Support lightweight compiler column elements w/ slots

the _CompileLabel class included ``__slots__`` but these
weren't used as the superclasses included slots.

Create a ``__slots__`` superclass for ``ClauseElement``,
creating a new class of compilable SQL elements that don't
include heavier features like caching, annotations and
cloning, which are meant to be used only in an ad-hoc
compiler fashion.   Create new ``CompilerColumnElement``
from that which serves in column-oriented contexts, but
similarly does not include any expression operator support
as it is intended to be used only to generate a string.

Apply this to both
``_CompileLabel`` as well as PostgreSQL ``_ColonCast``,
which does not actually subclass ``ColumnElement`` as this
class has memoized attributes that aren't worth changing,
and does not include SQL operator capabilities as these
are not needed for these compiler-only objects.

this allows us to more inexpensively add new ad-hoc
labels / casts etc. at compile time, as we will be seeking
to expand out the typecasts that are needed for PostgreSQL
dialects in a subsequent patch.

Change-Id: I52973ae3295cb6e2eb0d7adc816c678a626643ed

1 files changed, 9 insertions, 0 deletions

diff --git a/test/dialect/postgresql/test_compiler.py b/test/dialect/postgresql/test_compiler.py
index 93513c39d..7e91f0ebb 100644
--- a/test/dialect/postgresql/test_compiler.py
+++ b/test/dialect/postgresql/test_compiler.py
@@ -38,6 +38,7 @@ from sqlalchemy.dialects.postgresql import array_agg as pg_array_agg
 from sqlalchemy.dialects.postgresql import ExcludeConstraint
 from sqlalchemy.dialects.postgresql import insert
 from sqlalchemy.dialects.postgresql import TSRANGE
+from sqlalchemy.dialects.postgresql.base import _ColonCast
 from sqlalchemy.dialects.postgresql.base import PGDialect
 from sqlalchemy.dialects.postgresql.psycopg2 import PGDialect_psycopg2
 from sqlalchemy.orm import aliased
@@ -98,6 +99,14 @@ class CompileTest(fixtures.TestBase, AssertsCompiledSQL):
 
     __dialect__ = postgresql.dialect()
 
+    def test_colon_cast_is_slots(self):
+
+        c1 = _ColonCast(column("q"), String(50))
+
+        assert not hasattr(c1, "__dict__")
+
+        self.assert_compile(c1, "q::VARCHAR(50)")
+
     def test_update_returning(self):
         dialect = postgresql.dialect()
         table1 = table(