2 files changed, 8 insertions, 3 deletions
diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index c60a8383f..5c1a159a6 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -768,7 +768,10 @@ class Connection(Connectable):
         else:
             if self._is_future:
                 raise exc.InvalidRequestError(
-                    "a transaction is already begun for this connection"
+                    "This connection has already initialized a SQLAlchemy "
+                    "Transaction() object via begin() or autobegin; can't "
+                    "call begin() here unless rollback() or commit() "
+                    "is called first."
                 )
             else:
                 return MarkerTransaction(self)
diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 9ec6783b0..b6dae6abc 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -612,8 +612,10 @@ class DefaultDialect(interfaces.Dialect):
             if trans_objs:
                 if connection._is_future:
                     raise exc.InvalidRequestError(
-                        "This connection has already begun a transaction; "
-                        "%s may not be altered until transaction end"
+                        "This connection has already initialized a SQLAlchemy "
+                        "Transaction() object via begin() or autobegin; "
+                        "%s may not be altered unless rollback() or commit() "
+                        "is called first."
                         % (", ".join(name for name, obj in trans_objs))
                     )
                 else: