How to enable dynamic requirements in Luigi?

Question

I have built a pipeline of Tasks in Luigi. Because this pipeline is going to be used in different contexts, it was possible that it would require to include more tasks at the beginning of or the end of the pipeline or even totally different dependencies between the tasks.

That's when I thought: "Hey, why declare the dependencies between the tasks in my config file?", so I added something like this to my config.py:

PIPELINE_DEPENDENCIES = {
     "TaskA": [],
     "TaskB": ["TaskA"],
     "TaskC": ["TaskA"],
     "TaskD": ["TaskB", "TaskC"]
}

I was annoyed by having those stacking up parameters throughout the tasks, so at some point I introduced just one parameter, task_config, that every Task has and where every information or data that's necessary for run() is stored. So I put PIPELINE_DEPENDENCIES right in there.

Finally, I would have every Task I defined inherit from both luigi.Task and a custom Mixin class, that would implement the dynamic requires(), which looks something like this:

class TaskRequirementsFromConfigMixin(object):
    task_config = luigi.DictParameter()

    def requires(self):
        required_tasks = self.task_config["PIPELINE_DEPENDENCIES"]
        requirements = [
            self._get_task_cls_from_str(required_task)(task_config=self.task_config)
            for required_task in required_tasks
        ]
        return requirements

    def _get_task_cls_from_str(self, cls_str):
        ...

Unfortunately, that doesn't work, as running the pipeline gives me the following:

===== Luigi Execution Summary =====

Scheduled 4 tasks of which:
* 4 were left pending, among these:
    * 4 was not granted run permission by the scheduler:
        - 1 TaskA(...)
        - 1 TaskB(...)
        - 1 TaskC(...)
        - 1 TaskD(...)

Did not run any tasks
This progress looks :| because there were tasks that were not granted run permission by the scheduler

===== Luigi Execution Summary =====

and a lot of

DEBUG: Not all parameter values are hashable so instance isn't coming from the cache

Although I am not sure if that's relevant.

So: 1. What's my mistake? Is it fixable? 2. Is there another way to achieve this?