Make a common approach for "emulated" types

Internal refinements to the :class:`.Enum`, :class:`.Interval`, and
:class:`.Boolean` types, which now extend a common mixin
:class:`.Emulated` that indicates a type that provides Python-side
emulation of a DB native type, switching out to the DB native type when a
supporting backend is in use.   The Postgresql :class:`.INTERVAL` type
when used directly will now include the correct type coercion rules for
SQL expressions that also take effect for :class:`.sqltypes.Interval`
(such as adding a date to an interval yields a datetime).

Change-Id: Ifb9f9d7cbd9f5990dcb2abb583193e9e92b789ad
Fixes: #4088

1 files changed, 15 insertions, 16 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/enumerated.py b/lib/sqlalchemy/dialects/mysql/enumerated.py
index 495bee5a8..e67177b2f 100644
--- a/lib/sqlalchemy/dialects/mysql/enumerated.py
+++ b/lib/sqlalchemy/dialects/mysql/enumerated.py
@@ -9,7 +9,7 @@ import re
 
 from .types import _StringType
 from ... import exc, sql, util
-from ... import types as sqltypes
+from ...sql import sqltypes
 
 
 class _EnumeratedValues(_StringType):
@@ -55,11 +55,13 @@ class _EnumeratedValues(_StringType):
         return strip_values
 
 
-class ENUM(sqltypes.Enum, _EnumeratedValues):
+class ENUM(sqltypes.NativeForEmulated, sqltypes.Enum, _EnumeratedValues):
     """MySQL ENUM type."""
 
     __visit_name__ = 'ENUM'
 
+    native_enum = True
+
     def __init__(self, *enums, **kw):
         """Construct an ENUM.
 
@@ -114,21 +116,21 @@ class ENUM(sqltypes.Enum, _EnumeratedValues):
         """
 
         kw.pop('strict', None)
-        validate_strings = kw.pop("validate_strings", False)
-        sqltypes.Enum.__init__(
-            self, validate_strings=validate_strings, *enums)
-        kw.pop('metadata', None)
-        kw.pop('schema', None)
-        kw.pop('name', None)
-        kw.pop('quote', None)
-        kw.pop('native_enum', None)
-        kw.pop('inherit_schema', None)
-        kw.pop('_create_events', None)
+        self._enum_init(enums, kw)
         _StringType.__init__(self, length=self.length, **kw)
 
+    @classmethod
+    def adapt_emulated_to_native(cls, impl, **kw):
+        """Produce a MySQL native :class:`.mysql.ENUM` from plain
+        :class:`.Enum`.
+
+        """
+        kw.setdefault("validate_strings", impl.validate_strings)
+        return cls(**kw)
+
     def _setup_for_values(self, values, objects, kw):
         values, length = self._init_values(values, kw)
-        return sqltypes.Enum._setup_for_values(self, values, objects, kw)
+        return super(ENUM, self)._setup_for_values(values, objects, kw)
 
     def _object_value_for_elem(self, elem):
         # mysql sends back a blank string for any value that
@@ -144,9 +146,6 @@ class ENUM(sqltypes.Enum, _EnumeratedValues):
         return util.generic_repr(
             self, to_inspect=[ENUM, _StringType, sqltypes.Enum])
 
-    def adapt(self, cls, **kw):
-        return sqltypes.Enum.adapt(self, cls, **kw)
-
 
 class SET(_EnumeratedValues):
     """MySQL SET type."""