Currently, it is not possible to update the estimator hyperparameters with the hyperparameters passed to TrainingStep if a Placeholder is used as input. The merging of hyperparameters can only be done if the hyperparameters passed to Training step is a dict.

Proposing to add a utility that translates a Placeholder to a usable jsonpath dict (with $). That translated placeholder_dict could be used as hyperparameters input to TrainingStep. With that, merging the constructor and estimator hyperparameters will be possible.

• ex: A Placeholder with schema

{ 'TrainingInput': { 'B': str, 'C': str } } 

will output the following:

{ 'B.$': '$$.Execution.Input.B', 'C.$': '$$.Execution.Input.C' }