[QGIS-Developer] How to create a simple processing script in QGIS 3.x, eg just a buffer

[Anita Graser]

Great, thanks for sharing, Andrew!

Anita

On Fri, Aug 10, 2018 at 2:05 PM Andrew C <andrew.cutts at gmail.com> wrote:

> Hi Anita,
>
> I worked it out. Here is a very simple replication of a buffer for
> scripting, in case anyone else needs it. I don't know if this is best
> practice but it works for a simple test. I didn't realise I could parse
> parameters['INPUT'] as an input.
>
> Thanks for your help
>
> Kind Regards
>
> Andrew
>
> import processing
> from qgis.PyQt.QtCore import QCoreApplication
> from qgis.core import QgsProcessingAlgorithm, QgsProcessing,
> QgsProcessingParameterFeatureSink,QgsProcessingParameterFeatureSource
>
> class testAlg(QgsProcessingAlgorithm):
>     OUTPUT = 'OUTPUT'
>     INPUT = 'INPUT'
>
>     def tr(self, text):
>         return QCoreApplication.translate('testalg', text)
>
>     def createInstance(self):
>         return type(self)()
>
>     def group(self):
>         return self.tr('Test')
>
>     def groupId(self):
>         return 'test'
>
>     def __init__(self):
>         super().__init__()
>
>     def initAlgorithm(self, config=None):
>
>         self.addParameter(
>             QgsProcessingParameterFeatureSource(
>                 self.INPUT,
>                 self.tr('Input layer'),
>                 [QgsProcessing.TypeVectorAnyGeometry]
>             )
>         )
>
>         self.addParameter(
>             QgsProcessingParameterFeatureSink(
>                 self.OUTPUT,
>                 self.tr('Output'),
>                 QgsProcessing.TypeVectorPolygon
>             )
>         )
>
>     def name(self):
>         return 'testalg'
>
>     def displayName(self):
>         return self.tr('Test Algorithm')
>
>     def processAlgorithm(self, parameters, context, feedback):
>
>         output = processing.run("native:buffer", {
>             'INPUT': parameters['INPUT'],
>             'DISTANCE': 10,
>             'SEGMENTS': 5,
>             'END_CAP_STYLE': 0,
>             'JOIN_STYLE': 0,
>             'MITER_LIMIT': 2,
>             'DISSOLVE': False,
>             'OUTPUT': parameters['OUTPUT']
>         }, context=context, feedback=feedback)['OUTPUT']
>
>         return {self.OUTPUT: output}
>
> On Thu, Aug 9, 2018 at 5:29 PM Andrew C <andrew.cutts at gmail.com> wrote:
>
>> Hi Anita,
>>
>> Thanks very much for this. I am still lost though. If I replace the sink
>> and add your code I don't get an output in the dialog box - it needs to be
>> a parameter for that to happen right?
>>
>> I found this
>> https://gis.stackexchange.com/questions/282773/writing-a-python-processing-script-with-qgis-3-0
>> which lead me to this
>> https://github.com/qgis/QGIS/blob/master/doc/porting_processing.dox
>>
>> Which is recommending "Best practice in 3.x Processing algorithms is to
>> use "feature sinks" instead of vector layer outputs"
>>
>>
>> Kind Regards
>> Andrew
>>
>>
>>
>> On Thu, Aug 9, 2018 at 2:23 PM Anita Graser <anitagraser at gmx.at> wrote:
>>
>>> Hi Andrew,
>>>
>>> On Thu, Aug 9, 2018 at 2:57 PM Andrew C <andrew.cutts at gmail.com> wrote:
>>>
>>>> I am not clear on how QgsFeatureSinkis working, perhaps that is why I
>>>> am not getting the buffered layer? Normally in a script I could try
>>>> printing variables I cannot see where these are printed either when I
>>>> tested.
>>>>
>>>
>>> I think you'd need an output layer instead of a feature sink. Have a
>>> look at
>>> https://github.com/qgis/QGIS/blob/master/python/plugins/processing/algs/qgis/PostGISExecuteAndLoadSQL.py
>>>
>>> self.addOutput(QgsProcessingOutputVectorLayer(
>>>
>>> self.OUTPUT,
>>>
>>> self.tr("Output layer"),
>>>
>>> QgsProcessing.TypeVectorAnyGeometry))
>>>
>>> Regards,
>>> Anita
>>>
>>>
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