SPL operator invocation: Issue with setting output assignment for reserved names/properties

[markheger]

Following code raises TypeError: Object of type 'Expression' is not JSON serializable

        topo = Topology()
        b = op.Source(topo, "spl.utility::Beacon",
            'tuple<uint64 seq, rstring category>',
            params = {'iterations':2})
        b.seq = b.output('IterationCount()')
        b.category = b.output('"category"') # b.category causes TypeError: Object of type 'Expression' is not JSON serializable

Applying an output assignment for an attribute named "category" is not possible.

category is a reserved word, since a property with this name exists
https://streamsxtopology.readthedocs.io/en/stable/streamsx.spl.op.html#streamsx.spl.op.Source.category

[ghost]

The solution from the user's perspective would be to allow assignments to attributes like category, params, ... etc. and to assign a category for grouping of processing logic.

Example:

topo = Topology()
b = op.Source(topo, "spl.utility::Beacon",
    'tuple<uint64 seq, rstring category, boolean params>',
    params = {'iterations':2})
# output assignments
b.seq = b.output('IterationCount()')
b.category = b.output('"category"')
b.params = b.output('true')
# category for grouping processing logic:
b.category = 'sources_category'   # can also happen before output assignment to category
# update operator parameters
b.params['period'] = 0.01
print(b.params) # {'iterations': 2, 'period': 0.01}
print(b.category) # 'sources_category'