Emit 2.0 deprecation warning for sub-transactions

The nesting pattern will be removed in 2.0, so the use of the
MarkerTransaction should emit a 2.0 deprecation warning
unconditionally.

Change-Id: I96aed22c4e5db9b59e9b28a7f2d1283cd99a9cb6

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index 028af9fbb..d36cf30e9 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -489,6 +489,7 @@ class Connection(Connectable):
                 code="8s2b",
             )
         else:
+            assert not self._is_future
             raise exc.PendingRollbackError(
                 "This connection is on an inactive %stransaction.  "
                 "Please rollback() fully before proceeding."
@@ -2190,6 +2191,15 @@ class MarkerTransaction(Transaction):
                 "Please issue a rollback first."
             )
 
+        assert not connection._is_future
+        util.warn_deprecated_20(
+            "Calling .begin() when a transaction is already begun, creating "
+            "a 'sub' transaction, is deprecated "
+            "and will be removed in 2.0.  See the documentation section "
+            "'Migrating from the nesting pattern' for background on how "
+            "to migrate from this pattern."
+        )
+
         self.connection = connection
         if connection._nested_transaction is not None:
             self._transaction = connection._nested_transaction