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, 4 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/dml.py b/lib/sqlalchemy/dialects/postgresql/dml.py
index 70d26a94b..50fd09528 100644
--- a/lib/sqlalchemy/dialects/postgresql/dml.py
+++ b/lib/sqlalchemy/dialects/postgresql/dml.py
@@ -30,6 +30,8 @@ class Insert(StandardInsert):
 
     """
 
+    stringify_dialect = "postgresql"
+
     @util.memoized_property
     def excluded(self):
         """Provide the ``excluded`` namespace for an ON CONFLICT statement
@@ -149,6 +151,8 @@ insert = public_factory(
 
 
 class OnConflictClause(ClauseElement):
+    stringify_dialect = "postgresql"
+
     def __init__(self, constraint=None, index_elements=None, index_where=None):
 
         if constraint is not None: