Subgroup: Processing

Class: QgsProcessingFeatureBasedAlgorithm¶

class qgis.core.QgsProcessingFeatureBasedAlgorithm¶

Bases: qgis._core.QgsProcessingAlgorithm

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).

New in version 3.0: Methods

`addOutput`
`addParameter`
`asPythonCommand`	Returns a Python command string which can be executed to run the algorithm using the specified `parameters`.
`canExecute`	Returns true if the algorithm can execute.
`checkParameterValues`	Checks the supplied `parameter` values to verify that they satisfy the requirements of this algorithm in the supplied `context`.
`countVisibleParameters`	Returns the number of visible (non-hidden) parameters defined by this algorithm.
`create`	Creates a copy of the algorithm, ready for execution.
`createCustomParametersWidget`	If an algorithm subclass implements a custom parameters widget, a copy of this widget should be constructed and returned by this method.
`createExpressionContext`	Creates an expression context relating to the algorithm.
`createInstance`
`destinationParameterDefinitions`	Returns a list of destination parameters definitions utilized by the algorithm.
`displayName`	Returns the translated algorithm name, which should be used for any user-visible display of the algorithm name.
`flags`	Returns the flags indicating how and when the algorithm operates and should be exposed to users.
`group`	Returns the name of the group this algorithm belongs to.
`groupId`	Returns the unique ID of the group this algorithm belongs to.
`hasHtmlOutputs`	Returns true if this algorithm generates HTML outputs.
`helpString`	Returns a localised help string for the algorithm.
`helpUrl`	Returns a url pointing to the algorithm’s help page.
`icon`	Returns an icon for the algorithm.
`id`	Returns the unique ID for the algorithm, which is a combination of the algorithm provider’s ID and the algorithms unique name (e.g.
`initAlgorithm`
`initParameters`	Initializes any extra parameters added by the algorithm subclass.
`inputLayerTypes`	Returns the valid input layer types for the source layer for this algorithm.
`name`	Returns the algorithm name, used for identifying the algorithm.
`outputCrs`	Maps the input source coordinate reference system (`inputCrs`) to a corresponding output CRS generated by the algorithm.
`outputDefinition`	Returns a matching output by `name`.
`outputDefinitions`	Returns an ordered list of output definitions utilized 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`
`parameterAsCompatibleSourceLayerPath`
`parameterAsCrs`
`parameterAsDouble`
`parameterAsEnum`
`parameterAsEnums`
`parameterAsExpression`
`parameterAsExtent`
`parameterAsExtentCrs`
`parameterAsExtentGeometry`
`parameterAsFields`
`parameterAsFile`
`parameterAsFileOutput`
`parameterAsInt`
`parameterAsLayer`
`parameterAsLayerList`
`parameterAsMatrix`
`parameterAsOutputLayer`
`parameterAsPoint`
`parameterAsPointCrs`
`parameterAsRange`
`parameterAsRasterLayer`
`parameterAsSink`
`parameterAsSource`
`parameterAsString`
`parameterAsVectorLayer`
`parameterDefinition`	Returns a matching parameter by `name`.
`parameterDefinitions`	Returns an ordered list of parameter definitions utilized by the algorithm.
`postProcess`	Should be called in the main thread following the completion of runPrepared().
`postProcessAlgorithm`
`prepare`	Prepares the algorithm for execution.
`prepareAlgorithm`
`processAlgorithm`
`processFeature`	Processes an individual input `feature` from the source.
`provider`	Returns the provider to which this algorithm belongs.
`removeParameter`
`request`	Returns the feature request used for fetching features to process from the source layer.
`run`	Executes the algorithm using the specified `parameters`.
`runPrepared`	Runs the algorithm, which has been prepared by an earlier call to prepare().
`setProvider`	Associates this algorithm with its provider.
`shortHelpString`	Returns a localised short helper string for the algorithm.
`sourceCrs`	Returns the source’s coordinate reference system.
`sourceFlags`	Returns the processing feature source flags to be used in the algorithm.
`svgIconPath`	Returns a path to an SVG version of the algorithm’s icon.
`tags`	Returns a list of tags which relate to the algorithm, and are used to assist users in searching for suitable algorithms.
`validateInputCrs`	Checks whether the coordinate reference systems for the specified set of `parameters` are valid for the algorithm.

Attributes

`FlagCanCancel`
`FlagCanRunInBackground`
`FlagDeprecated`
`FlagHideFromModeler`
`FlagHideFromToolbox`
`FlagRequiresMatchingCrs`
`FlagSupportsBatch`

addOutput()¶

addParameter()¶

createInstance()¶

initAlgorithm(self, configuration: Dict[str, Any] = QVariantMap())¶

initParameters(self, configuration: Dict[str, Any] = QVariantMap())¶: 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.

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.

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).

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).

outputLayerType(self) → QgsProcessing.SourceType¶: Returns the layer type for layers generated by this algorithm, if this is possible to determine in advance.

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.

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).

parameterAsBool()¶

parameterAsCompatibleSourceLayerPath()¶

parameterAsCrs()¶

parameterAsDouble()¶

parameterAsEnum()¶

parameterAsEnums()¶

parameterAsExpression()¶

parameterAsExtent()¶

parameterAsExtentCrs()¶

parameterAsExtentGeometry()¶

parameterAsFields()¶

parameterAsFile()¶

parameterAsFileOutput()¶

parameterAsInt()¶

parameterAsLayer()¶

parameterAsLayerList()¶

parameterAsMatrix()¶

parameterAsOutputLayer()¶

parameterAsPoint()¶

parameterAsPointCrs()¶

parameterAsRange()¶

parameterAsRasterLayer()¶

parameterAsSink()¶

parameterAsSource()¶

parameterAsString()¶

parameterAsVectorLayer()¶

postProcessAlgorithm()¶

prepareAlgorithm()¶

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

processFeature(self, feature: QgsFeature, context: QgsProcessingContext, feedback: QgsProcessingFeedback) → 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 the modified feature. Returning an invalid feature (e.g. a default constructed QgsFeature) 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.

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 cancelation 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.

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.

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

sourceFlags(self) → QgsProcessingFeatureSource.Flag¶: Returns the processing feature source flags to be used in the algorithm.