Debugging Error: Algorithm gdal:dissolve not found in PyQGIS

Question

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

Answer 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:

• Using QGIS Processing algorithms from PyQGIS standalone scripts (outside of GUI)
• PyQGIS 101: Running Processing tools
• QGIS User Guide | Calling algorithms from the Python console

Answer 2

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

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