Allow dialect-specific stringification

Dialect-specific constructs such as
:meth:`_postgresql.Insert.on_conflict_do_update` can now stringify in-place
without the need to specify an explicit dialect object.  The constructs,
when called upon for ``str()``, ``print()``, etc. now have internal
direction to call upon their appropriate dialect rather than the
"default"dialect which doesn't know how to stringify these.   The approach
is also adapted to generic schema-level create/drop such as
:class:`_schema.AddConstraint`, which will adapt its stringify dialect to
one indicated by the element within it, such as the
:class:`_postgresql.ExcludeConstraint` object.

mostly towards being able to provide doctest-style
examples for "on conflict" constructs using print statements.

Change-Id: I4b855516fe6dee2df77744c1bb21a373d7fbab93

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index ab635cd56..98ea8648a 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -100,6 +100,8 @@ class SchemaItem(SchemaEventTarget, visitors.Visitable):
 
     __visit_name__ = "schema_item"
 
+    create_drop_stringify_dialect = "default"
+
     def _init_items(self, *args, **kw):
         """Initialize the list of child items for this SchemaItem."""
         for item in args: