Include import error message when greenlet is not installed

CaselIT · CaselIT · commit 533f5718904b · 2021-12-09T21:48:29.000+01:00
Fixes: sqlalchemy#7419 Change-Id: I0c604875a80287acff3bab732f67601a5e2db98c
diff --git 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 @@ def _not_implemented():
 
         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
