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

3 files changed, 18 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/ddl.py b/lib/sqlalchemy/sql/ddl.py
index 3bd831292..f1012292b 100644
--- a/lib/sqlalchemy/sql/ddl.py
+++ b/lib/sqlalchemy/sql/ddl.py
@@ -373,6 +373,10 @@ class _CreateDropBase(DDLElement):
         self.element = element
         self.bind = bind
 
+    @property
+    def stringify_dialect(self):
+        return self.element.create_drop_stringify_dialect
+
     def _create_rule_disable(self, compiler):
         """Allow disable of _create_rule using a callable.
 
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 03476633a..86611baeb 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -199,6 +199,9 @@ class ClauseElement(
     """
 
     supports_execution = False
+
+    stringify_dialect = "default"
+
     _from_objects = []
     bind = None
     description = None
@@ -435,6 +438,7 @@ class ClauseElement(
         return self
 
     @util.preload_module("sqlalchemy.engine.default")
+    @util.preload_module("sqlalchemy.engine.url")
     def compile(self, bind=None, dialect=None, **kw):
         """Compile this SQL expression.
 
@@ -482,14 +486,20 @@ class ClauseElement(
 
         """
 
-        default = util.preloaded.engine_default
         if not dialect:
             if bind:
                 dialect = bind.dialect
             elif self.bind:
                 dialect = self.bind.dialect
             else:
-                dialect = default.StrCompileDialect()
+                if self.stringify_dialect == "default":
+                    default = util.preloaded.engine_default
+                    dialect = default.StrCompileDialect()
+                else:
+                    url = util.preloaded.engine_url
+                    dialect = url.URL.create(
+                        self.stringify_dialect
+                    ).get_dialect()()
 
         return self._compiler(dialect, **kw)
 
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: