diff -r a94156400269 Lib/concurrent/futures/_base.py
--- a/Lib/concurrent/futures/_base.py	Wed Apr 20 18:26:40 2016 +0200
+++ b/Lib/concurrent/futures/_base.py	Thu Apr 21 18:59:03 2016 +0800
@@ -3,6 +3,7 @@
 
 __author__ = 'Brian Quinlan (brian@sweetapp.com)'
 
+import abc
 import collections
 import logging
 import threading
@@ -53,7 +54,7 @@
     """The operation exceeded the given deadline."""
     pass
 
-class _Waiter(object):
+class _Waiter:
     """Provides the event that wait() and as_completed() block on."""
     def __init__(self):
         self.event = threading.Event()
@@ -135,7 +136,7 @@
         super().add_cancelled(future)
         self._decrement_pending_calls()
 
-class _AcquireFutures(object):
+class _AcquireFutures:
     """A context manager that does an ordered acquire of Future conditions."""
 
     def __init__(self, futures):
@@ -279,7 +280,7 @@
     done.update(waiter.finished_futures)
     return DoneAndNotDoneFutures(done, set(fs) - done)
 
-class Future(object):
+class Future:
     """Represents the result of an asynchronous computation."""
 
     def __init__(self):
@@ -506,9 +507,10 @@
             self._condition.notify_all()
         self._invoke_callbacks()
 
-class Executor(object):
+class Executor(metaclass=abc.ABCMeta):
     """This is an abstract base class for concrete asynchronous executors."""
 
+    @abc.abstractmethod
     def submit(self, fn, *args, **kwargs):
         """Submits a callable to be executed with the given arguments.
 
@@ -518,7 +520,7 @@
         Returns:
             A Future representing the given call.
         """
-        raise NotImplementedError()
+        raise NotImplementedError
 
     def map(self, fn, *iterables, timeout=None, chunksize=1):
         """Returns an iterator equivalent to map(fn, iter).