I'd like to write a simple algorithm to sum up a numeric column in an input vector layer. I can't connect the FIELD input to the SOURCE layer. I tried to add 'parent' parameter but it is unknown keyword argument, however it is listed in the algfactory.py. Here is my code:

from qgis import processing
from qgis.processing import alg
from qgis.core import QgsProject

@alg(name='AlgSumCol', label='Sum of a column', group='examplescripts',
     group_label='Example scripts')
@alg.input(type=alg.SOURCE, name='INPUT', label='Input vector layer')
@alg.input(type=alg.FIELD, name='FIELD', label='Input field', parent='INPUT')
@alg.output(type=alg.NUMBER, name='SUM', label='Sum')

def AlgSumCol(instance, parameters, context, feedback, inputs):
    """ Sum up a numeric column """
    source = instance.parameterAsSource(parameters, 'INPUT', context)
    field_name = instance.parameterAsString(parameters, 'FIELD', context)
    features = source.getFeatures()
    total = 0
    for feat in features:
        total += feat[field_name]
    return {'SUM': total}

I would like to have the names of numeric attributes of the INPUT layer in the FIELD to select, but I get an error: TypeError: 'parent' is an unknown keyword argument.

If I remove parent keyword argument and change alg.FIELD to arg.STRING, it works but I have to fill the column name manually.

How can I connect the two inputs and limit to numeric attributes? I can do it using the QgsProcessingAlgorithm class, but I would like to use @arg shorter code.

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