Context manager to avoid running a task twice

avoid_twice.py

#!/usr/bin/env python
import unittest


class Avoid_twice():
    tasks_filename = '/tmp/avoid_twice'

    def __init__(self, task_name):
        self.current_task = task_name

    def __enter__(self):
        with open(self.tasks_filename) as i:
            return self.current_task in i.read()

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None:
            with open(self.tasks_filename, 'a') as o:
                o.write(self.current_task + '\n')


class AvoidTwiceTestCases(unittest.TestCase):

    def test_once(self):

        l = []
        with Avoid_twice('task_name') as done:
            if not done:
                l.append(42)

        with Avoid_twice('task_name') as done:
            if not done:
                l.append(42)

        self.assertEqual(l, [42])


if __name__ == '__main__':
    unittest.main()