[jobs] Add _str_ method

This code defines the `_str_` method for the JobResult class in
order to improve the log messages generated by the redis class
rq.worker.

arthur/jobs.py

@@ -92,6 +92,10 @@ def __init__(self, job_id, task_id, backend, category, last_uuid,
         self.offset = offset
         self.nresumed = nresumed
 
+    def __str__(self):
+        return "job id: %s, task id: %s, backend: %s, category: %s, nitems: %s" % \
+               (self.job_id, self.task_id, self.backend, self.category, self.nitems)
+
 
 class PercevalJob:
     """Class for wrapping Perceval jobs.

tests/test_jobs.py

@@ -254,6 +254,8 @@ def test_job_result_init(self):
 
         self.assertEqual(result.offset, 128)
         self.assertEqual(result.nresumed, 10)
+        self.assertEqual(str(result), 'job id: arthur-job-1234567890, task id: mytask, '
+                                      'backend: mock_backend, category: category, nitems: 58')
 
 
 class TestPercevalJob(TestBaseRQ):