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

3 files changed, 6 insertions, 11 deletions

diff --git a/taskflow/examples/hello_world.py b/taskflow/examples/hello_world.py
index 22f6a3b..f8e0bb2 100644
--- a/taskflow/examples/hello_world.py
+++ b/taskflow/examples/hello_world.py
@@ -25,17 +25,12 @@ top_dir = os.path.abspath(os.path.join(os.path.dirname(__file__),
                                        os.pardir))
 sys.path.insert(0, top_dir)
 
-try:
-    import eventlet  # noqa
-    EVENTLET_AVAILABLE = True
-except ImportError:
-    EVENTLET_AVAILABLE = False
-
 from taskflow import engines
 from taskflow.patterns import linear_flow as lf
 from taskflow.patterns import unordered_flow as uf
 from taskflow import task
 from taskflow.types import futures
+from taskflow.utils import eventlet_utils
 
 
 # INTRO: This is the defacto hello world equivalent for taskflow; it shows how
@@ -86,7 +81,7 @@ song.add(PrinterTask("conductor@begin",
                      show_name=False, inject={'output': "*dong*"}))
 
 # Run in parallel using eventlet green threads...
-if EVENTLET_AVAILABLE:
+if eventlet_utils.EVENTLET_AVAILABLE:
     with futures.GreenThreadPoolExecutor() as executor:
         e = engines.load(song, executor=executor, engine='parallel')
         e.run()
diff --git a/taskflow/examples/parallel_table_multiply.py b/taskflow/examples/parallel_table_multiply.py
index 88562a2..f4550c2 100644
--- a/taskflow/examples/parallel_table_multiply.py
+++ b/taskflow/examples/parallel_table_multiply.py
@@ -33,7 +33,7 @@ from taskflow import engines
 from taskflow.patterns import unordered_flow as uf
 from taskflow import task
 from taskflow.types import futures
-from taskflow.utils import async_utils
+from taskflow.utils import eventlet_utils
 
 # INTRO: This example walks through a miniature workflow which does a parallel
 # table modification where each row in the table gets adjusted by a thread, or
@@ -97,7 +97,7 @@ def main():
     f = make_flow(tbl)
 
     # Now run it (using the specified executor)...
-    if async_utils.EVENTLET_AVAILABLE:
+    if eventlet_utils.EVENTLET_AVAILABLE:
         executor = futures.GreenThreadPoolExecutor(max_workers=5)
     else:
         executor = futures.ThreadPoolExecutor(max_workers=5)
diff --git a/taskflow/examples/resume_vm_boot.py b/taskflow/examples/resume_vm_boot.py
index f400d0d..7d34a95 100644
--- a/taskflow/examples/resume_vm_boot.py
+++ b/taskflow/examples/resume_vm_boot.py
@@ -39,7 +39,7 @@ from taskflow.patterns import graph_flow as gf
 from taskflow.patterns import linear_flow as lf
 from taskflow import task
 from taskflow.types import futures
-from taskflow.utils import async_utils
+from taskflow.utils import eventlet_utils
 from taskflow.utils import persistence_utils as p_utils
 
 import example_utils as eu  # noqa
@@ -238,7 +238,7 @@ with eu.get_backend() as backend:
 
     # Set up how we want our engine to run, serial, parallel...
     executor = None
-    if async_utils.EVENTLET_AVAILABLE:
+    if eventlet_utils.EVENTLET_AVAILABLE:
         executor = futures.GreenThreadPoolExecutor(5)
 
     # Create/fetch a logbook that will track the workflows work.