2

I have difficulties with the processing package. I try to run a simple QGIS dissolve in PyQGIS 3. When I run the line :

from qgis.core import QgsVectorLayer

import processing
from processing.core.Processing import Processing
Processing.initialize()

layer = QgsVectorLayer(layerPath, 'layer 1', 'ogr')

parameters = {'INPUT': layer, 'OUTPUT': 'memory:'}

dissolved = processing.run('gdal:dissolve', parameters)

In QGIS Python Console, it works fine but when I try it in my script, it sends me the error message in the title.

I run my scripts with a batch that imports all necessary libraries.

The full message is following :

File "C:\Program Files\QGIS 3.2\apps\qgis\python\plugins\processing\tools\general.py", line 96, in run
    return Processing.runAlgorithm(algOrName, parameters, onFinish, feedback, context)
File "C:\Program Files\QGIS 3.2\apps\qgis\python\plugins\processing\core\Processing.py", line 125, in runAlgorithm
    raise QgsProcessingException(msg)
_core.QgsProcessingException: Error: Algorithm gdal:dissolve not found

2 Answers 2

1

To check whether PyQGIS has a certain algorithm one can use the following code:

from qgis import processing

for alg in QgsApplication.processingRegistry().algorithms():
    if 'dissolve' in alg.id(): #here change the 'dissolve' into what you are looking for
        print(alg.id(), " : " , alg.displayName())

The statement above should provide such an output:

gdal:dissolve  :  Dissolve
grass7:v.dissolve  :  v.dissolve
native:dissolve  :  Dissolve

But your problem is not about the algorithm, it is about the deployment of the PyQGIS standalone scripts.

So, you may try this instead:

import sys
from qgis.core import (
     QgsApplication, 
     QgsProcessingFeedback, 
     QgsVectorLayer
)
from qgis.analysis import QgsNativeAlgorithms

# See https://gis.stackexchange.com/a/155852/4972 for details about the prefix 
QgsApplication.setPrefixPath('/usr', True)
qgs = QgsApplication([], False)
qgs.initQgis()

# Append the path where processing plugin can be found
sys.path.append('/docs/dev/qgis/build/output/python/plugins')

import processing
from processing.core.Processing import Processing
Processing.initialize()
QgsApplication.processingRegistry().addProvider(QgsNativeAlgorithms())

layer = QgsVectorLayer(layerPath, 'layer 1', 'ogr')

parameters = {'INPUT': layer, 'OUTPUT': 'memory:'}
dissolved = processing.run('gdal:dissolve', parameters)

Another thing is that you are lacking parameters for "gdal:dissolve", because there should be : INPUT, FIELD, GEOMETRY, EXPLODE_COLLECTIONS, KEEP_ATTRIBUTES, COUNT_FEATURES, COMPUTE_AREA, COMPUTE_STATISTICS, STATISTICS_ATTRIBUTE, OPTIONS, OUTPUT.

Check them with this statement processing.algorithmHelp("gdal:dissolve"):

Dissolve (gdal:dissolve)
----------------
Input parameters
----------------

INPUT: Input layer

    Parameter type: QgsProcessingParameterFeatureSource

    Accepted data types:
        - str: layer ID
        - str: layer name
        - str: layer source
        - QgsProcessingFeatureSourceDefinition
        - QgsProperty
        - QgsVectorLayer

FIELD: Dissolve field

    Parameter type: QgsProcessingParameterField

    Accepted data types:
        - str
        - QgsProperty

GEOMETRY: Geometry column name

    Parameter type: QgsProcessingParameterString

    Accepted data types:
        - str
        - QgsProperty

EXPLODE_COLLECTIONS: Produce one feature for each geometry in any kind of geometry collection in the source file

    Parameter type: QgsProcessingParameterBoolean

    Accepted data types:
        - bool
        - int
        - str
        - QgsProperty

KEEP_ATTRIBUTES: Keep input attributes

    Parameter type: QgsProcessingParameterBoolean

    Accepted data types:
        - bool
        - int
        - str
        - QgsProperty

COUNT_FEATURES: Count dissolved features

    Parameter type: QgsProcessingParameterBoolean

    Accepted data types:
        - bool
        - int
        - str
        - QgsProperty

COMPUTE_AREA: Compute area and perimeter of dissolved features

    Parameter type: QgsProcessingParameterBoolean

    Accepted data types:
        - bool
        - int
        - str
        - QgsProperty

COMPUTE_STATISTICS: Compute min/max/sum/mean for attribute

    Parameter type: QgsProcessingParameterBoolean

    Accepted data types:
        - bool
        - int
        - str
        - QgsProperty

STATISTICS_ATTRIBUTE: Numeric attribute to calculate statistics on

    Parameter type: QgsProcessingParameterField

    Accepted data types:
        - str
        - QgsProperty

OPTIONS: Additional creation options

    Parameter type: QgsProcessingParameterString

    Accepted data types:
        - str
        - QgsProperty

OUTPUT: Dissolved

    Parameter type: QgsProcessingParameterVectorDestination

    Accepted data types:
        - str
        - QgsProperty
        - QgsProcessingOutputLayerDefinition

----------------
Outputs
----------------

OUTPUT:  <QgsProcessingOutputVectorLayer>
    Dissolved

References:

0

Maybe try 'qgis:dissolve' instead of 'gdal:dissolve.

Or check that Processing Toolbox / Settings / Providers / GDAL is activated?

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