Deprecate empty or_() and and_()

Creating an :func:`.and_` or :func:`.or_` construct with no arguments or
empty ``*args`` will now emit a deprecation warning, as the SQL produced is
a no-op (i.e. it renders as a blank string). This behavior is considered to
be non-intuitive, so for empty or possibly empty :func:`.and_` or
:func:`.or_` constructs, an appropriate default boolean should be included,
such as ``and_(True, *args)`` or ``or_(False, *args)``.   As has been the
case for many major versions of SQLAlchemy, these particular boolean
values will not render if the ``*args`` portion is non-empty.

As there are some internal cases where an empty and_() construct is used
in order to build an optional WHERE expression, a private
utility function is added to suit this use case.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #5054
Closes: #5062
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5062
Pull-request-sha: 5ca2f27281977d74e390148c0fb8deaa0e0e4ad9

Change-Id: I599b9c8befa64d9a59a35ad7dd84ff400e3aa647

1 files changed, 9 insertions, 6 deletions

diff --git a/test/sql/test_delete.py b/test/sql/test_delete.py
index 1f4c49c56..5dcc0d112 100644
--- a/test/sql/test_delete.py
+++ b/test/sql/test_delete.py
@@ -15,6 +15,7 @@ from sqlalchemy.engine import default
 from sqlalchemy.testing import assert_raises_message
 from sqlalchemy.testing import AssertsCompiledSQL
 from sqlalchemy.testing import eq_
+from sqlalchemy.testing import expect_deprecated
 from sqlalchemy.testing import fixtures
 from sqlalchemy.testing.schema import Column
 from sqlalchemy.testing.schema import Table
@@ -77,12 +78,14 @@ class DeleteTest(_DeleteTestBase, fixtures.TablesTest, AssertsCompiledSQL):
     def test_where_empty(self):
         table1 = self.tables.mytable
 
-        self.assert_compile(
-            table1.delete().where(and_()), "DELETE FROM mytable"
-        )
-        self.assert_compile(
-            table1.delete().where(or_()), "DELETE FROM mytable"
-        )
+        with expect_deprecated():
+            self.assert_compile(
+                table1.delete().where(and_()), "DELETE FROM mytable"
+            )
+        with expect_deprecated():
+            self.assert_compile(
+                table1.delete().where(or_()), "DELETE FROM mytable"
+            )
 
     def test_prefix_with(self):
         table1 = self.tables.mytable