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

6 files changed, 29 insertions, 10 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index ff855a539..83f0f0c83 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -940,9 +940,7 @@ class Connection(Connectable):
         try:
             meth = object._execute_on_connection
         except AttributeError:
-            raise exc.InvalidRequestError(
-                "Unexecutable object type: %s" %
-                type(object))
+            raise exc.ObjectNotExecutableError(object)
         else:
             return meth(self, multiparams, params)
 
diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 1bb575984..bd6e37d6c 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -554,8 +554,9 @@ class DefaultExecutionContext(interfaces.ExecutionContext):
 
         self.compiled = compiled
 
-        if not compiled.can_execute:
-            raise exc.ArgumentError("Not an executable clause")
+        # this should be caught in the engine before
+        # we get here
+        assert compiled.can_execute
 
         self.execution_options = compiled.statement._execution_options.union(
             connection._execution_options)
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."""
diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 095c84f03..85d5ff6da 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -215,7 +215,10 @@ class Compiled(object):
         pass
 
     def _execute_on_connection(self, connection, multiparams, params):
-        return connection._execute_compiled(self, multiparams, params)
+        if self.can_execute:
+            return connection._execute_compiled(self, multiparams, params)
+        else:
+            raise exc.ObjectNotExecutableError(self.statement)
 
     @property
     def sql_compiler(self):
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index e277b28a4..75d5368d5 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -259,7 +259,10 @@ class ClauseElement(Visitable):
             return self
 
     def _execute_on_connection(self, connection, multiparams, params):
-        return connection._execute_clauseelement(self, multiparams, params)
+        if self.supports_execution:
+            return connection._execute_clauseelement(self, multiparams, params)
+        else:
+            raise exc.ObjectNotExecutableError(self)
 
     def unique_params(self, *optionaldict, **kwargs):
         """Return a copy with :func:`bindparam()` elements replaced.
diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index e364b2e7f..98a96fd56 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -71,9 +71,6 @@ class SchemaItem(SchemaEventTarget, visitors.Visitable):
 
     __visit_name__ = 'schema_item'
 
-    def _execute_on_connection(self, connection, multiparams, params):
-        return connection._execute_default(self, multiparams, params)
-
     def _init_items(self, *args):
         """Initialize the list of child items for this SchemaItem."""
 
@@ -1941,6 +1938,9 @@ class DefaultGenerator(_NotAColumnExpr, SchemaItem):
             bind = _bind_or_error(self)
         return bind._execute_default(self, **kwargs)
 
+    def _execute_on_connection(self, connection, multiparams, params):
+        return connection._execute_default(self, multiparams, params)
+
     @property
     def bind(self):
         """Return the connectable associated with this default."""