Is it possible to use an optional variable (value2) like

@qgsfunction(args='auto', group='Custom')
def my_test(value1, value2=True, feature, parent)

in a custom function? Using QGIS 3.8

...likeenter image description here

2 Answers 2


Not sure of a direct way but there is the handlesnull flag which when set to true would let you input NULL as a parameter value.

@qgsfunction(args='auto', group='Custom', handlesnull=True)
def my_test(value1, value2, feature, parent):

You might want to include some logic in your code to determine what happens when a parameter value is NULL.


I found a workaround to accept optional argument with default value. You need to accept any number of arguments then parse the arguments yourself

To do so, in the decorator set args to '-1' so any number of arguments are accepted, then the first parameter of your function will be a list of all the arguments

@qgsfunction(args='-1', group='Custom')
def my_test(values, feature, parent):
    if len(values) < 1 or len(values) > 2:
        parent.setEvalErrorString("Error: invalid number of arguments in the function my_test")
    value1 = values[0]
    value2 = values[1] if len(values) == 2 else 'Default Value'
    #your own code here

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