Here is an example. Please note that this is just a coding example and has no physical meaning.
This example will extract the principal stresses S1 and S3, evaluate S1-S3 and scale this by a sqrt(2)/2 factor.

Code is:

def post_started(sender, analysis):# Do not edit this line
    define_dpf_workflow(analysis)

# Uncomment this function to enable retrieving results from the table/chart
def table_retrieve_result(value):# Do not edit this line
    import mech_dpf
    import Ans.DataProcessing as dpf
    wf = dpf.Workflow(this.WorkflowId)
    wf.Connect('contour_selector', value)
    this.Evaluate()

def define_dpf_workflow(analysis):
    import mech_dpf
    import Ans.DataProcessing as dpf
    mech_dpf.setExtAPI(ExtAPI)
    data_source = dpf.DataSources(analysis.ResultFileName)

    timePointsOp =dpf.operators.metadata.time_freq_provider()
    timePointsOp.inputs.data_sources.Connect(data_source)
    timepoints = dpf.operators.metadata.time_freq_support_get_attribute(time_freq_support=timePointsOp,property_name="time_freqs",)

    prin1 = dpf.operators.result.stress_principal_1()
    prin1.inputs.data_sources.Connect(data_source)
    prin1.inputs.time_scoping.Connect(timepoints)

    prin3 = dpf.operators.result.stress_principal_3()
    prin3.inputs.data_sources.Connect(data_source)
    prin3.inputs.time_scoping.Connect(timepoints)

    minus_fc = dpf.operators.math.minus_fc()
    minus_fc.inputs.field_or_fields_container_A.Connect(prin1.outputs.fields_container)
    minus_fc.inputs.field_or_fields_container_B.Connect(prin3.outputs.fields_container)

    scale = dpf.operators.math.scale_fc()
    scale.inputs.fields_container.Connect(minus_fc)
    scale.inputs.ponderation.Connect(sqrt(2)/2)

    dpf_workflow = dpf.Workflow()
    dpf_workflow.Add(scale)
    dpf_workflow.SetOutputContour(scale)
    dpf_workflow.Record('wf_id', False)
    this.WorkflowId = dpf_workflow.GetRecordedId()