Implement new ClauseElement role and coercion system

A major refactoring of all the functions handle all detection of
Core argument types as well as perform coercions into a new class hierarchy
based on "roles", each of which identify a syntactical location within a
SQL statement.  In contrast to the ClauseElement hierarchy that identifies
"what" each object is syntactically, the SQLRole hierarchy identifies
the "where does it go" of each object syntactically.   From this we define
a consistent type checking and coercion system that establishes well
defined behviors.

This is a breakout of the patch that is reorganizing select()
constructs to no longer be in the FromClause hierarchy.

Also includes a rename of as_scalar() into scalar_subquery(); deprecates
automatic coercion to scalar_subquery().

Partially-fixes: #4617
Change-Id: I26f1e78898693c6b99ef7ea2f4e7dfd0e8e1a1bd

1 files changed, 5 insertions, 4 deletions

diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index c5e5fd8a1..9df0c932f 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -17,6 +17,9 @@ from .visitors import ClauseVisitor
 from .. import exc
 from .. import util
 
+coercions = None  # type: types.ModuleType
+elements = None  # type: types.ModuleType
+type_api = None  # type: types.ModuleType
 
 PARSE_AUTOCOMMIT = util.symbol("PARSE_AUTOCOMMIT")
 NO_ARG = util.symbol("NO_ARG")
@@ -589,8 +592,7 @@ class ColumnCollection(util.OrderedProperties):
 
     __hash__ = None
 
-    @util.dependencies("sqlalchemy.sql.elements")
-    def __eq__(self, elements, other):
+    def __eq__(self, other):
         l = []
         for c in getattr(other, "_all_columns", other):
             for local in self._all_columns:
@@ -636,8 +638,7 @@ class ColumnSet(util.ordered_column_set):
     def __add__(self, other):
         return list(self) + list(other)
 
-    @util.dependencies("sqlalchemy.sql.elements")
-    def __eq__(self, elements, other):
+    def __eq__(self, other):
         l = []
         for c in other:
             for local in self: