Avoid assignment from None (#1113)

With this commit we reenable the pylint warning `assignment-from-none`
which checks whether a `None` return value from a function is assigned
to a variable. We also audited all such issues in the code and disabled
the warning accordingly.

Relates #838

.pylintrc

@@ -177,8 +177,7 @@ disable=print-statement,
         W0613,
         W0621,
         invalid-docstring-quote,
-        raise-missing-from,
-        assignment-from-none
+        raise-missing-from
 
 
 # Enable the message, report, category or checker with the given id(s). You can

esrally/driver/driver.py

@@ -517,6 +517,8 @@ def joinpoint_reached(self, worker_id, worker_local_timestamp, task_allocations)
             if self.finished():
                 self.telemetry.on_benchmark_stop()
                 self.logger.info("All steps completed.")
+                # Some metrics store implementations return None because no external representation is required.
+                # pylint: disable=assignment-from-none
                 m = self.metrics_store.to_externalizable(clear=True)
                 self.logger.debug("Closing metrics store...")
                 self.metrics_store.close()
@@ -539,6 +541,8 @@ def move_to_next_task(self, workers_curr_step):
             # Assumption: We don't have a lot of clock skew between reaching the join point and sending the next task
             #             (it doesn't matter too much if we're a few ms off).
             waiting_period = 1.0
+        # Some metrics store implementations return None because no external representation is required.
+        # pylint: disable=assignment-from-none
         m = self.metrics_store.to_externalizable(clear=True)
         self.target.on_task_finished(m, waiting_period)
         # Using a perf_counter here is fine also in the distributed case as we subtract it from `master_received_msg_at` making it