Cannot set default values for workflow template arguments

[tzielaski]

Pre-bug-report checklist

1. This bug can be reproduced using pure Argo YAML

• Yes 👉 Please report a bug to the Argo Workflows GitHub 👈
• No

If yes, it is more likely to be an Argo bug unrelated to Hera. Please double check before submitting an issue to Hera.

2. I have searched for existing issues

• Yes

3. This bug occurs in Hera when...

• exporting to YAML
• submitting to Argo
• running on Argo with the Hera runner
• other:

Bug report

Describe the bug
Setting default values for workflow template's arguments doesn't work but is supported by Argo if specified directly in the YAML.

To Reproduce
Full Hera code to reproduce the bug:

from hera.workflows import DAG, script, WorkflowTemplate, Parameter


@script()
def foo(a, b=42, c=None):
    print(a, b, c)


with WorkflowTemplate(
        name="test-default-param",
        entrypoint="d",
        arguments=[
            Parameter(name='a', default='1', description='This should have a default')
        ]
) as w:
    with DAG(
            name="d",
            inputs=[
                Parameter(name='a', default='1')
            ]):
        foo(name="b-unset-c-unset", arguments={"a": "{{ inputs.parameters.a }}"})

generates

apiVersion: argoproj.io/v1alpha1
kind: WorkflowTemplate
metadata:
  name: test-default-param
spec:
  arguments:
    parameters:
    - description: This should have a default
      name: a
  entrypoint: d
  templates:
  - dag:
      tasks:
      - arguments:
          parameters:
          - name: a
            value: '{{ inputs.parameters.a }}'
        name: b-unset-c-unset
        template: foo
    inputs:
      parameters:
      - default: '1'
        name: a
    name: d
  - inputs:
      parameters:
      - name: a
      - default: '42'
        name: b
      - default: 'null'
        name: c
    name: foo
    script:
      command:
      - python
      image: python:3.8
      source: |-
        import os
        import sys
        sys.path.append(os.getcwd())
        import json
        try: a = json.loads(r'''{{inputs.parameters.a}}''')
        except: a = r'''{{inputs.parameters.a}}'''
        try: b = json.loads(r'''{{inputs.parameters.b}}''')
        except: b = r'''{{inputs.parameters.b}}'''
        try: c = json.loads(r'''{{inputs.parameters.c}}''')
        except: c = r'''{{inputs.parameters.c}}'''

        print(a, b, c)

The default value is not present in the UI:

Expected behavior
The generated YAML contains default values:

apiVersion: argoproj.io/v1alpha1
kind: WorkflowTemplate
metadata:
  name: test-default-param
spec:
  arguments:
    parameters:
    - description: This should have a default
      default: '1'
      name: a
  entrypoint: d
  templates:
  - dag:
      tasks:
      - arguments:
          parameters:
          - name: a
            value: '{{ inputs.parameters.a }}'
        name: b-unset-c-unset
        template: foo
    inputs:
      parameters:
      - default: '1'
        name: a
    name: d
  - inputs:
      parameters:
      - name: a
      - default: '42'
        name: b
      - default: 'null'
        name: c
    name: foo
    script:
      command:
      - python
      image: python:3.8
      source: |-
        import os
        import sys
        sys.path.append(os.getcwd())
        import json
        try: a = json.loads(r'''{{inputs.parameters.a}}''')
        except: a = r'''{{inputs.parameters.a}}'''
        try: b = json.loads(r'''{{inputs.parameters.b}}''')
        except: b = r'''{{inputs.parameters.b}}'''
        try: c = json.loads(r'''{{inputs.parameters.c}}''')
        except: c = r'''{{inputs.parameters.c}}'''

        print(a, b, c)

and the default value is set in Argo UI

Environment

• Hera Version: 5.16.2
• Python Version: 3.12.5
• Argo Version: 3.6.0