Conditional task based on artifact existence?

[cjbland]

Is it possible to add a condition to a task in a DAG that is based on whether or not an artifact from the previous step was produced? The situation I'm running into is our main task finds no output and thus writes no output files. I do not want the follow on tasks to execute if no file was produced. Any thoughts or pointers would be appreciated.

A contrived example that loosely resembles what we are trying to do is:

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
  generateName: dag-conditional-artifacts-
spec:
  entrypoint: main
  templates:
    - name: main
      dag:
        tasks:
          - name: flip-coin
            template: flip-coin
          - name: heads
            depends: flip-coin
            template: heads
            # We only want this task to execute if the output artifact exist
            when: "{{tasks.flip-coin.outputs.artifacts.output}} == exists"
    - name: flip-coin
      script:
        image: python:alpine3.6
        command: [ python ]
        source: |
          import os
          import random
          val = random.randint(0, 1)
          if val == 0:
            fd = open("output.txt", "w")
            fd.write("heads")
            fd.close()
          # We are not writing anything out if val is not 0
        outputs:
          artifacts:
          - name: output
            path: /output.txt
            optional: true
    - name: heads
      script:
        image: python:alpine3.6
        command: [ python ]
        inputs:
          artifacts:
          - name: output
            path: /output.txt
            optional: true
        source: |
          with open("/output.txt", "r") as f:
            print(r.read())

[agilgur5]

I believe this exact syntax should be possible, based on the Conditionals example. Did it not work?

[kuddai]

In the example it is based upon output existence (its value). It is not obvious (there are no example) how to check in when expression whether artificat exists or not

  when: "{{tasks.flip-coin.outputs.artifacts.output}} what goes here? is it a string value of file or is it path to the file"