@@ -2,8 +2,17 @@

 import asyncio

 import concurrent.futures as concurrent

-import distributed

-import ipyparallel

+try:

+    import ipyparallel

+    with_ipyparallel = True

+except ModuleNotFoundError:

+    with_ipyparallel = False

+

+try:

+    import distributed

+    with_distributed = True

+except ModuleNotFoundError:

+    with_distributed = False

 class Runner:

@@ -130,12 +139,12 @@ def ensure_async_executor(executor, ioloop):

         executor = concurrent.ProcessPoolExecutor()

     elif isinstance(executor, concurrent.Executor):

         pass

-    elif isinstance(executor, ipyparallel.Client):

+    elif with_ipyparallel and isinstance(executor, ipyparallel.Client):

         executor = executor.executor()

-    elif isinstance(executor, distributed.Client):

+    elif with_distributed and isinstance(executor, distributed.Client):

         executor = executor.get_executor()

     else:

-        raise TypeError('Only concurrent.futures.Executor, distributed.Client,'

+        raise TypeError('Only a concurrent.futures.Executor, distributed.Client,'

                         ' or ipyparallel.Client can be used.')

     return _AsyncExecutor(executor, ioloop)

@@ -179,14 +188,14 @@ class _AsyncExecutor:

     @property

     def ncores(self):

         ex = self.executor

-        if isinstance(ex, ipyparallel.client.view.ViewExecutor):

+        if with_ipyparallel and isinstance(ex, ipyparallel.client.view.ViewExecutor):

             return len(ex.view)

         elif isinstance(ex, (concurrent.ProcessPoolExecutor,

                              concurrent.ThreadPoolExecutor)):

             return ex._max_workers  # not public API!

         elif isinstance(ex, SequentialExecutor):

             return 1

-        elif isinstance(ex, distributed.cfexecutor.ClientExecutor):

+        elif with_distributed and isinstance(ex, distributed.cfexecutor.ClientExecutor):

             # XXX: check if not sum(n for n in ex._client.ncores().values())

             return len(ex._client.ncores())

         else: