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, 11 insertions, 1 deletions

diff --git a/test/sql/test_deprecations.py b/test/sql/test_deprecations.py
index ec7b2ac38..4e88dcdb8 100644
--- a/test/sql/test_deprecations.py
+++ b/test/sql/test_deprecations.py
@@ -1,6 +1,7 @@
 #! coding: utf-8
 
 from sqlalchemy import alias
+from sqlalchemy import and_
 from sqlalchemy import bindparam
 from sqlalchemy import CHAR
 from sqlalchemy import column
@@ -14,6 +15,7 @@ from sqlalchemy import join
 from sqlalchemy import literal_column
 from sqlalchemy import MetaData
 from sqlalchemy import null
+from sqlalchemy import or_
 from sqlalchemy import select
 from sqlalchemy import sql
 from sqlalchemy import String
@@ -42,7 +44,7 @@ from sqlalchemy.testing.schema import Column
 from sqlalchemy.testing.schema import Table
 
 
-class DeprecationWarningsTest(fixtures.TestBase):
+class DeprecationWarningsTest(fixtures.TestBase, AssertsCompiledSQL):
     __backend__ = True
 
     def test_ident_preparer_force(self):
@@ -122,6 +124,14 @@ class DeprecationWarningsTest(fixtures.TestBase):
         ):
             select([column("x")], for_update=True)
 
+    def test_empty_and_or(self):
+        with testing.expect_deprecated(
+            r"Invoking and_\(\) without arguments is deprecated, and "
+            r"will be disallowed in a future release.   For an empty "
+            r"and_\(\) construct, use and_\(True, \*args\)"
+        ):
+            self.assert_compile(or_(and_()), "")
+
 
 class ConvertUnicodeDeprecationTest(fixtures.TestBase):