more autocommit messaging

Further clarified connection-level logging to indicate the BEGIN, ROLLBACK
and COMMIT log messages do not actually indicate a real transaction when
the AUTOCOMMIT isolation level is in use; messaging has been extended to
include the BEGIN message itself, and the messaging has also been fixed to
accommodate when the :class:`.Engine` level
:paramref:`.create_engine.isolation_level` parameter was used directly.

Fixes: #7853
Change-Id: Iafc78070737ad117f84262e4bde84b81a81e4ea1

2 files changed, 19 insertions, 3 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index 061794bde..6a79cdf02 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -925,9 +925,14 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
         )
 
     def _is_autocommit_isolation(self) -> bool:
+        opt_iso = self._execution_options.get("isolation_level", None)
         return bool(
-            self._execution_options.get("isolation_level", None)
-            == "AUTOCOMMIT"
+            opt_iso == "AUTOCOMMIT"
+            or (
+                opt_iso is None
+                and self.engine.dialect._on_connect_isolation_level
+                == "AUTOCOMMIT"
+            )
         )
 
     def get_transaction(self) -> Optional[RootTransaction]:
@@ -949,7 +954,13 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
 
     def _begin_impl(self, transaction: RootTransaction) -> None:
         if self._echo:
-            self._log_info("BEGIN (implicit)")
+            if self._is_autocommit_isolation():
+                self._log_info(
+                    "BEGIN (implicit; DBAPI should not BEGIN due to "
+                    "autocommit mode)"
+                )
+            else:
+                self._log_info("BEGIN (implicit)")
 
         self.__in_begin = True
 
diff --git a/lib/sqlalchemy/engine/interfaces.py b/lib/sqlalchemy/engine/interfaces.py
index 1b178641e..aa75da614 100644
--- a/lib/sqlalchemy/engine/interfaces.py
+++ b/lib/sqlalchemy/engine/interfaces.py
@@ -677,9 +677,14 @@ class Dialect(EventTarget):
 
     """
 
+    # NOTE: this does not take into effect engine-level isolation level.
+    # not clear if this should be changed, seems like it should
     default_isolation_level: Optional[_IsolationLevel]
     """the isolation that is implicitly present on new connections"""
 
+    # create_engine()  -> isolation_level  currently goes here
+    _on_connect_isolation_level: Optional[_IsolationLevel]
+
     execution_ctx_cls: Type["ExecutionContext"]
     """a :class:`.ExecutionContext` class used to handle statement execution"""