pep484 ORM / SQL result support

after some experimentation it seems mypy is more amenable
to the generic types being fully integrated rather than
having separate spin-off types.   so key structures
like Result, Row, Select become generic.  For DML
Insert, Update, Delete, these are spun into type-specific
subclasses ReturningInsert, ReturningUpdate, ReturningDelete,
which is fine since the "row-ness" of these constructs
doesn't happen until returning() is called in any case.

a Tuple based model is then integrated so that these
objects can carry along information about their return
types.  Overloads at the .execute() level carry through
the Tuple from the invoked object to the result.

To suit the issue of AliasedClass generating attributes
that are dynamic, experimented with a custom subclass
AsAliased, but then just settled on having aliased()
lie to the type checker and return `Type[_O]`, essentially.
will need some type-related accessors for with_polymorphic()
also.

Additionally, identified an issue in Update when used
"mysql style" against a join(), it basically doesn't work
if asked to UPDATE two tables on the same column name.
added an error message to the specific condition where
it happens with a very non-specific error message that we
hit a thing we can't do right now, suggest multi-table
update as a possible cause.

Change-Id: I5eff7eefe1d6166ee74160b2785c5e6a81fa8b95

1 files changed, 18 insertions, 6 deletions

diff --git a/lib/sqlalchemy/engine/cursor.py b/lib/sqlalchemy/engine/cursor.py
index ccf573675..ff69666b7 100644
--- a/lib/sqlalchemy/engine/cursor.py
+++ b/lib/sqlalchemy/engine/cursor.py
@@ -24,6 +24,7 @@ from typing import Optional
 from typing import Sequence
 from typing import Tuple
 from typing import TYPE_CHECKING
+from typing import TypeVar
 from typing import Union
 
 from .result import MergedResult
@@ -55,11 +56,12 @@ if typing.TYPE_CHECKING:
     from .interfaces import ExecutionContext
     from .result import _KeyIndexType
     from .result import _KeyMapRecType
-    from .result import _KeyMapType
     from .result import _KeyType
     from .result import _ProcessorsType
     from ..sql.type_api import _ResultProcessorType
 
+_T = TypeVar("_T", bound=Any)
+
 # metadata entry tuple indexes.
 # using raw tuple is faster than namedtuple.
 MD_INDEX: Literal[0] = 0  # integer index in cursor.description
@@ -214,7 +216,9 @@ class CursorResultMetaData(ResultMetaData):
         return md
 
     def __init__(
-        self, parent: CursorResult, cursor_description: _DBAPICursorDescription
+        self,
+        parent: CursorResult[Any],
+        cursor_description: _DBAPICursorDescription,
     ):
         context = parent.context
         self._tuplefilter = None
@@ -1158,7 +1162,7 @@ class _NoResultMetaData(ResultMetaData):
 _NO_RESULT_METADATA = _NoResultMetaData()
 
 
-class CursorResult(Result):
+class CursorResult(Result[_T]):
     """A Result that is representing state from a DBAPI cursor.
 
     .. versionchanged:: 1.4  The :class:`.CursorResult``
@@ -1179,6 +1183,15 @@ class CursorResult(Result):
 
     """
 
+    __slots__ = (
+        "context",
+        "dialect",
+        "cursor",
+        "cursor_strategy",
+        "_echo",
+        "connection",
+    )
+
     _metadata: Union[CursorResultMetaData, _NoResultMetaData]
     _no_result_metadata = _NO_RESULT_METADATA
     _soft_closed: bool = False
@@ -1231,7 +1244,6 @@ class CursorResult(Result):
                 make_row = _make_row_2
             else:
                 make_row = _make_row
-
             self._set_memoized_attribute("_row_getter", make_row)
 
         else:
@@ -1726,12 +1738,12 @@ class CursorResult(Result):
     def _raw_row_iterator(self):
         return self._fetchiter_impl()
 
-    def merge(self, *others: Result) -> MergedResult:
+    def merge(self, *others: Result[Any]) -> MergedResult[Any]:
         merged_result = super().merge(*others)
         setup_rowcounts = not self._metadata.returns_rows
         if setup_rowcounts:
             merged_result.rowcount = sum(
-                cast(CursorResult, result).rowcount
+                cast("CursorResult[Any]", result).rowcount
                 for result in (self,) + others
             )
         return merged_result