Hfrhyu

I've just started to use the new (3.6.1) processing script format (see https://github.com/qgis/QGIS-Enhancement-Proposals/issues/134 or https://anitagraser.com/2019/03/02/easy-processing-scripts-comeback-in-qgis-3-6/) and tried to put 2 processing algorithmen in it (centroid and buffer), but without success. The script stopped with a :( QGIS crashed. May be someone can help me to get the right idea to connect various processing steps within the new format. Here is my script:

from qgis.processing import alg
import processing
@alg(name="centroids_and_buffer", label=alg.tr("Centroids and buffer"), group="examplescripts", group_label=alg.tr("Example Scripts"))
@alg.input(type=alg.SOURCE, name="INPUT", label="Input layer")
@alg.input(type=alg.SINK, name="OUTPUT", label="Output layer") 
def testalg(instance, parameters, context, feedback, inputs):
 """
 Description goes here. (Don't delete this! Removing this comment will cause errors.)
 """
 source = instance.parameterAsVectorLayer(parameters, "INPUT", context)
 output = instance.parameterAsOutputLayer(parameters, "OUTPUT", context)

 centroids = processing.run("native:centroids", 'INPUT': source,'ALL_PARTS': True, 'OUTPUT': 'memory:')
 #now the buffer
 alg_params = 'DISSOLVE': False,
 'DISTANCE': 0.001,
 'END_CAP_STYLE': 0,
 'INPUT': centroids['OUTPUT'],
 'JOIN_STYLE': 0,
 'MITER_LIMIT': 2,
 'SEGMENTS': 5,
 'OUTPUT': 'memory:'
 buffer= processing.run("native:buffer", alg_params)
 print(buffer['OUTPUT'])
 #output = buffer['OUTPUT']
 return "OUTPUT": buffer['OUTPUT']

Thanks to the qgis-user@lists.osgeo.org I could solve the problem. There is a brief description how to proceed with the new @alg decorator at: https://docs.qgis.org/testing/en/docs/user_manual/processing/scripts.html#the-alg-decorator. Finally the script looks like this:

from qgis.processing import alg
import processing

@alg(name="centroids_and_buffer", label=alg.tr("Centroids and buffer"), group="examplescripts", group_label=alg.tr("Example Scripts"))
@alg.input(type=alg.SOURCE, name="INPUT", label="Input layer")
@alg.input(type=alg.VECTOR_LAYER_DEST, name='BUFFER_OUTPUT', label='Buffer output')

def centroids_and_buffer(instance, parameters, context, feedback, inputs):
 """
 Description goes here. (Don't delete this! Removing this comment will cause errors.)
 """
 source = instance.parameterAsVectorLayer(parameters, "INPUT", context)
 if feedback.isCanceled():
 return 

 centroids = processing.run("native:centroids", 
 'INPUT': parameters['INPUT'],
 'ALL_PARTS': True, 
 'OUTPUT': 'memory:',
 is_child_algorithm=True,
 context=context,
 feedback=feedback)

 if feedback.isCanceled():
 return 

 alg_params = 'DISSOLVE': False,
 'DISTANCE': 0.01,
 'END_CAP_STYLE': 0,
 'INPUT': centroids['OUTPUT'],
 'JOIN_STYLE': 0,
 'MITER_LIMIT': 2,
 'SEGMENTS': 5,
 'OUTPUT': parameters['BUFFER_OUTPUT']

 buffer = processing.run("native:buffer", alg_params,
 is_child_algorithm=True,
 context=context,
 feedback=feedback)

 if feedback.isCanceled():
 return 

 return 'BUFFER_OUTPUT': buffer['OUTPUT']