Stages with conditional dependency

Correct me if this already exists, I seem to see some merges from 2018 that may be related (#646 ) but see no examples.

Essentially I have a stage that prepares a model, of which I would like to specify multiple options as parameters. Each model has a potentially unique preprocessing step, BUT some models share an additional preprocessing step.

For example, param model modulates stage predict, which for some models requires no previous stage, but for others requires a stage preprocess. How can I ensure that preprocess is run for the required models but not rerun it because it is expensive. If I have the preprocess step also conditioned on param model, it will rerun the step even if I switch between models where it does not need to be rerun.

Thanks for any wisdom.

The issue does not identify a file, test, or entry point to inspect. Begin by locating DVC's stage parameter and dependency evaluation logic, then determine whether conditional dependencies can run only for selected models without rerunning shared preprocessing; done would include documented behavior and coverage for both dependent and independent models.