[QGIS-Developer] Vector destination vs feature sink

[Anita Graser]

Thank you Matthias, those future optimization possibilities sound really exciting.


Based on all your responses so far, I think I'd summarize that Processing script developers should always go for the feature sink parameter QgsProcessingParameterFeatureSink. 


Given the info Nyall shared with me, the vector destination parameter is only used in implementations of third party tools (such as Saga/grass/otb) which are limited to outputting "plain old vector files". That's why the QgsProcessingParameterVectorDestination class exists. 


This has been very helpful. I'll go update my materials accordingly. Thank you all.


Regards,
Anita




On Sunday, 23. October 2022 15:45:30 (+02:00), Matthias Kuhn wrote:


Hi Anita,


A QgsFeatureSink is an interface for accepting features. This can be a QgsVectorLayer but does not have to be one. It can also be a spatial index, a vector file writer or others.
I.e. your algorithm specifies that it produces "features" which can be sent to a vector layer, another algorithm or [you-name-it]. It doesn't care if it's a vector layer, which also has other properties like style.
This even leaves room for future optimization of processing pipelines, for example, features could be sent through various algorithms without waiting for the first one to be finished.


I hope that sheds some light
Matthias




On Sun, Oct 23, 2022 at 1:48 PM Anita Graser via QGIS-Developer <qgis-developer at lists.osgeo.org> wrote:

Thank you very much, Thomas. This code comparison approach is very useful indeed. 

If FeatureSink covers all functions of VectorDestination and more, that still leaves me wonder what is the downside of a FeatureSink


"Note: Consider using the more flexible QgsProcessingParameterFeatureSink wherever possible."


i.e. where is it not possible to use a Feature Sink? Only in older versions of QGIS? 


Regards,
Anita


On Saturday, 22. October 2022 23:32:11 (+02:00), Thomas Gratier wrote:


Hi,


Although, I do know the intend to recommend one against the other, doing some code differences helps to sort out things.
They both inherits from same classes but the constructor for QgsProcessingParameterFeatureSink accepts an additionnal arg bool supportsAppend=false


You also find the additional methods in QgsProcessingParameterFeatureSink to manipulate the same supportsAppend.


```
a = QgsProcessingParameterVectorDestination(
  'OUTPUT',
  'Sortie'
)

b = QgsProcessingParameterFeatureSink(
  'OUTPUT',
  'Sortie'
)

methods_a = dir(a)
methods_b = dir(b)
common_methods = set(methods_a).intersection(methods_b)
print(common_methods)

only_in_a = set(methods_a).difference(methods_b)

print(only_in_a)

only_in_b = set(methods_b).difference(methods_a)
print(only_in_b)
```


Regards


Thomas Gratier


Le ven. 21 oct. 2022 à 18:07, Anita Graser via QGIS-Developer <qgis-developer at lists.osgeo.org> a écrit :

Thank you Stefan,


I saw that note but was hoping some developer could shed light on what they mean by "more flexible" and "wherever possible". I guess otherwise it's just trial and error. 


Regards,
Anita


On Wednesday, 19. October 2022 07:44:05 (+02:00), Stefan Giese (WhereGroup) wrote:



Hi Anita,

they are very similar, but the API Documentation says to QgsProcessingParameterVectorDestination: "Note: Consider using the more flexible QgsProcessingParameterFeatureSink wherever possible." (https://api.qgis.org/api/3.22/classQgsProcessingParameterVectorDestination.html#details)

Best regards

Stefan


Am 18.10.2022 um 20:35 schrieb Anita Graser via QGIS-Developer:

Hi.

Does any one of you know the real difference between QgsProcessingParameterVectorDestination versus QgsProcessingParameterFeatureSink? They seem to be used interchangably in the resources I can find.

Thank you.

Anita


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