Include import error message when greenlet is not installed

Fixes: #7419
Change-Id: I0c604875a80287acff3bab732f67601a5e2db98c

diff --git a/lib/sqlalchemy/util/concurrency.py b/lib/sqlalchemy/util/concurrency.py
index 3518f874dc8b05d7f5925569f27258abe15ebbac..2397c11897a24278eaaa09b2b44e2c76c3d57f4e 100644 (file)
--- a/lib/sqlalchemy/util/concurrency.py
+++ b/lib/sqlalchemy/util/concurrency.py
@@ -7,10 +7,11 @@
 
 
 have_greenlet = False
-
+greenlet_error = None
 try:
     import greenlet  # noqa F401
-except ImportError:
+except ImportError as e:
+    greenlet_error = str(e)
     pass
 else:
     have_greenlet = True
@@ -37,6 +38,9 @@ if not have_greenlet:
 
         raise ValueError(
             "the greenlet library is required to use this function."
+            " %s" % greenlet_error
+            if greenlet_error
+            else ""
         )
 
     def is_exit_exception(e):  # noqa F811