pep484: schema API

implement strict typing for schema.py

this module has lots of public API, lots of old decisions
and very hard to follow construction sequences in many
cases, and is also where we get a lot of new feature requests,
so strict typing should help keep things clean.

among improvements here, fixed the pool .info getters
and also figured out how to get ColumnCollection and
related to be covariant so that we may set them up
as returning Column or ColumnClause without any conflicts.

DDL was affected, noting that superclasses of DDLElement
(_DDLCompiles, added recently) can now be passed into
"ddl_if" callables; reorganized ddl into ExecutableDDLElement
as a new name for DDLElement and _DDLCompiles renamed to
BaseDDLElement.

setting up strict also located an API use case that
is completely broken, which is connection.execute(some_default)
returns a scalar value.   This case has been deprecated
and new paths have been set up so that connection.scalar()
may be used.  This likely wasn't possible in previous
versions because scalar() would assume a CursorResult.

The scalar() change also impacts Session as we have explicit
support (since someone had reported it as a regression)
for session.execute(Sequence()) to work.  They will get the
same deprecation message (which omits the word "Connection",
just uses ".execute()" and ".scalar()") and they can then
use Session.scalar() as well.  Getting this to type
correctly while still supporting ORM use cases required
some refactoring, and I also set up a keyword only delimeter
for Session.execute() and related as execution_options /
bind_arguments should always be keyword only, applied these
changes to AsyncSession as well.

Additionally simpify Table __init__ now that we are Python
3 only, we can have positional plus explicit kwargs finally.
Simplify Column.__init__ as well again taking advantage
of kw only arguments.

Fill in most/all __init__ methods in sqltypes.py as
the constructor for types is most of the API.   should
likely do this for dialect-specific types as well.

Apply _InfoType for all info attributes as should have been
done originally and update descriptor decorators.

Change-Id: I3f9f8ff3f1c8858471ff4545ac83d68c88107527

1 files changed, 47 insertions, 17 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 805758283..20938fd5a 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -77,6 +77,7 @@ from ..util.typing import Literal
 
 if typing.TYPE_CHECKING:
     from ._typing import _ColumnExpressionArgument
+    from ._typing import _InfoType
     from ._typing import _PropagateAttrsType
     from ._typing import _TypeEngineArgument
     from .cache_key import _CacheKeyTraversalType
@@ -85,6 +86,7 @@ if typing.TYPE_CHECKING:
     from .compiler import SQLCompiler
     from .functions import FunctionElement
     from .operators import OperatorType
+    from .schema import _ServerDefaultType
     from .schema import Column
     from .schema import DefaultGenerator
     from .schema import FetchedValue
@@ -444,9 +446,8 @@ class ClauseElement(
         connection: Connection,
         distilled_params: _CoreMultiExecuteParams,
         execution_options: _ExecuteOptions,
-        _force: bool = False,
     ) -> Result:
-        if _force or self.supports_execution:
+        if self.supports_execution:
             if TYPE_CHECKING:
                 assert isinstance(self, Executable)
             return connection._execute_clauseelement(
@@ -455,6 +456,22 @@ class ClauseElement(
         else:
             raise exc.ObjectNotExecutableError(self)
 
+    def _execute_on_scalar(
+        self,
+        connection: Connection,
+        distilled_params: _CoreMultiExecuteParams,
+        execution_options: _ExecuteOptions,
+    ) -> Any:
+        """an additional hook for subclasses to provide a different
+        implementation for connection.scalar() vs. connection.execute().
+
+        .. versionadded:: 2.0
+
+        """
+        return self._execute_on_connection(
+            connection, distilled_params, execution_options
+        ).scalar()
+
     def unique_params(
         self: SelfClauseElement,
         __optionaldict: Optional[Dict[str, Any]] = None,
@@ -1481,6 +1498,7 @@ class ColumnElement(
     def _make_proxy(
         self,
         selectable: FromClause,
+        *,
         name: Optional[str] = None,
         key: Optional[str] = None,
         name_is_truncatable: bool = False,
@@ -4032,12 +4050,14 @@ class NamedColumn(ColumnElement[_T]):
 
     def _make_proxy(
         self,
-        selectable,
-        name=None,
-        name_is_truncatable=False,
-        disallow_is_literal=False,
-        **kw,
-    ):
+        selectable: FromClause,
+        *,
+        name: Optional[str] = None,
+        key: Optional[str] = None,
+        name_is_truncatable: bool = False,
+        disallow_is_literal: bool = False,
+        **kw: Any,
+    ) -> typing_Tuple[str, ColumnClause[_T]]:
         c = ColumnClause(
             coercions.expect(roles.TruncatedLabelRole, name or self.name)
             if name_is_truncatable
@@ -4188,7 +4208,13 @@ class Label(roles.LabeledColumnExprRole[_T], NamedColumn[_T]):
     def _from_objects(self) -> List[FromClause]:
         return self.element._from_objects
 
-    def _make_proxy(self, selectable, name=None, **kw):
+    def _make_proxy(
+        self,
+        selectable: FromClause,
+        *,
+        name: Optional[str] = None,
+        **kw: Any,
+    ) -> typing_Tuple[str, ColumnClause[_T]]:
         name = self.name if not name else name
 
         key, e = self.element._make_proxy(
@@ -4279,7 +4305,7 @@ class ColumnClause(
 
     onupdate: Optional[DefaultGenerator] = None
     default: Optional[DefaultGenerator] = None
-    server_default: Optional[FetchedValue] = None
+    server_default: Optional[_ServerDefaultType] = None
     server_onupdate: Optional[FetchedValue] = None
 
     _is_multiparam_column = False
@@ -4422,12 +4448,14 @@ class ColumnClause(
 
     def _make_proxy(
         self,
-        selectable,
-        name=None,
-        name_is_truncatable=False,
-        disallow_is_literal=False,
-        **kw,
-    ):
+        selectable: FromClause,
+        *,
+        name: Optional[str] = None,
+        key: Optional[str] = None,
+        name_is_truncatable: bool = False,
+        disallow_is_literal: bool = False,
+        **kw: Any,
+    ) -> typing_Tuple[str, ColumnClause[_T]]:
         # the "is_literal" flag normally should never be propagated; a proxied
         # column is always a SQL identifier and never the actual expression
         # being evaluated. however, there is a case where the "is_literal" flag
@@ -4699,7 +4727,9 @@ class AnnotatedColumnElement(Annotated):
         return self._Annotated__element.key
 
     @util.memoized_property
-    def info(self):
+    def info(self) -> _InfoType:
+        if TYPE_CHECKING:
+            assert isinstance(self._Annotated__element, Column)
         return self._Annotated__element.info
 
     @util.memoized_property