Add back a 'eventlet_utils' helper utility module

Recreate a very simple eventlet utility module that
has only a few features; one function checks if eventlet is
available and if not raise an exception; and a constant that
can be used by calling code (such as tests or other optional
functionality) to check if eventlet is useable before
proceeding.

Change-Id: I32df0702eeae7c7c78972c9796156dd824b2f123

2 files changed, 38 insertions, 3 deletions

diff --git a/taskflow/utils/async_utils.py b/taskflow/utils/async_utils.py
index 2fa3b5f..71eafa1 100644
--- a/taskflow/utils/async_utils.py
+++ b/taskflow/utils/async_utils.py
@@ -19,11 +19,11 @@ from concurrent.futures import _base
 
 try:
     from eventlet.green import threading as greenthreading
-    EVENTLET_AVAILABLE = True
 except ImportError:
-    EVENTLET_AVAILABLE = False
+    pass
 
 from taskflow.types import futures
+from taskflow.utils import eventlet_utils as eu
 
 
 _DONE_STATES = frozenset([
@@ -94,7 +94,8 @@ def _partition_futures(fs):
 
 
 def _wait_for_any_green(fs, timeout=None):
-    assert EVENTLET_AVAILABLE, 'eventlet is needed to wait on green futures'
+    eu.check_for_eventlet(RuntimeError('Eventlet is needed to wait on'
+                                       ' green futures'))
 
     with _base._AcquireFutures(fs):
         done, not_done = _partition_futures(fs)
diff --git a/taskflow/utils/eventlet_utils.py b/taskflow/utils/eventlet_utils.py
new file mode 100644
index 0000000..68dd355
--- /dev/null
+++ b/taskflow/utils/eventlet_utils.py
@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+
+#    Copyright (C) 2015 Yahoo! Inc. All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    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
+
+
+def check_for_eventlet(exc=None):
+    """Check if eventlet is available and if not raise a runtime error.
+
+    :param exc: exception to raise instead of raising a runtime error
+    :type exc: exception
+    """
+    if not EVENTLET_AVAILABLE:
+        if exc is None:
+            raise RuntimeError('Eventlet is not current available')
+        else:
+            raise exc