add AsyncResult.timedelta

for computing various comparisons of timestamps in AsyncResults

IPython/parallel/client/asyncresult.py

@@ -267,6 +267,41 b' class AsyncResult(object):'
     # Sugar methods and attributes
     #-------------------------------------
     
+    def timedelta(self, start, end, start_key=min, end_key=max):
+        """compute the difference between two sets of timestamps
+        
+        The default behavior is to use the earliest of the first
+        and the latest of the second list, but this can be changed
+        by passing a different
+        
+        Parameters
+        ----------
+        
+        start : one or more datetime objects (e.g. ar.submitted)
+        end : one or more datetime objects (e.g. ar.received)
+        start_key : callable
+            Function to call on `start` to extract the relevant
+            entry [defalt: min]
+        end_key : callable
+            Function to call on `end` to extract the relevant
+            entry [default: max]
+        
+        Returns
+        -------
+        
+        dt : float
+            The time elapsed (in seconds) between the two selected timestamps.
+        """
+        if not isinstance(start, datetime):
+            # handle single_result AsyncResults, where ar.stamp is single object,
+            # not a list
+            start = start_key(start)
+        if not isinstance(end, datetime):
+            # handle single_result AsyncResults, where ar.stamp is single object,
+            # not a list
+            end = end_key(end)
+        return (end - start).total_seconds()
+        
     @property
     def progress(self):
         """the number of tasks which have been completed at this point.
@@ -313,10 +348,11 b' class AsyncResult(object):'
         Only reliable if Client was spinning/waiting when the task finished, because
         the `received` timestamp is created when a result is pulled off of the zmq queue,
         which happens as a result of `client.spin()`.
+        
+        For similar comparison of other timestamp pairs, check out AsyncResult.timedelta.
+        
         """
-        received = max([ md['received'] for md in self._metadata ])
-        submitted = min([ md['submitted'] for md in self._metadata ])
-        return (received - submitted).total_seconds()
+        return self.timedelta(self.submitted, self.received)
     
     def wait_interactive(self, interval=1., timeout=None):
         """interactive wait, printing progress at regular intervals"""