Prevent __init__ from being called for Alias, subclasses

The :class:`.Alias` class and related subclasses :class:`.CTE`,
:class:`.Lateral` and :class:`.TableSample` have been reworked so that it is
not possible for a user to construct the objects directly.  These constructs
require that the standalone construction function or selectable-bound method
be used to instantiate new objects.

Fixes: #4509
Change-Id: I74ae4786cb3ae625dab33b00bfd6bdc4e1219139

1 files changed, 17 insertions, 0 deletions

diff --git a/test/sql/test_tablesample.py b/test/sql/test_tablesample.py
index d2c57c930..7e0600a66 100644
--- a/test/sql/test_tablesample.py
+++ b/test/sql/test_tablesample.py
@@ -1,12 +1,16 @@
 from sqlalchemy import Column
+from sqlalchemy import column
 from sqlalchemy import Integer
 from sqlalchemy import String
 from sqlalchemy import Table
+from sqlalchemy import table
 from sqlalchemy import tablesample
 from sqlalchemy.engine import default
 from sqlalchemy.sql import func
 from sqlalchemy.sql import select
 from sqlalchemy.sql import text
+from sqlalchemy.sql.selectable import TableSample
+from sqlalchemy.testing import assert_raises_message
 from sqlalchemy.testing import AssertsCompiledSQL
 from sqlalchemy.testing import fixtures
 
@@ -59,3 +63,16 @@ class TableSampleTest(fixtures.TablesTest, AssertsCompiledSQL):
             "SELECT alias.people_id FROM "
             "people AS alias TABLESAMPLE system(1)",
         )
+
+    def test_no_alias_construct(self):
+        a = table("a", column("x"))
+
+        assert_raises_message(
+            NotImplementedError,
+            "The TableSample class is not intended to be constructed "
+            "directly.  "
+            r"Please use the tablesample\(\) standalone",
+            TableSample,
+            a,
+            "foo",
+        )