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, 18 insertions, 7 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index 594a19344..a325da929 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -12,7 +12,6 @@ import typing
 from typing import Any
 from typing import Callable
 from typing import cast
-from typing import Dict
 from typing import Iterator
 from typing import List
 from typing import Mapping
@@ -65,13 +64,15 @@ if typing.TYPE_CHECKING:
     from ..pool import Pool
     from ..pool import PoolProxiedConnection
     from ..sql import Executable
+    from ..sql._typing import _InfoType
     from ..sql.base import SchemaVisitor
     from ..sql.compiler import Compiled
-    from ..sql.ddl import DDLElement
+    from ..sql.ddl import ExecutableDDLElement
     from ..sql.ddl import SchemaDropper
     from ..sql.ddl import SchemaGenerator
     from ..sql.functions import FunctionElement
     from ..sql.schema import ColumnDefault
+    from ..sql.schema import DefaultGenerator
     from ..sql.schema import HasSchemaAttr
     from ..sql.schema import SchemaItem
 
@@ -561,7 +562,7 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
         raise exc.ResourceClosedError("This Connection is closed")
 
     @property
-    def info(self) -> Dict[str, Any]:
+    def info(self) -> _InfoType:
         """Info dictionary associated with the underlying DBAPI connection
         referred to by this :class:`_engine.Connection`, allowing user-defined
         data to be associated with the connection.
@@ -1157,7 +1158,17 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
          first row returned.
 
         """
-        return self.execute(statement, parameters, execution_options).scalar()
+        distilled_parameters = _distill_params_20(parameters)
+        try:
+            meth = statement._execute_on_scalar
+        except AttributeError as err:
+            raise exc.ObjectNotExecutableError(statement) from err
+        else:
+            return meth(
+                self,
+                distilled_parameters,
+                execution_options or NO_OPTIONS,
+            )
 
     def scalars(
         self,
@@ -1200,7 +1211,7 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
          * :class:`_expression.TextClause` and
            :class:`_expression.TextualSelect`
          * :class:`_schema.DDL` and objects which inherit from
-           :class:`_schema.DDLElement`
+           :class:`_schema.ExecutableDDLElement`
 
         :param parameters: parameters which will be bound into the statement.
          This may be either a dictionary of parameter names to values,
@@ -1244,7 +1255,7 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
 
     def _execute_default(
         self,
-        default: ColumnDefault,
+        default: DefaultGenerator,
         distilled_parameters: _CoreMultiExecuteParams,
         execution_options: _ExecuteOptionsParameter,
     ) -> Any:
@@ -1303,7 +1314,7 @@ class Connection(ConnectionEventsTarget, inspection.Inspectable["Inspector"]):
 
     def _execute_ddl(
         self,
-        ddl: DDLElement,
+        ddl: ExecutableDDLElement,
         distilled_parameters: _CoreMultiExecuteParams,
         execution_options: _ExecuteOptionsParameter,
     ) -> CursorResult: