Check for supports_execution at ClauseElement base

Raise a more descriptive exception / message when ClauseElement
or non-SQLAlchemy objects that are not "executable" are erroneously
passed to ``.execute()``; a new exception ObjectNotExecutableError
is raised consistently in all cases.

Change-Id: I2dd393121e2c7e5b6b9e40286a2f25670876e8e4
Fixes: #3786

1 files changed, 14 insertions, 0 deletions

diff --git a/lib/sqlalchemy/exc.py b/lib/sqlalchemy/exc.py
index 272984229..2a7f84de3 100644
--- a/lib/sqlalchemy/exc.py
+++ b/lib/sqlalchemy/exc.py
@@ -26,6 +26,20 @@ class ArgumentError(SQLAlchemyError):
     """
 
 
+class ObjectNotExecutableError(ArgumentError):
+    """Raised when an object is passed to .execute() that can't be
+    executed as SQL.
+
+    .. versionadded:: 1.1
+
+    """
+
+    def __init__(self, target):
+        super(ObjectNotExecutableError, self).__init__(
+            "Not an executable object: %r" % target
+        )
+
+
 class NoSuchModuleError(ArgumentError):
     """Raised when a dynamically-loaded module (usually a database dialect)
     of a particular name cannot be located."""