global step_container needs explanation

The Step class has this strange pattern which I can not make sense of:

some classmethods like

@classmethod
def get_exec_graph(cls):
    """return the execution graph of the last step append
    """
    return dask.delayed(cls.task_launcher)(step_container[-1].step_tasks)

use the global variable step_container which is declared as a singleton list without explanation:

step_container = singleton_list()

The problem is that in some way that I did not find, a list gets to be passed to the task_launcher method of Step class. For example, if I print some info in the task_launcher like this

def task_launcher(self,
                    *dependencies,
                    task_name: Optional[str] = None,
                    log_err: Optional[str] = None,
                    log_out: Optional[str] = None,
                    scheduler_type: str = "debug",
                    pbs_worker_name: str = "i2-worker",
                    # [...]
                    **kwargs) -> str:
    """this function encapsulate tasks to be delayed
    [...]
    """
    print(">>>>>")
    print("task_name", end=": ")
    print(task_name)
    print("self", end=": ")
    print(self)
    print("type(self)", end=": ")
    print(type(self))

    if task_name and self.older_i2_state:
        if task_name in self.older_i2_state and self.older_i2_state[
                task_name] == "done":
            return "done"
    dependencies_complete = all([dep == "done" for dep in dependencies])
    if task_name and dependencies and not dependencies_complete:
        task_status = "unlaunchable"
        self.save_task_status(task_name, task_status)
        return task_status
    # [...]

I get

>>>>>
task_name: check_inputs
self: CheckInputsClassifWorkflow
type(self): <class 'iota2.steps.check_inputs_step.CheckInputsClassifWorkflow'>
>>>>>
task_name: preprocessing_T31TCJ
self: SensorsPreprocess
type(self): <class 'iota2.steps.sensors_preprocess.SensorsPreprocess'>
>>>>>
task_name: None
self: ['done']
type(self): <class 'list'>
>>>>>
task_name: common_mask_T31TCJ
self: CommonMasks
type(self): <class 'iota2.steps.common_masks.CommonMasks'>

(note how task_laucher get called on the ['done'] list)

I think this is related to a bug I'm getting on my new regression builder, but all of this is rather obscure to me and I'm failing to debug myself in this darkness.

Could someone enlighten me?