qgis-processing: how to specify optional outputs?

Question

Suppose a script has a variable number of outputs, which may or may not be produced depending on settings:

##Input_layer=vector
##Output_1=output vector
##Produce_output_2=boolean
##Output_2=output vector

from qgis.core import *

def create_my_output(path):
    # whatever...

create_my_output(Output_1)

if Produce_output_2:
    create_my_output(Output_2)

If this script is run with "Produce output 2" unchecked then QGIS displays a warning dialog saying Output 2 was not correctly produced, even though not producing Output 2 was clearly intended behaviour.

Is there a way to tell QGIS that an output is optional?

Answer 1

First alternative:

You could uncheck the Open output file after running algorithm option if you uncheck Produce output 2.

As you mentioned in your comment, QGIS cannot load this output if it doesn't get processed by the algorithm, therefore disabling the option to load it should work without an error message showing up.

---

Second alternative:

Use output file instead of output vector for "Output_2". QGIS will not load this immediately if processed, which means you can add runandload into the algorithm. And if it's not processed, QGIS will not make a fuss. I made a simple buffer script using your definitions:

##Input_layer=vector
##Output_1=output vector
##Output_2=output file
##Produce_output_2=boolean

processing.runalg("qgis:fixeddistancebuffer", Input_layer, 0.01, 10, False, Output_1)

if Produce_output_2 == True:
    processing.runandload("qgis:fixeddistancebuffer", Input_layer, 1.00, 10, False, Output_2)
else:
    pass

Hope this helps somewhat!