Establish deprecation path for CursorResult.keys()

Established a deprecation path for calling upon the
:meth:`_cursor.CursorResult.keys` method for a statement that returns no
rows to provide support for legacy patterns used by the "records" package
as well as any other non-migrated applications. Previously, this would
raise :class:`.ResourceClosedException` unconditionally in the same way as
it does when attempting to fetch rows. While this is the correct behavior
going forward, the :class:`_cursor.LegacyCursorResult` object will now in
this case return an empty list for ``.keys()`` as it did in 1.3, while also
emitting a 2.0 deprecation warning. The :class:`_cursor.CursorResult`, used
when using a 2.0-style "future" engine, will continue to raise as it does
now.

Fixes: #6427
Change-Id: I4148f28c88039e4141deeab28b1a5994e6d6e098

2 files changed, 47 insertions, 14 deletions

diff --git a/test/sql/test_deprecations.py b/test/sql/test_deprecations.py
index 4af1f65e3..db2042f5b 100644
--- a/test/sql/test_deprecations.py
+++ b/test/sql/test_deprecations.py
@@ -1467,11 +1467,11 @@ class ConnectionlessCursorResultTest(fixtures.TablesTest):
             "This result object does not return rows.",
             result.fetchone,
         )
-        assert_raises_message(
-            exc.ResourceClosedError,
-            "This result object does not return rows.",
-            result.keys,
-        )
+
+        with testing.expect_deprecated_20(
+            r"Calling the .keys\(\) method on a result set that does not "
+        ):
+            eq_(result.keys(), [])
 
 
 class CursorResultTest(fixtures.TablesTest):
@@ -1554,6 +1554,22 @@ class CursorResultTest(fixtures.TablesTest):
         ):
             eq_(r._mapping[users.c.user_name], "jack")
 
+    def test_keys_no_rows(self, connection):
+
+        for i in range(2):
+            r = connection.execute(
+                text("update users set user_name='new' where user_id=10")
+            )
+
+            with testing.expect_deprecated(
+                r"Calling the .keys\(\) method on a result set that does not "
+                r"return rows is deprecated and will raise "
+                r"ResourceClosedError in SQLAlchemy 2.0."
+            ):
+                list_ = r.keys()
+                eq_(list_, [])
+                list_.append("Don't cache me")
+
     def test_column_accessor_basic_text(self, connection):
         users = self.tables.users
 
diff --git a/test/sql/test_resultset.py b/test/sql/test_resultset.py
index c0c16512c..2054b3cf1 100644
--- a/test/sql/test_resultset.py
+++ b/test/sql/test_resultset.py
@@ -627,7 +627,12 @@ class CursorResultTest(fixtures.TablesTest):
             lambda: r._mapping["foo"],
         )
 
-    def test_graceful_fetch_on_non_rows(self):
+    @testing.combinations(
+        (True,),
+        (False,),
+        argnames="future",
+    )
+    def test_graceful_fetch_on_non_rows(self, future):
         """test that calling fetchone() etc. on a result that doesn't
         return rows fails gracefully.
 
@@ -642,6 +647,10 @@ class CursorResultTest(fixtures.TablesTest):
         users = self.tables.users
 
         conn = testing.db.connect()
+        if future:
+            conn = conn.execution_options(future_result=True)
+        keys_lambda = lambda r: r.keys()  # noqa: E731
+
         for meth in [
             lambda r: r.fetchone(),
             lambda r: r.fetchall(),
@@ -649,19 +658,27 @@ class CursorResultTest(fixtures.TablesTest):
             lambda r: r.scalar(),
             lambda r: r.fetchmany(),
             lambda r: r._getter("user"),
-            lambda r: r.keys(),
+            keys_lambda,
             lambda r: r.columns("user"),
             lambda r: r.cursor_strategy.fetchone(r, r.cursor),
         ]:
             trans = conn.begin()
             result = conn.execute(users.insert(), dict(user_id=1))
-            assert_raises_message(
-                exc.ResourceClosedError,
-                "This result object does not return rows. "
-                "It has been closed automatically.",
-                meth,
-                result,
-            )
+
+            if not future and meth is keys_lambda:
+                with testing.expect_deprecated(
+                    r"Calling the .keys\(\) method on a result set that does "
+                    r"not return rows is deprecated"
+                ):
+                    eq_(meth(result), [])
+            else:
+                assert_raises_message(
+                    exc.ResourceClosedError,
+                    "This result object does not return rows. "
+                    "It has been closed automatically.",
+                    meth,
+                    result,
+                )
             trans.rollback()
 
     def test_fetchone_til_end(self, connection):