How to make a celery task fail from within the task?

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To mark a task as failed without raising an exception, update the task state to FAILURE and then raise an Ignore exception, because returning any value will record the task as successful, an example:

from celery import Celery, states
from celery.exceptions import Ignore

app = Celery('tasks', broker="amqp://guest@localhost//")

@app.task(bind=True)
def run_simulation(self):
    if some_condition:
        # manually update the task state
        self.update_state(
            state = states.FAILURE,
            meta="REASON FOR FAILURE"
        )

        # ignore the task so no other state is recorded
        raise Ignore()

But the best way is to raise an exception from your task, you can create a custom exception to track these failures:

class TaskFailure(Exception):
   pass

And raise this exception from your task:

if some_condition:
    raise TaskFailure('Failure reason')

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