How to use python output as input for next step in argo workflow?

Based on the last line of the output of a python script (I cannot adapt the output-format) I want to trigger multiple new steps in argo-wf. How can I ignore all output lines except the last one in below example? I cannot adapt thy python-code so I guess I have to include an additional step to exclude all lines except last one.

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
  generateName: loops-param-result-
spec:
  entrypoint: loop-param-result-example
  templates:
  - name: loop-param-result-example
    steps:
    - - name: generate
        template: gen-number-list
    - - name: sleep
        template: sleep-n-sec
        arguments:
          parameters:
          - name: seconds
            value: "{{item}}"
        withParam: "{{steps.generate.outputs.result}}"

  - name: gen-number-list
    script:
      image: python:alpine3.6
      command: [python]
      source: |
        import json
        import sys
        print("abc")
        print("def")
        print("1, 2, 3")
  - name: sleep-n-sec
    inputs:
      parameters:
      - name: seconds
    container:
      image: alpine:latest
      command: [sh, -c]
      args: ["echo sleeping for {{inputs.parameters.seconds}} seconds; sleep {{inputs.parameters.seconds}}; echo done"]

As I am not able to adapt the python-script I cannot use the standard-solution as shown below:

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
  generateName: loops-param-result-
spec:
  entrypoint: loop-param-result-example
  templates:
  - name: loop-param-result-example
    steps:
    - - name: generate
        template: gen-number-list
    - - name: sleep
        template: sleep-n-sec
        arguments:
          parameters:
          - name: seconds
            value: "{{item}}"
        withParam: "{{steps.generate.outputs.result}}"

  - name: gen-number-list
    script:
      image: python:alpine3.6
      command: [python]
      source: |
        import json
        import sys
        json.dump([i for i in range(20, 31)], sys.stdout)
  - name: sleep-n-sec
    inputs:
      parameters:
      - name: seconds
    container:
      image: alpine:latest
      command: [sh, -c]
      args: ["echo sleeping for {{inputs.parameters.seconds}} seconds; sleep {{inputs.parameters.seconds}}; echo done"]

I never got the

command: [ python ]
source |
    import json

to work, however, this works for me:

    command: [ python, -c ]
    args: &script
    - |
        import json