"Object of type ... is not JSON serializable" when ListParameters of Task instances

[maxgalli]

Hi, I would like to do feed a class that inherits from luigi.Task with a luigi.ListParameters where the items are instances of classes that inherit from luigi.Task as well, like in the following small reproducer:

import luigi

class SubTask(luigi.Task):
    param = luigi.Parameter()

    def run(self):
        with self.output().open('w') as f:
            f.write(f"SubTask {self.param}\n")

class MainTask(luigi.Task):
    subtasks = luigi.ListParameter()

    def requires(self):
        return self.subtasks

    def run(self):
        with self.output().open('w') as f:
            f.write("MainTask\n")

if __name__ == "__main__":
    subA = SubTask(param='A')
    subB = SubTask(param='B')
    luigi.build([MainTask(subtasks=[subA, subB])], local_scheduler=True)

However, this seems to raise the following error:

...

TypeError: Object of type SubTask is not JSON serializable

The only way I can think of to make this work consists in instantiating subA and subB before defining MainTask, create a dictionary where the instances are values and then simply pass a list of strings when instantiating MainTask, like e.g.:

import luigi

class SubTask(luigi.Task):
    param = luigi.Parameter()

    def run(self):
        with self.output().open('w') as f:
            f.write(f"SubTask {self.param}\n")

subA = SubTask(param='A')
subB = SubTask(param='B')

dct = {
    'A': subA,
    'B': subB
}

class MainTask(luigi.Task):
    subtasks = luigi.ListParameter()

    def requires(self):
        return [dct[i] for i in self.subtasks]

    def run(self):
        with self.output().open('w') as f:
            f.write("MainTask\n")

luigi.build([MainTask(subtasks=['A', 'B'])], local_scheduler=True)

So I have two questions:

• is there a nicest way to do this which does not require to create instances of SubTask before defining MainTask?
• would it be a nice addition to luigi the possibility to run the first snippet without incurring in the above mention error?

Thank you,

Massimiliano