From 03d6747f8ff1bd62c12c6fffef5444f97977f2bb Mon Sep 17 00:00:00 2001
From: Erik De Bonte <erikd@microsoft.com>
Date: Thu, 31 Mar 2022 08:15:58 -0700
Subject: [PATCH] PEP 681: Move descriptor-field support to Rejected Ideas
 (#2477)

Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
---
 pep-0681.rst | 67 ++++++++++++++++------------------------------------
 1 file changed, 21 insertions(+), 46 deletions(-)

diff --git a/pep-0681.rst b/pep-0681.rst
index e9871fb667f..1d3dc0a32fc 100644
--- a/pep-0681.rst
+++ b/pep-0681.rst
@@ -213,7 +213,6 @@ customization of default behaviors:
       eq_default: bool = True,
       order_default: bool = False,
       kw_only_default: bool = False,
-      transform_descriptor_types: bool = False,
       field_descriptors: tuple[type | Callable[..., Any], ...] = (),
   ) -> Callable[[_T], _T]: ...
 
@@ -229,14 +228,6 @@ customization of default behaviors:
   assumed to be True or False if it is omitted by the caller. If not
   specified, ``kw_only_default`` will default to False (the default
   assumption for dataclass).
-* ``transform_descriptor_types`` affects fields annotated with
-  descriptor types that define a ``__set__`` method. If True, the type
-  of each parameter on the synthesized ``__init__`` method
-  corresponding to such a field will be the type of the value
-  parameter to the descriptor's ``__set__`` method. If False, the
-  descriptor type will be used. If not specified,
-  ``transform_descriptor_types`` will default to False (the default
-  behavior of dataclass).
 * ``field_descriptors`` specifies a static list of supported classes
   that describe fields. Some libraries also supply functions to
   allocate instances of field descriptors, and those functions may
@@ -342,37 +333,6 @@ Metaclass example
       id: int
       name: str
 
-``transform_descriptor_types`` example
-``````````````````````````````````````
-
-Because ``transform_descriptor_types`` is set to ``True``, the
-``target`` parameter on the synthesized ``__init__`` method will be of
-type ``float`` (the type of ``__set__``\ 's ``value`` parameter)
-instead of ``Descriptor``.
-
-.. code-block:: python
-
-  @typing.dataclass_transform(transform_descriptor_types=True)
-  def create_model() -> Callable[[Type[_T]], Type[_T]]: ...
-
-  # We anticipate that most descriptor classes used with
-  # transform_descriptor_types will be generic with __set__ functions
-  # whose value parameters are based on the generic's type vars.
-  # However, this is not required.
-  class Descriptor:
-      def __get__(self, instance: object, owner: Any) -> int:
-            ...
-
-      # The setter and getter can have different types (asymmetric).
-      # The setter's value type is used for the __init__ parameter.
-      # The getter's return type is ignored.
-      def __set__(self, instance: object, value: float):
-          ...
-
-  @create_model
-  class CustomerModel:
-      target: Descriptor
-
 
 Field descriptors
 -----------------
@@ -506,7 +466,6 @@ For example:
     "eq_default": True,
     "order_default": False,
     "kw_only_default": False,
-    "transform_descriptor_types": False,
     "field_descriptors": (),
   }
 
@@ -597,11 +556,6 @@ If multiple ``dataclass_transform`` decorators are found, either on a
 single function/class or within a class hierarchy, the resulting
 behavior is undefined. Library authors should avoid these scenarios.
 
-The ``__set__`` method on descriptors is not expected to be
-overloaded. If such overloads are found when
-``transform_descriptor_types`` is ``True``, the resulting behavior is
-undefined.
-
 
 Reference Implementation
 ========================
@@ -711,6 +665,26 @@ We chose not to support this feature, since it is specific to
 SQLAlchemy. Users can manually set ``default=None`` on these fields
 instead.
 
+Descriptor-typed field support
+------------------------------
+
+We considered adding a boolean parameter on ``dataclass_transform``
+to enable better support for fields with descriptor types, which is
+common in SQLAlchemy. When enabled, the type of each parameter on the
+synthesized ``__init__`` method corresponding to a descriptor-typed
+field would be the type of the value parameter to the descriptor's
+``__set__`` method rather than the descriptor type itself. Similarly,
+when setting the field, the ``__set__`` value type would be expected.
+And when getting the value of the field, its type would be expected to
+match the return type of ``__get__``.
+
+This idea was based on the belief that ``dataclass`` did not properly
+support descriptor-typed fields. In fact it does, but type checkers
+(at least mypy and pyright) did not reflect the runtime behavior which
+led to our misunderstanding. For more details, see the
+`Pyright bug <#pyright-descriptor-bug_>`__.
+
+
 ``converter`` field descriptor parameter
 ----------------------------------------
 
@@ -748,6 +722,7 @@ References
 .. _#kw-only-docs: https://docs.python.org/3/library/dataclasses.html#dataclasses.KW_ONLY
 .. _#kw-only-issue: https://bugs.python.org/issue43532
 .. _#class-var: https://docs.python.org/3/library/dataclasses.html#class-variables
+.. _#pyright-descriptor-bug: https://github.com/microsoft/pyright/issues/3245
 
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