initial reorganize for static typing

start applying foundational annotations to key
elements.

two main elements addressed here:

1. removal of public_factory() and replacement with
   explicit functions.  this just works much better with
   typing.

2. typing support for column expressions and operators.
   The biggest part of this involves stubbing out all the
   ColumnOperators methods under ColumnElement in a
   TYPE_CHECKING section.  Took me a while to see this
   method vs. much more complicated things I thought
   I needed.

Also for this version implementing #7519, ColumnElement
types against the Python type and not TypeEngine.  it is
hoped this leads to easier transferrence between ORM/Core
as well as eventual support for result set typing.
Not clear yet how well this approach will work and what
new issues it may introduce.

given the current approach we now get full, rich typing for
scenarios like this:

from sqlalchemy import column, Integer, String, Boolean

c1 = column('a', String)

c2 = column('a', Integer)

expr1 = c2.in_([1, 2, 3])

expr2 = c2 / 5

expr3 = -c2

expr4_a = ~(c2 == 5)

expr4_b = ~column('q', Boolean)

expr5 = c1 + 'x'

expr6 = c2 + 10

Fixes: #7519
Fixes: #6810
Change-Id: I078d9f57955549f6f7868314287175f6c61c44cb

1 files changed, 8 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index dbcbc8a1b..87829d643 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -14,11 +14,13 @@ This is a semi-private module; the main configurational API of the ORM is
 available in :class:`~sqlalchemy.orm.`.
 
 """
-
 from collections import deque
 from functools import reduce
 from itertools import chain
 import sys
+from typing import Generic
+from typing import Type
+from typing import TypeVar
 import weakref
 
 from . import attributes
@@ -58,6 +60,9 @@ from ..util import HasMemoized
 _mapper_registries = weakref.WeakKeyDictionary()
 
 
+_MC = TypeVar("_MC")
+
+
 def _all_registries():
     with _CONFIGURE_MUTEX:
         return set(_mapper_registries)
@@ -88,6 +93,7 @@ class Mapper(
     ORMEntityColumnsClauseRole,
     sql_base.MemoizedHasCacheKey,
     InspectionAttr,
+    Generic[_MC],
 ):
     """Defines an association between a Python class and a database table or
     other relational structure, so that ORM operations against the class may
@@ -115,7 +121,7 @@ class Mapper(
     )
     def __init__(
         self,
-        class_,
+        class_: Type[_MC],
         local_table=None,
         properties=None,
         primary_key=None,