Uma camada de texto
`QgsProcessingParameterAuthConfig`	`alg.AUTH_CFG`	Permite que os usuários selecionem entre as configurações de autenticação disponíveis ou criem novas configurações de autenticação
`QgsProcessingParameterBand`	`alg.BAND`	Uma banda de uma camada raster
`QgsProcessingParameterBoolean`	`alg.BOOL`	Um valor booleano
`QgsProcessingParameterColor`	`alg.COLOR`	Uma cor
`QgsProcessingParameterCoordinateOperation`	`alg.COORDINATE_OPERATION`	Uma operação de coordenadas (para transformações SRC)
`QgsProcessingParameterCrs`	`alg.CRS`	Um Sistema de Referência de Coordenadas
`QgsProcessingParameterDatabaseSchema`	`alg.DATABASE_SCHEMA`	A database schema
`QgsProcessingParameterDatabaseTable`	`alg.DATABASE_TABLE`	A database table
`QgsProcessingParameterDateTime`	`alg.DATETIME`	A datetime (or a pure date or time)
`QgsProcessingParameterDistance`	`alg.DISTANCE`	Um parâmetro numérico duplo para valores de distância
`QgsProcessingParameterEnum`	`alg.ENUM`	Uma enumeração, permitindo a seleção de um conjunto de valores predefinidos
`QgsProcessingParameterExpression`	`alg.EXPRESSION`	Uma expressão
`QgsProcessingParameterExtent`	`alg.EXTENT`	Uma extensão espacial definida por xmin, xmax, ymin, ymax
`QgsProcessingParameterField`	`alg.FIELD`	Um campo na tabela de atributos de uma camada vetorial
`QgsProcessingParameterFile`	`alg.FILE`	Um nome de arquivo de um arquivo existente
`QgsProcessingParameterFileDestination`	`alg.FILE_DEST`	Um nome de arquivo para um arquivo de saída recém-criado
`QgsProcessingParameterFolderDestination`	`alg.FOLDER_DEST`	Uma pasta (pasta de destino)
`QgsProcessingParameterGeometry`	`alg.GEOMETRY`	Uma geometria
`QgsProcessingParameterNumber`	`alg.INT`	Um número inteiro
`QgsProcessingParameterLayout`	`alg.LAYOUT`	Um layout
`QgsProcessingParameterLayoutItem`	`alg.LAYOUT_ITEM`	Um item de layout
`QgsProcessingParameterMapLayer`	`alg.MAPLAYER`	Uma camada do mapa
`QgsProcessingParameterMapTheme`	`alg.MAP_THEME`	A project map theme
`QgsProcessingParameterMatrix`	`alg.MATRIX`	Uma matriz
`QgsProcessingParameterMeshLayer`	`alg.MESH_LAYER`	Uma camada de malha
`QgsProcessingParameterMultipleLayers`	`alg.MULTILAYER`	A set of layers
`QgsProcessingParameterNumber`	`alg.NUMBER`	Um valor numérico
`QgsProcessingParameterPoint`	`alg.POINT`	Um ponto
`QgsProcessingParameterPointCloudDestination`	`alg.POINTCLOUD_LAYER_DEST`	A point cloud layer destination parameter, for specifying the destination path for a point cloud layer created by the algorithm
`QgsProcessingParameterPointCloudLayer`	`alg.POINTCLOUD_LAYER`	Uma camada de nuvens de pontos
`QgsProcessingParameterProviderConnection`	`alg.PROVIDER_CONNECTION`	An available connection for a database provider
`QgsProcessingParameterRange`	`alg.RANGE`	Um intervalo de números
`QgsProcessingParameterRasterLayer`	`alg.RASTER_LAYER`	Uma camada raster
`QgsProcessingParameterRasterDestination`	`alg.RASTER_LAYER_DEST`	A raster layer destination parameter, for specifying the destination path for a raster layer created by the algorithm
`QgsProcessingParameterScale`	`alg.SCALE`	Uma escala do mapa
`QgsProcessingParameterFeatureSink`	`alg.SINK`	A feature sink
`QgsProcessingParameterFeatureSource`	`alg.SOURCE`	A feature source
`QgsProcessingParameterString`	`alg.STRING`	A text string
`QgsProcessingParameterVectorLayer`	`alg.VECTOR_LAYER`	Uma camada vetorial
`QgsProcessingParameterVectorDestination`	`alg.VECTOR_LAYER_DEST`	A vector layer destination parameter, for specifying the destination path for a vector layer created by the algorithm

26.9.3.2. Tipos de saída 

Classes	Alg constant	Descrição
`QgsProcessingOutputBoolean`	`alg.BOOL`	Um valor booleano
`QgsProcessingOutputNumber`	`alg.DISTANCE`	Um parâmetro numérico duplo para valores de distância
`QgsProcessingOutputFile`	`alg.FILE`	Um nome de arquivo de um arquivo existente
`QgsProcessingOutputFolder`	`alg.FOLDER`	Uma pasta
`QgsProcessingOutputHtml`	`alg.HTML`	HTML
`QgsProcessingOutputNumber`	`alg.INT`	Um número inteiro
`QgsProcessingOutputLayerDefinition`	`alg.LAYERDEF`	Uma definição de camada
`QgsProcessingOutputMapLayer`	`alg.MAPLAYER`	Uma camada do mapa
`QgsProcessingOutputMultipleLayers`	`alg.MULTILAYER`	A set of layers
`QgsProcessingOutputNumber`	`alg.NUMBER`	Um valor numérico
`QgsProcessingOutputPointCloudLayer`	`alg.POINTCLOUD_LAYER`	Uma camada de nuvens de pontos
`QgsProcessingOutputRasterLayer`	`alg.RASTER_LAYER`	Uma camada raster
`QgsProcessingOutputString`	`alg.STRING`	A text string
`QgsProcessingOutputVectorLayer`	`alg.VECTOR_LAYER`	Uma camada vetorial

26.9.4. Handing algorithm output 

Quando você declara uma saída representando uma camada (raster ou vetor), o algoritmo tentará adicioná-la ao QGIS assim que terminar.

Raster layer output: QgsProcessingParameterRasterDestination / alg.RASTER_LAYER_DEST.
Vector layer output: QgsProcessingParameterVectorDestination / alg.VECTOR_LAYER_DEST.

So even if the processing.run() method does not add the layers it creates to the user’s current project, the two output layers (buffer and raster buffer) will be loaded, since they are saved to the destinations entered by the user (or to temporary destinations if the user does not specify destinations).

If a layer is created as output of an algorithm, it should be declared as such. Otherwise, you will not be able to properly use the algorithm in the modeler, since what is declared will not match what the algorithm really creates.

You can return strings, numbers and more by specifying them in the result dictionary (as demonstrated for “NUMBEROFFEATURES”), but they should always be explicitly defined as outputs from your algorithm. We encourage algorithms to output as many useful values as possible, since these can be valuable for use in later algorithms when your algorithm is used as part of a model.

26.9.5. Comunicação com o usuário 

If your algorithm takes a long time to process, it is a good idea to inform the user about the progress. You can use feedback (QgsProcessingFeedback) for this.

The progress text and progressbar can be updated using two methods: setProgressText(text) and setProgress(percent).

You can provide more information by using pushCommandInfo(text), pushDebugInfo(text), pushInfo(text) and reportError(text).

If your script has a problem, the correct way of handling it is to raise a QgsProcessingException. You can pass a message as an argument to the constructor of the exception. Processing will take care of handling it and communicating with the user, depending on where the algorithm is being executed from (toolbox, modeler, Python console, …)

26.9.6. Documentando seus scripts 

You can document your scripts by overloading the helpString() and helpUrl() methods of QgsProcessingAlgorithm.

26.9.7. Bandeiras 

You can override the flags() method of QgsProcessingAlgorithm to tell QGIS more about your algorithm. You can for instance tell QGIS that the script shall be hidden from the modeler, that it can be canceled, that it is not thread safe, and more.

Dica

By default, Processing runs algorithms in a separate thread in order to keep QGIS responsive while the processing task runs. If your algorithm is regularly crashing, you are probably using API calls which are not safe to do in a background thread. Try returning the QgsProcessingAlgorithm.FlagNoThreading flag from your algorithm’s flags() method to force Processing to run your algorithm in the main thread instead.

26.9.8. Melhores práticas para algoritmos de script escrito 

Here’s a quick summary of ideas to consider when creating your script algorithms and, especially, if you want to share them with other QGIS users. Following these simple rules will ensure consistency across the different Processing elements such as the toolbox, the modeler or the batch processing interface.

Não coloque resultados das camadas. Vamos trabalhar o Processamento com seus resultados e carregar suas camadas se necessárias.
Sempre declare as saídas que seu algoritmo cria.
Do not show message boxes or use any GUI element from the script. If you want to communicate with the user, use the methods of the feedback object (QgsProcessingFeedback) or throw a QgsProcessingException.

There are already many processing algorithms available in QGIS. You can find code on the QGIS repo.