Conditionals

Conditional execution of steps and tasks is available through the basic when clause, along with more complex expressions and Task functions.

Basic when Clauses

Argo uses govaluate in its when expressions - Hera is compatible with these when expression if you write them in literal strings, however, without prior Argo knowledge, you may not know the chain of keys to access steps and their parameters.

You can instead use Parameters and the special result parameter in an f-string to make your code more readable:

run_script(
    ...,
    when=f'{previous_step.get_parameter("some-parameter")} == "some-value"'
)

is equivalent to

run_script(
    ...,
    when='{{steps.previous-step.outputs.parameter.some-parameter}} == "some-value"'
)

And

run_script(
    ...,
    when=f'{previous_step.result} == "some-value"'
)

is equivalent to

run_script(
    ...,
    when='{{steps.previous-step.outputs.result}} == "some-value"'
)

Complex when Clauses

If you want to use more complex when clauses than simple comparisons, involving Argo expression functions such as filter or map or even sprig functions, you should check out Hera’s expr module documentation.

Improved Task Conditionals

When using DAGs, there are many convenience functions we can use:

• on_success
• on_failure
• on_error
• on_other_result
• when_any_succeeded
• when_all_failed
• on_workflow_status

These functions (except on_workflow_status) are essentially aliases to Task’s next function, with the on parameter populated with the corrected TaskResult, so they can be used to set stricter dependencies than with the rshift (>>) operator.

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