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

2 files changed, 11 insertions, 0 deletions

diff --git a/lib/sqlalchemy/util/__init__.py b/lib/sqlalchemy/util/__init__.py
index e6e4907ab..a8cdc5ef7 100644
--- a/lib/sqlalchemy/util/__init__.py
+++ b/lib/sqlalchemy/util/__init__.py
@@ -140,6 +140,7 @@ from .langhelpers import portable_instancemethod  # noqa
 from .langhelpers import quoted_token_parser  # noqa
 from .langhelpers import safe_reraise  # noqa
 from .langhelpers import set_creation_order  # noqa
+from .langhelpers import string_or_unprintable  # noqa
 from .langhelpers import symbol  # noqa
 from .langhelpers import unbound_method_to_callable  # noqa
 from .langhelpers import warn  # noqa
diff --git a/lib/sqlalchemy/util/langhelpers.py b/lib/sqlalchemy/util/langhelpers.py
index 7d1321e0b..7a7faff60 100644
--- a/lib/sqlalchemy/util/langhelpers.py
+++ b/lib/sqlalchemy/util/langhelpers.py
@@ -79,6 +79,16 @@ class safe_reraise(object):
             compat.reraise(type_, value, traceback)
 
 
+def string_or_unprintable(element):
+    if isinstance(element, compat.string_types):
+        return element
+    else:
+        try:
+            return str(element)
+        except Exception:
+            return "unprintable element %r" % element
+
+
 def clsname_as_plain_name(cls):
     return " ".join(
         n.lower() for n in re.findall(r"([A-Z][a-z]+)", cls.__name__)