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

Adds an output definition to the algorithm.

addParameter

Adds a parameter definition to the algorithm.

createInstance

Creates a new instance of the algorithm class.

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.

inputParameterDescription

Returns the translated description of the parameter corresponding to the input layer.

inputParameterName

Returns the name of the parameter corresponding to the input layer.

invalidRasterError

Returns a user-friendly string to use as an error when a raster layer input could not be loaded.

invalidSinkError

Returns a user-friendly string to use as an error when a sink parameter could not be created.

invalidSourceError

Returns a user-friendly string to use as an error when a source parameter could not be loaded.

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

Evaluates the parameter with matching name to a static boolean value.

parameterAsBoolean

Evaluates the parameter with matching name to a static boolean value.

parameterAsColor

Evaluates the parameter with matching name to a color, or returns an invalid color if the parameter was not set.

parameterAsCompatibleSourceLayerPath

Evaluates the parameter with matching name to a source vector layer file path of compatible format.

parameterAsCompatibleSourceLayerPathAndLayerName

Evaluates the parameter with matching name to a source vector layer file path and layer name of compatible format.

parameterAsConnectionName

Evaluates the parameter with matching name to a connection name string.

parameterAsCrs

Evaluates the parameter with matching name to a coordinate reference system.

parameterAsDatabaseTableName

Evaluates the parameter with matching name to a database table name string.

parameterAsDateTime

Evaluates the parameter with matching name to a DateTime, or returns an invalid date time if the parameter was not set.

parameterAsDouble

Evaluates the parameter with matching name to a static double value.

parameterAsEnum

Evaluates the parameter with matching name to a enum value.

parameterAsEnumString

Evaluates the parameter with matching name to a static enum string.

parameterAsEnumStrings

Evaluates the parameter with matching name to list of static enum strings.

parameterAsEnums

Evaluates the parameter with matching name to list of enum values.

parameterAsExpression

Evaluates the parameter with matching name to an expression.

parameterAsExtent

Evaluates the parameter with matching name to a rectangular extent.

parameterAsExtentCrs

Returns the coordinate reference system associated with an extent parameter value.

parameterAsExtentGeometry

Evaluates the parameter with matching name to a rectangular extent, and returns a geometry covering this extent.

parameterAsFields

Evaluates the parameter with matching name to a list of fields.

parameterAsFile

Evaluates the parameter with matching name to a file/folder name.

parameterAsFileList

Evaluates the parameter with matching name to a list of files (for QgsProcessingParameterMultipleLayers in QgsProcessing:TypeFile mode).

parameterAsFileOutput

Evaluates the parameter with matching name to a file based output destination.

parameterAsGeometry

Evaluates the parameter with matching name to a geometry.

parameterAsGeometryCrs

Returns the coordinate reference system associated with a geometry parameter value.

parameterAsInt

Evaluates the parameter with matching name to a static integer value.

parameterAsInts

Evaluates the parameter with matching name to a list of integer values.

parameterAsLayer

Evaluates the parameter with matching name to a map layer.

parameterAsLayerList

Evaluates the parameter with matching name to a list of map layers.

parameterAsLayout

Evaluates the parameter with matching name to a print layout.

parameterAsLayoutItem

Evaluates the parameter with matching name to a print layout item, taken from the specified layout.

parameterAsMatrix

Evaluates the parameter with matching name to a matrix/table of values.

parameterAsMeshLayer

Evaluates the parameter with matching name to a mesh layer.

parameterAsOutputLayer

Evaluates the parameter with matching name to a output layer destination.

parameterAsPoint

Evaluates the parameter with matching name to a point.

parameterAsPointCrs

Returns the coordinate reference system associated with an point parameter value.

parameterAsRange

Evaluates the parameter with matching name to a range of values.

parameterAsRasterLayer

Evaluates the parameter with matching name to a raster layer.

parameterAsSchema

Evaluates the parameter with matching name to a database schema name string.

parameterAsSink

Evaluates the parameter with matching name to a feature sink.

parameterAsSource

Evaluates the parameter with matching name to a feature source.

parameterAsString

Evaluates the parameter with matching name to a static string value.

parameterAsVectorLayer

Evaluates the parameter with matching name to a vector layer.

postProcessAlgorithm

Allows the algorithm to perform any required cleanup tasks.

prepareAlgorithm

Prepares the algorithm to run using the specified parameters.

prepareSource

Read the source from parameters and context and set it

processAlgorithm

param parameters

processFeature

Processes an individual input feature from the source.

removeParameter

Removes the parameter with matching name from the algorithm, and deletes any existing definition.

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.

sinkProperties

param sink

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(self, outputDefinition: QgsProcessingOutputDefinition) → bool

Adds an output definition to the algorithm. Ownership of the definition is transferred to the algorithm. Returns True if the output could be successfully added, or False if the output could not be added (e.g. as a result of a duplicate name).

This should usually be called from a subclass’ initAlgorithm() implementation.

Note that in some cases output creation can be automatically performed when calling addParameter(). See the notes in addParameter() for a description of when this occurs.

See also

addParameter()

See also

initAlgorithm()

addParameter(self, parameterDefinition: QgsProcessingParameterDefinition, createOutput: bool = True) → bool

Adds a parameter definition to the algorithm. Ownership of the definition is transferred to the algorithm. Returns True if parameter could be successfully added, or False if the parameter could not be added (e.g. as a result of a duplicate name).

This should usually be called from a subclass’ initAlgorithm() implementation.

If the createOutput argument is True, then a corresponding output definition will also be created (and added to the algorithm) where appropriate. E.g. when adding a QgsProcessingParameterVectorDestination and createOutput is True, then a QgsProcessingOutputVectorLayer output will be created and added to the algorithm. There is no need to call addOutput() to manually add a corresponding output for this vector. If createOutput is False then this automatic output creation will not occur.

See also

initAlgorithm()

See also

addOutput()

createInstance(self)QgsProcessingAlgorithm

Creates a new instance of the algorithm class.

This method should return a ‘pristine’ instance of the algorithm class.

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]

inputParameterDescription(self) → str

Returns the translated description of the parameter corresponding to the input layer.

By default this is a translated “Input layer” string.

New in version 3.12.

Return type

str

inputParameterName(self) → str

Returns the name of the parameter corresponding to the input layer.

By default this is the standard “INPUT” parameter name.

New in version 3.12.

Return type

str

invalidRasterError(parameters: Dict[str, Any], name: str) → str

Returns a user-friendly string to use as an error when a raster layer input could not be loaded.

The parameters argument should give the algorithms parameter map, and the name should correspond to the invalid source parameter name.

New in version 3.2.

invalidSinkError(parameters: Dict[str, Any], name: str) → str

Returns a user-friendly string to use as an error when a sink parameter could not be created.

The parameters argument should give the algorithms parameter map, and the name should correspond to the invalid source parameter name.

New in version 3.2.

invalidSourceError(parameters: Dict[str, Any], name: str) → str

Returns a user-friendly string to use as an error when a source parameter could not be loaded.

The parameters argument should give the algorithms parameter map, and the name should correspond to the invalid source parameter name.

New in version 3.2.

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(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → bool

Evaluates the parameter with matching name to a static boolean value.

parameterAsBoolean(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → bool

Evaluates the parameter with matching name to a static boolean value.

New in version 3.8.

parameterAsColor(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → QColor

Evaluates the parameter with matching name to a color, or returns an invalid color if the parameter was not set.

New in version 3.10.

parameterAsCompatibleSourceLayerPath(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, compatibleFormats: Iterable[str], preferredFormat: str = '', feedback: QgsProcessingFeedback = None) → str

Evaluates the parameter with matching name to a source vector layer file path of compatible format.

If the parameter is evaluated to an existing layer, and that layer is not of the format listed in the compatibleFormats argument, then the layer will first be exported to a compatible format in a temporary location. The function will then return the path to that temporary file.

compatibleFormats should consist entirely of lowercase file extensions, e.g. ‘shp’.

The preferredFormat argument is used to specify to desired file extension to use when a temporary layer export is required.

When an algorithm is capable of handling multi-layer input files (such as Geopackage), it is preferable to use parameterAsCompatibleSourceLayerPathAndLayerName() which may avoid conversion in more situations.

parameterAsCompatibleSourceLayerPathAndLayerName(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, compatibleFormats: Iterable[str], preferredFormat: str = '', feedback: QgsProcessingFeedback = None) → Tuple[str, str]

Evaluates the parameter with matching name to a source vector layer file path and layer name of compatible format.

If the parameter is evaluated to an existing layer, and that layer is not of the format listed in the compatibleFormats argument, then the layer will first be exported to a compatible format in a temporary location. The function will then return the path to that temporary file.

compatibleFormats should consist entirely of lowercase file extensions, e.g. ‘shp’.

The preferredFormat argument is used to specify to desired file extension to use when a temporary layer export is required. This defaults to shapefiles, because shapefiles are the future (don’t believe the geopackage hype!).

This method should be preferred over parameterAsCompatibleSourceLayerPath() when an algorithm is able to correctly handle files with multiple layers. Unlike parameterAsCompatibleSourceLayerPath(), it will not force a conversion in this case and will return the target layer name in the layerName argument.

Parameters
  • parameters – input parameter value map

  • name – name of target parameter

  • context – processing context

  • compatibleFormats – a list of lowercase file extensions compatible with the algorithm

  • preferredFormat – preferred format extension to use if conversion if required

  • feedback – feedback object

Returns

  • path to source layer, or nearly converted compatible layer

  • layerName: will be set to the target layer name for multi-layer sources (e.g. Geopackage)

New in version 3.10.

parameterAsConnectionName(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a connection name string.

New in version 3.14.

parameterAsCrs(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsCoordinateReferenceSystem

Evaluates the parameter with matching name to a coordinate reference system.

parameterAsDatabaseTableName(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a database table name string.

New in version 3.14.

parameterAsDateTime(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → QDateTime

Evaluates the parameter with matching name to a DateTime, or returns an invalid date time if the parameter was not set.

New in version 3.14.

parameterAsDouble(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → float

Evaluates the parameter with matching name to a static double value.

parameterAsEnum(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → int

Evaluates the parameter with matching name to a enum value.

parameterAsEnumString(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a static enum string.

New in version 3.18.

parameterAsEnumStrings(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[str]

Evaluates the parameter with matching name to list of static enum strings.

New in version 3.18.

parameterAsEnums(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[int]

Evaluates the parameter with matching name to list of enum values.

parameterAsExpression(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to an expression.

parameterAsExtent(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, crs: QgsCoordinateReferenceSystem = QgsCoordinateReferenceSystem())QgsRectangle

Evaluates the parameter with matching name to a rectangular extent.

If crs is set, and the original coordinate reference system of the parameter can be determined, then the extent will be automatically reprojected so that it is in the specified crs. In this case the extent of the reproject rectangle will be returned.

parameterAsExtentCrs(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsCoordinateReferenceSystem

Returns the coordinate reference system associated with an extent parameter value.

parameterAsExtentGeometry(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, crs: QgsCoordinateReferenceSystem = QgsCoordinateReferenceSystem())QgsGeometry

Evaluates the parameter with matching name to a rectangular extent, and returns a geometry covering this extent.

If crs is set, and the original coordinate reference system of the parameter can be determined, then the extent will be automatically reprojected so that it is in the specified crs. Unlike parameterAsExtent(), the reprojected rectangle returned by this function will no longer be a rectangle itself (i.e. this method returns the geometry of the actual reprojected rectangle, while parameterAsExtent() returns just the extent of the reprojected rectangle).

parameterAsFields(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[str]

Evaluates the parameter with matching name to a list of fields.

parameterAsFile(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a file/folder name.

parameterAsFileList(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[str]

Evaluates the parameter with matching name to a list of files (for QgsProcessingParameterMultipleLayers in QgsProcessing:TypeFile mode).

New in version 3.10.

parameterAsFileOutput(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a file based output destination.

parameterAsGeometry(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, crs: QgsCoordinateReferenceSystem = QgsCoordinateReferenceSystem())QgsGeometry

Evaluates the parameter with matching name to a geometry.

If crs is set then the geometry will be automatically reprojected so that it is in the specified crs.

parameterAsGeometryCrs(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsCoordinateReferenceSystem

Returns the coordinate reference system associated with a geometry parameter value.

parameterAsInt(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → int

Evaluates the parameter with matching name to a static integer value.

parameterAsInts(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[int]

Evaluates the parameter with matching name to a list of integer values.

New in version 3.4.

parameterAsLayer(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsMapLayer

Evaluates the parameter with matching name to a map layer.

Layers will either be taken from context’s active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

parameterAsLayerList(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[QgsMapLayer]

Evaluates the parameter with matching name to a list of map layers.

parameterAsLayout(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsPrintLayout

Evaluates the parameter with matching name to a print layout.

Warning

This method is not safe to run in a background thread, so it must either be used within a prepareAlgorithm implementation (which runs in the main thread), or the algorithm must return the FlagNoThreading flag.

New in version 3.8.

parameterAsLayoutItem(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, layout: QgsPrintLayout)QgsLayoutItem

Evaluates the parameter with matching name to a print layout item, taken from the specified layout.

Warning

This method is not safe to run in a background thread, so it must either be used within a prepareAlgorithm implementation (which runs in the main thread), or the algorithm must return the FlagNoThreading flag.

New in version 3.8.

parameterAsMatrix(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[Any]

Evaluates the parameter with matching name to a matrix/table of values. Tables are collapsed to a 1 dimensional list.

parameterAsMeshLayer(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsMeshLayer

Evaluates the parameter with matching name to a mesh layer.

Layers will either be taken from context’s active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

New in version 3.6.

parameterAsOutputLayer(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a output layer destination.

parameterAsPoint(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, crs: QgsCoordinateReferenceSystem = QgsCoordinateReferenceSystem())QgsPointXY

Evaluates the parameter with matching name to a point.

If crs is set then the point will be automatically reprojected so that it is in the specified crs.

parameterAsPointCrs(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsCoordinateReferenceSystem

Returns the coordinate reference system associated with an point parameter value.

parameterAsRange(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → List[float]

Evaluates the parameter with matching name to a range of values.

parameterAsRasterLayer(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsRasterLayer

Evaluates the parameter with matching name to a raster layer.

Layers will either be taken from context’s active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

parameterAsSchema(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a database schema name string.

New in version 3.14.

parameterAsSink(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext, fields: QgsFields, geometryType: QgsWkbTypes.Type = QgsWkbTypes.NoGeometry, crs: QgsCoordinateReferenceSystem = QgsCoordinateReferenceSystem(), sinkFlags: Union[QgsFeatureSink.SinkFlags, QgsFeatureSink.SinkFlag] = QgsFeatureSink.SinkFlags(), createOptions: Dict[str, Any] = {}, datasourceOptions: Iterable[str] = [], layerOptions: Iterable[str] = []) → Tuple[QgsFeatureSink, str]

Evaluates the parameter with matching name to a feature sink.

Sinks will either be taken from context’s active project, or created from external providers and stored temporarily in the context.

The fields, geometryType and crs parameters dictate the properties of the resulting feature sink.

The destinationIdentifier argument will be set to a string which can be used to retrieve the layer corresponding to the sink, e.g. via calling QgsProcessingUtils.mapLayerFromString().

The createOptions argument is used to pass on creation options such as layer name.

The datasourceOptions and layerOptions arguments is used to pass on GDAL-specific format driver options.

This function creates a new object and the caller takes responsibility for deleting the returned object.

Raises

: – py:class:QgsProcessingException

parameterAsSource(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsProcessingFeatureSource

Evaluates the parameter with matching name to a feature source.

Sources will either be taken from context’s active project, or loaded from external sources and stored temporarily in the context.

This function creates a new object and the caller takes responsibility for deleting the returned object.

parameterAsString(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext) → str

Evaluates the parameter with matching name to a static string value.

parameterAsVectorLayer(self, parameters: Dict[str, Any], name: str, context: QgsProcessingContext)QgsVectorLayer

Evaluates the parameter with matching name to a vector layer.

Layers will either be taken from context’s active project, or loaded from external sources and stored temporarily in the context. In either case, callers do not need to handle deletion of the returned layer.

postProcessAlgorithm(self, context: QgsProcessingContext, feedback: QgsProcessingFeedback) → Dict[str, Any]

Allows the algorithm to perform any required cleanup tasks. The returned variant map includes the results evaluated by the algorithm. These may be output layer references, or calculated values such as statistical calculations.

The context argument specifies the context in which the algorithm was run.

Postprocess progress should be reported using the supplied feedback object. Additionally, well-behaved algorithms should periodically check feedback to determine whether the post processing should be canceled and exited early.

postProcessAlgorithm should be used to handle any thread-sensitive cleanup which is required by the algorithm. It will always be called from the same thread that context has thread affinity with. While this will generally be the main thread, it is not guaranteed. For instance, algorithms which are run as a step in a larger model or as a subcomponent of a script-based algorithm will call postProcessAlgorithm from the same thread as that model/script it being executed in.

postProcessAlgorithm will not be called if the prepareAlgorithm() step failed (returned False), or if an exception was raised by the processAlgorithm() step.

Returns

A map of algorithm outputs. These may be output layer references, or calculated values such as statistical calculations. Implementations which return a non-empty map will override any results returned by processAlgorithm().

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

Prepares the algorithm to run using the specified parameters. Algorithms should implement their logic for evaluating parameter values here. The evaluated parameter results should be stored in member variables ready for a call to processAlgorithm().

The context argument specifies the context in which the algorithm is being run.

prepareAlgorithm should be used to handle any thread-sensitive preparation which is required by the algorithm. It will always be called from the same thread that context has thread affinity with. While this will generally be the main thread, it is not guaranteed. For instance, algorithms which are run as a step in a larger model or as a subcomponent of a script-based algorithm will call prepareAlgorithm from the same thread as that model/script it being executed in.

Note that the processAlgorithm step uses a temporary context with affinity for the thread in which the algorithm is executed, making it safe for processAlgorithm implementations to load sources and sinks without issue. Implementing prepareAlgorithm is only required if special thread safe handling is required by the algorithm.

Algorithm preparation progress should be reported using the supplied feedback object. Additionally, well-behaved algorithms should periodically check feedback to determine whether the algorithm should be canceled and exited early.

If the preparation was successful algorithms must return True. If a False value is returned this indicates that the preparation could not be completed, and the algorithm execution will be canceled.

Returns

True if preparation was successful.

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(self, name: str)

Removes the parameter with matching name from the algorithm, and deletes any existing definition.

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

sinkProperties(self, sink: str, parameters: Dict[str, Any], context: QgsProcessingContext, sourceProperties: Dict[str, QgsProcessingAlgorithm.VectorProperties])QgsProcessingAlgorithm.VectorProperties
Parameters
  • sink (str) –

  • parameters (Dict[str) –

  • context (QgsProcessingContext) –

  • sourceProperties (Dict[str) –

Return type

QgsProcessingAlgorithm.VectorProperties

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) –