Class: QgsProcessingFeatureBasedAlgorithm

class qgis.core.QgsProcessingFeatureBasedAlgorithm

Bases: QgsProcessingAlgorithm

Constructor for QgsProcessingFeatureBasedAlgorithm.

An abstract QgsProcessingAlgorithm base class for processing algorithms which operate “feature-by-feature”.

Feature based algorithms are algorithms which operate on individual features in isolation. These are algorithms where one feature is output for each input feature, and the output feature result for each input feature is not dependent on any other features present in the source.

For instance, algorithms like “centroids” and “buffers” are feature based algorithms since the centroid or buffer of a feature is calculated for each feature in isolation. An algorithm like “dissolve” is NOT suitable for a feature based algorithm as the dissolved output depends on multiple input features and these features cannot be processed in isolation.

Using QgsProcessingFeatureBasedAlgorithm as the base class for feature based algorithms allows shortcutting much of the common algorithm code for handling iterating over sources and pushing features to output sinks. It also allows the algorithm execution to be optimised in future (for instance allowing automatic multi-thread processing of the algorithm, or use of the algorithm in “chains”, avoiding the need for temporary outputs in multi-step models).

Methods

addOutput

addParameter

createInstance

flags

rtype

QgsProcessingAlgorithm.Flags

initAlgorithm

param configuration

initParameters

Initializes any extra parameters added by the algorithm subclass.

inputLayerTypes

Returns the valid input layer types for the source layer for this algorithm.

invalidRasterError

invalidSinkError

invalidSourceError

outputCrs

Maps the input source coordinate reference system (inputCrs) to a corresponding output CRS generated by the algorithm.

outputFields

Maps the input source fields (inputFields) to corresponding output fields generated by the algorithm.

outputLayerType

Returns the layer type for layers generated by this algorithm, if this is possible to determine in advance.

outputName

Returns the translated, user visible name for any layers created by this algorithm.

outputWkbType

Maps the input WKB geometry type (inputWkbType) to the corresponding output WKB type generated by the algorithm.

parameterAsBool

parameterAsBoolean

parameterAsColor

parameterAsCompatibleSourceLayerPath

parameterAsCompatibleSourceLayerPathAndLayerName

parameterAsCrs

parameterAsDouble

parameterAsEnum

parameterAsEnums

parameterAsExpression

parameterAsExtent

parameterAsExtentCrs

parameterAsExtentGeometry

parameterAsFields

parameterAsFile

parameterAsFileList

parameterAsFileOutput

parameterAsInt

parameterAsInts

parameterAsLayer

parameterAsLayerList

parameterAsLayout

parameterAsLayoutItem

parameterAsMatrix

parameterAsMeshLayer

parameterAsOutputLayer

parameterAsPoint

parameterAsPointCrs

parameterAsRange

parameterAsRasterLayer

parameterAsSink

parameterAsSource

parameterAsString

parameterAsVectorLayer

postProcessAlgorithm

prepareAlgorithm

prepareSource

Read the source from parameters and context and set it

processAlgorithm

param parameters

processFeature

Processes an individual input feature from the source.

removeParameter

request

Returns the feature request used for fetching features to process from the source layer.

sinkFlags

Returns the feature sink flags to be used for the output.

sourceCrs

Returns the source’s coordinate reference system.

sourceFlags

Returns the processing feature source flags to be used in the algorithm.

supportInPlaceEdit

Checks whether this algorithm supports in-place editing on the given layer Default implementation for feature based algorithms run some basic compatibility checks based on the geometry type of the layer.

addOutput()
addParameter()
createInstance()
flags(self) → QgsProcessingAlgorithm.Flags
Return type

QgsProcessingAlgorithm.Flags

initAlgorithm(self, configuration: Dict[str, Any] = {})
Parameters

configuration (Dict[str) –

initParameters(self, configuration: Dict[str, Any] = {})

Initializes any extra parameters added by the algorithm subclass. There is no need to declare the input source or output sink, as these are automatically created by QgsProcessingFeatureBasedAlgorithm.

Parameters

configuration (Dict[str) –

inputLayerTypes(self) → List[int]

Returns the valid input layer types for the source layer for this algorithm. By default vector layers with any geometry types (excluding non-spatial, geometryless layers) are accepted.

Return type

List[int]

invalidRasterError()
invalidSinkError()
invalidSourceError()
outputCrs(self, inputCrs: QgsCoordinateReferenceSystem) → QgsCoordinateReferenceSystem

Maps the input source coordinate reference system (inputCrs) to a corresponding output CRS generated by the algorithm. The default behavior is that the algorithm maintains the same CRS as the input source.

This is called once by the base class when creating the output sink for the algorithm (i.e. it is not called once per feature processed).

Parameters

inputCrs (QgsCoordinateReferenceSystem) –

Return type

QgsCoordinateReferenceSystem

outputFields(self, inputFields: QgsFields) → QgsFields

Maps the input source fields (inputFields) to corresponding output fields generated by the algorithm. The default behavior is that the algorithm maintains the same fields as are input. Algorithms which add, remove or modify existing fields should override this method and implement logic here to indicate which fields are output by the algorithm.

This is called once by the base class when creating the output sink for the algorithm (i.e. it is not called once per feature processed).

Parameters

inputFields (QgsFields) –

Return type

QgsFields

outputLayerType(self) → QgsProcessing.SourceType

Returns the layer type for layers generated by this algorithm, if this is possible to determine in advance.

Return type

QgsProcessing.SourceType

outputName(self) → str

Returns the translated, user visible name for any layers created by this algorithm. This name will be used as the default name when loading the resultant layer into a QGIS project.

Return type

str

outputWkbType(self, inputWkbType: QgsWkbTypes.Type) → QgsWkbTypes.Type

Maps the input WKB geometry type (inputWkbType) to the corresponding output WKB type generated by the algorithm. The default behavior is that the algorithm maintains the same WKB type. This is called once by the base class when creating the output sink for the algorithm (i.e. it is not called once per feature processed).

Parameters

inputWkbType (QgsWkbTypes.Type) –

Return type

QgsWkbTypes.Type

parameterAsBool()
parameterAsBoolean()
parameterAsColor()
parameterAsCompatibleSourceLayerPath()
parameterAsCompatibleSourceLayerPathAndLayerName()
parameterAsCrs()
parameterAsDouble()
parameterAsEnum()
parameterAsEnums()
parameterAsExpression()
parameterAsExtent()
parameterAsExtentCrs()
parameterAsExtentGeometry()
parameterAsFields()
parameterAsFile()
parameterAsFileList()
parameterAsFileOutput()
parameterAsInt()
parameterAsInts()
parameterAsLayer()
parameterAsLayerList()
parameterAsLayout()
parameterAsLayoutItem()
parameterAsMatrix()
parameterAsMeshLayer()
parameterAsOutputLayer()
parameterAsPoint()
parameterAsPointCrs()
parameterAsRange()
parameterAsRasterLayer()
parameterAsSink()
parameterAsSource()
parameterAsString()
parameterAsVectorLayer()
postProcessAlgorithm()
prepareAlgorithm()
prepareSource(self, parameters: Dict[str, Any], context: QgsProcessingContext)

Read the source from parameters and context and set it

New in version 3.4.

Parameters
processAlgorithm(self, parameters: Dict[str, Any], context: QgsProcessingContext, feedback: QgsProcessingFeedback) → Dict[str, Any]
Parameters
Return type

Dict[str, Any]

processFeature(self, feature: QgsFeature, context: QgsProcessingContext, feedback: QgsProcessingFeedback) → List[QgsFeature]

Processes an individual input feature from the source. Algorithms should implement their logic in this method for performing the algorithm’s operation (e.g. replacing the feature’s geometry with the centroid of the original feature geometry for a ‘centroid’ type algorithm).

Implementations should return a list containing the modified feature. Returning an empty an list will indicate that this feature should be ‘skipped’, and will not be added to the algorithm’s output. Subclasses can use this approach to filter the incoming features as desired.

Additionally, multiple features can be returned for a single input feature. Each returned feature will be added to the algorithm’s output. This allows for “explode” type algorithms where a single input feature results in multiple output features.

The provided feedback object can be used to push messages to the log and for giving feedback to users. Note that handling of progress reports and algorithm cancellation is handled by the base class and subclasses do not need to reimplement this logic.

Algorithms can throw a QgsProcessingException if a fatal error occurred which should prevent the algorithm execution from continuing. This can be annoying for users though as it can break valid model execution - so use with extreme caution, and consider using feedback to instead report non-fatal processing failures for features instead.

Parameters
Return type

List[QgsFeature]

removeParameter()
request(self) → QgsFeatureRequest

Returns the feature request used for fetching features to process from the source layer. The default implementation requests all attributes and geometry.

Return type

QgsFeatureRequest

sinkFlags(self) → QgsFeatureSink.SinkFlags

Returns the feature sink flags to be used for the output.

New in version 3.4.1.

Return type

QgsFeatureSink.SinkFlags

sourceCrs(self) → QgsCoordinateReferenceSystem

Returns the source’s coordinate reference system. This will only return a valid CRS when called from a subclasses’ processFeature() implementation.

Return type

QgsCoordinateReferenceSystem

sourceFlags(self) → QgsProcessingFeatureSource.Flag

Returns the processing feature source flags to be used in the algorithm.

Return type

QgsProcessingFeatureSource.Flag

supportInPlaceEdit(self, layer: QgsMapLayer) → bool

Checks whether this algorithm supports in-place editing on the given layer Default implementation for feature based algorithms run some basic compatibility checks based on the geometry type of the layer.

Return type

bool

Returns

True if the algorithm supports in-place editing

New in version 3.4.

Parameters

layer (QgsMapLayer) –