Implement Mypy plugin

Rudimentary and experimental support for Mypy has been added in the form of
a new plugin, which itself depends on new typing stubs for SQLAlchemy. The
plugin allows declarative mappings in their standard form to both be
compatible with Mypy as well as to provide typing support for mapped
classes and instances.

Fixes: #4609
Change-Id: Ia035978c02ad3a5c0e5b3c6c30044dd5a3155170

1 files changed, 80 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index b96b3b61e..2e48695f5 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -334,7 +334,85 @@ def _queryable_attribute_unreduce(key, mapped_class, parententity, entity):
         return getattr(entity, key)
 
 
-class InstrumentedAttribute(QueryableAttribute):
+if util.py3k:
+    from typing import TypeVar, Generic
+
+    _T = TypeVar("_T")
+    _Generic_T = Generic[_T]
+else:
+    _Generic_T = type("_Generic_T", (), {})
+
+
+class Mapped(QueryableAttribute, _Generic_T):
+    """Represent an ORM mapped :term:`descriptor` attribute for typing purposes.
+
+    This class represents the complete descriptor interface for any class
+    attribute that will have been :term:`instrumented` by the ORM
+    :class:`_orm.Mapper` class. When used with typing stubs, it is the final
+    type that would be used by a type checker such as mypy to provide the full
+    behavioral contract for the attribute.
+
+    .. tip::
+
+        The :class:`_orm.Mapped` class represents attributes that are handled
+        directly by the :class:`_orm.Mapper` class. It does not include other
+        Python descriptor classes that are provided as extensions, including
+        :ref:`hybrids_toplevel` and the :ref:`associationproxy_toplevel`.
+        While these systems still make use of ORM-specific superclasses
+        and structures, they are not :term:`instrumented` by the
+        :class:`_orm.Mapper` and instead provide their own functionality
+        when they are accessed on a class.
+
+    When using the :ref:`SQLAlchemy Mypy plugin <mypy_toplevel>`, the
+    :class:`_orm.Mapped` construct is used in typing annotations to indicate to
+    the plugin those attributes that are expected to be mapped; the plugin also
+    applies :class:`_orm.Mapped` as an annotation automatically when it scans
+    through declarative mappings in :ref:`orm_declarative_table` style. For
+    more indirect mapping styles such as
+    :ref:`imperative table <orm_imperative_table_configuration>` it is
+    typically applied explicitly to class level attributes that expect
+    to be mapped based on a given :class:`_schema.Table` configuration.
+
+    :class:`_orm.Mapped` is defined in the
+    `sqlalchemy2-stubs <https://pypi.org/project/sqlalchemy2-stubs>`_ project
+    as a :pep:`484` generic class which may subscribe to any arbitrary Python
+    type, which represents the Python type handled by the attribute::
+
+        class MyMappedClass(Base):
+            __table_ = Table(
+                "some_table", Base.metadata,
+                Column("id", Integer, primary_key=True),
+                Column("data", String(50)),
+                Column("created_at", DateTime)
+            )
+
+            id : Mapped[int]
+            data: Mapped[str]
+            created_at: Mapped[datetime]
+
+    For complete background on how to use :class:`_orm.Mapped` with
+    pep-484 tools like Mypy, see the link below for background on SQLAlchemy's
+    Mypy plugin.
+
+    .. versionadded:: 1.4
+
+    .. seealso::
+
+        :ref:`mypy_toplevel` - complete background on Mypy integration
+
+    """
+
+    def __get__(self, instance, owner):
+        raise NotImplementedError()
+
+    def __set__(self, instance, value):
+        raise NotImplementedError()
+
+    def __delete__(self, instance):
+        raise NotImplementedError()
+
+
+class InstrumentedAttribute(Mapped):
     """Class bound instrumented attribute which adds basic
     :term:`descriptor` methods.
 
@@ -1369,6 +1447,7 @@ class CollectionAttributeImpl(AttributeImpl):
         value,
         initiator=None,
         passive=PASSIVE_OFF,
+        check_old=None,
         pop=False,
         _adapt=True,
     ):