Use importutils.try_import for optional eventlet imports

Instead of doing try: except ImportError for eventlet imports
use the oslo.utils importutils.try_import function that safely
does the same with less verbosity for the optional usage of
eventlet imports/code that we have.

Change-Id: I7eaa7c5908ffb04282892c9f6af04044b73f4f8c

taskflow/types/futures.py

@@ -20,15 +20,13 @@ import threading
 from concurrent import futures as _futures
 from concurrent.futures import process as _process
 from concurrent.futures import thread as _thread
+from oslo_utils import importutils
 from oslo_utils import reflection
 
-try:
-    from eventlet.green import threading as greenthreading
-    from eventlet import greenpool
-    from eventlet import patcher as greenpatcher
-    from eventlet import queue as greenqueue
-except ImportError:
-    pass
+greenpatcher = importutils.try_import('eventlet.patcher')
+greenpool = importutils.try_import('eventlet.greenpool')
+greenqueue = importutils.try_import('eventlet.queue')
+greenthreading = importutils.try_import('eventlet.green.threading')
 
 from taskflow.types import timing
 from taskflow.utils import eventlet_utils as eu

taskflow/utils/async_utils.py

@@ -16,11 +16,9 @@
 
 from concurrent import futures as _futures
 from concurrent.futures import _base
+from oslo_utils import importutils
 
-try:
-    from eventlet.green import threading as greenthreading
-except ImportError:
-    pass
+greenthreading = importutils.try_import('eventlet.green.threading')
 
 from taskflow.types import futures
 from taskflow.utils import eventlet_utils as eu

taskflow/utils/eventlet_utils.py

@@ -14,11 +14,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-try:
-    import eventlet as _eventlet  # noqa
-    EVENTLET_AVAILABLE = True
-except ImportError:
-    EVENTLET_AVAILABLE = False
+from oslo_utils import importutils
+
+_eventlet = importutils.try_import('eventlet')
+
+EVENTLET_AVAILABLE = bool(_eventlet)
 
 
 def check_for_eventlet(exc=None):