Hello there! I was suprised to find that a tasks instances are reused when executed; that is, the

.run

method is called with the same instance but different input values. In other words, if I change a member variable inside

.run

, this change is available to the next

run

call. Perhaps a minimal example can explain the situation a bit better:

from prefect import task, Task, Flow


@task
def generate_numbers():
    return list(range(10))


class MyTask(Task):

    def __init__(self, *, value=None, **kwargs):
        super().__init__(**kwargs)
        self.value = None

    def run(self, *, number):
        print(f'Hello I am {hex(id(self))} number is {number}')
        if self.value is None:
            self.value = number
        else:
            print(f'What? Who set this value={self.value}?')
        return number + 1


instance = MyTask()

with Flow("My First Flow") as flow:
    n = generate_numbers()
    n1 = instance.map(number=n)

flow.run()

This outputs:

[2019-12-02 11:46:43,162] INFO - prefect.FlowRunner | Beginning Flow run for 'My First Flow'
[2019-12-02 11:46:43,162] INFO - prefect.FlowRunner | Starting flow run.
[2019-12-02 11:46:43,166] INFO - prefect.TaskRunner | Task 'generate_numbers': Starting task run...
[2019-12-02 11:46:43,166] INFO - prefect.TaskRunner | Task 'generate_numbers': finished task run for task with final state: 'Success'
[2019-12-02 11:46:43,167] INFO - prefect.TaskRunner | Task 'MyTask': Starting task run...
[2019-12-02 11:46:43,168] INFO - prefect.TaskRunner | Task 'MyTask[8]': Starting task run...
Hello I am 0x11ddaef50 number is 8
[2019-12-02 11:46:43,169] INFO - prefect.TaskRunner | Task 'MyTask[8]': finished task run for task with final state: 'Success'
[2019-12-02 11:46:43,169] INFO - prefect.TaskRunner | Task 'MyTask[1]': Starting task run...
Hello I am 0x11ddaef50 number is 1
What? Who set this value=8?
...

I assume that this is specific to the local executor. In general, I don’t think this is a problem, but I was wondering if there is any documentation / warning somewhere in the docs that I may have missed ?

Hey @David Ojeda ! While this isn’t explicitly stated anywhere, I think it falls under the category of “don’t rely on statefulness within your flow” as described further here: https://docs.prefect.io/core/tutorials/task-guide.html#best-practices-in-writing-tasks

Ah, yes… That’s what I was looking for!