Fix inferred type of async_sessionmaker

Improved the typing for :class:`.sessionmaker` and
:class:`.asyncsessionmaker`, so that the default type of their return value
will be :class:`.Session` or :class:`.AsyncSession`, without the need to
type this explicitly. Previously, Mypy would not automaticaly infer these
return types from its generic base.

As part of this change, arguments for :class:`.Session`,
:class:`.AsyncSession`, :class:`.sessionmaker` and
:class:`.asyncsessionmaker` beyond the initial "bind" argument have been
made keyword-only, which includes parameters that have always been
documented as keyword arguments, such as :paramref:`.Session.autoflush`,
:paramref:`.Session.class_`, etc.

Pull request courtesy Sam Bull.

Closes: #8842
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8842
Pull-request-sha: fa6d1a8468e98b40e12f82ed7ddaddc1fde060ac

Change-Id: Iaaabc4572a87489d61617d970f62b9b50db4fac7

1 files changed, 27 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/session.py b/lib/sqlalchemy/orm/session.py
index 849d37ab2..15cd58792 100644
--- a/lib/sqlalchemy/orm/session.py
+++ b/lib/sqlalchemy/orm/session.py
@@ -1362,6 +1362,7 @@ class Session(_SessionClassMethods, EventTarget):
     def __init__(
         self,
         bind: Optional[_SessionBind] = None,
+        *,
         autoflush: bool = True,
         future: Literal[True] = True,
         expire_on_commit: bool = True,
@@ -4621,9 +4622,35 @@ class sessionmaker(_SessionClassMethods, Generic[_S]):
 
     class_: Type[_S]
 
+    @overload
+    def __init__(
+        self,
+        bind: Optional[_SessionBind] = ...,
+        *,
+        class_: Type[_S],
+        autoflush: bool = ...,
+        expire_on_commit: bool = ...,
+        info: Optional[_InfoType] = ...,
+        **kw: Any,
+    ):
+        ...
+
+    @overload
+    def __init__(
+        self: "sessionmaker[Session]",
+        bind: Optional[_SessionBind] = ...,
+        *,
+        autoflush: bool = ...,
+        expire_on_commit: bool = ...,
+        info: Optional[_InfoType] = ...,
+        **kw: Any,
+    ):
+        ...
+
     def __init__(
         self,
         bind: Optional[_SessionBind] = None,
+        *,
         class_: Type[_S] = Session,  # type: ignore
         autoflush: bool = True,
         expire_on_commit: bool = True,