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

1 files changed, 16 insertions, 1 deletions

diff --git a/lib/sqlalchemy/engine/cursor.py b/lib/sqlalchemy/engine/cursor.py
index ff9dd0d6a..cf77d0835 100644
--- a/lib/sqlalchemy/engine/cursor.py
+++ b/lib/sqlalchemy/engine/cursor.py
@@ -1201,7 +1201,19 @@ class _NoResultMetaData(ResultMetaData):
         self._we_dont_return_rows()
 
 
+class _LegacyNoResultMetaData(_NoResultMetaData):
+    @property
+    def keys(self):
+        util.warn_deprecated_20(
+            "Calling the .keys() method on a result set that does not return "
+            "rows is deprecated and will raise ResourceClosedError in "
+            "SQLAlchemy 2.0.",
+        )
+        return []
+
+
 _NO_RESULT_METADATA = _NoResultMetaData()
+_LEGACY_NO_RESULT_METADATA = _LegacyNoResultMetaData()
 
 
 class BaseCursorResult(object):
@@ -1259,7 +1271,7 @@ class BaseCursorResult(object):
             self._set_memoized_attribute("_row_getter", make_row)
 
         else:
-            self._metadata = _NO_RESULT_METADATA
+            self._metadata = self._no_result_metadata
 
     def _init_metadata(self, context, cursor_description):
 
@@ -1763,6 +1775,7 @@ class CursorResult(BaseCursorResult, Result):
 
     _cursor_metadata = CursorResultMetaData
     _cursor_strategy_cls = CursorFetchStrategy
+    _no_result_metadata = _NO_RESULT_METADATA
 
     def _fetchiter_impl(self):
         fetchone = self.cursor_strategy.fetchone
@@ -1838,6 +1851,8 @@ class LegacyCursorResult(CursorResult):
     _cursor_metadata = LegacyCursorResultMetaData
     _cursor_strategy_cls = CursorFetchStrategy
 
+    _no_result_metadata = _LEGACY_NO_RESULT_METADATA
+
     def close(self):
         """Close this :class:`_engine.LegacyCursorResult`.