PEP 749: Rename SOURCE to STRING, go back on __annotations__ setting (#4000)

Author: JelleZijlstra

peps/pep-0749.rst

@@ -30,9 +30,11 @@ specification:
   that can be run in a "fake globals" environment. Instead, we add a fourth format,
   ``VALUE_WITH_FAKE_GLOBALS``, to allow third-party implementors of annotate functions to
   indicate what formats they support.
-* Setting the ``__annotations__`` attribute directly will not affect the ``__annotate__`` attribute.
+* Deleting the ``__annotations__`` attribute directly will also clear ``__annotate__``.
 * We add functionality to allow evaluating type alias values and type parameter bounds and defaults
   (which were added by :pep:`695` and :pep:`696`) using PEP 649-like semantics.
+* The ``SOURCE`` format is renamed to ``STRING`` to improve clarity and reduce the risk of
+  user confusion.
 
 Motivation
 ==========
@@ -229,13 +231,13 @@ The module will contain the following functionality:
   dictionary. This is intended to be used for evaluating deferred attributes introduced by
   :pep:`695` and :pep:`696`; see below for details. *func* may be ``None``
   for convenience; if ``None`` is passed, the function also returns ``None``.
-* ``annotations_to_source(annotations: dict[str, object]) -> dict[str, str]``: a function that
+* ``annotations_to_string(annotations: dict[str, object]) -> dict[str, str]``: a function that
   converts each value in an annotations dictionary to a string representation.
   This is useful for
   implementing the ``SOURCE`` format in cases where the original source is not available,
   such as in the functional syntax for :py:class:`typing.TypedDict`.
-* ``value_to_source(value: object) -> str``: a function that converts a single value to a
-  string representation. This is used by ``annotations_to_source``.
+* ``value_to_string(value: object) -> str``: a function that converts a single value to a
+  string representation. This is used by ``annotations_to_string``.
   It uses ``repr()`` for most values, but for types it returns the fully qualified name.
   It is also useful as a helper for the ``repr()`` of a number of objects in the
   :py:mod:`typing` and :py:mod:`collections.abc` modules.
@@ -629,29 +631,23 @@ Third-party code that implements ``__annotate__`` functions should raise
 and the function is not prepared to be run in a "fake globals" environment.
 This should be mentioned in the data model documentation for ``__annotate__``.
 
-Effect of setting ``__annotations__``
-=====================================
+Effect of deleting ``__annotations__``
+======================================
 
 :pep:`649` specifies:
 
   Setting ``o.__annotations__`` to a legal value
   automatically sets ``o.__annotate__`` to ``None``.
 
-We would prefer to keep ``__annotate__`` unchanged when ``__annotations__``
-is written to. Conceptually, ``__annotate__`` provides the ground truth
-and ``__annotations__`` is merely a cache, and we shouldn't throw away the
-ground truth if the cache is modified.
-
-The motivation for :pep:`649`'s behavior is to keep the two attributes in sync.
-However, this is impossible in general; if the ``__annotations__`` dictionary
-is modified in place, this will not be reflected in the ``__annotate__`` attribute.
-The overall mental model for this area will be simpler if setting ``__annotations__``
-has no effect on ``__annotate__``.
+However, the PEP does not say what happens if the ``__annotations__`` attribute
+is deleted (using ``del``). It seems most consistent that deleting the attribute
+will also delete ``__annotate__``.
 
 Specification
 -------------
 
-The value of ``__annotate__`` is not changed when ``__annotations__`` is set.
+Deleting the ``__annotations__`` attribute on functions, modules, and classes
+results in setting ``__annotate__`` to None.
 
 Deferred evaluation of PEP 695 and 696 objects
 ==============================================
@@ -730,6 +726,34 @@ can use the ``ForwardRef.evaluate`` method.
 If use cases come up in the future, we could add additional functionality,
 such as a new method that re-evaluates the annotation from scratch.
 
+Renaming ``SOURCE`` to ``STRING``
+=================================
+
+The ``SOURCE`` format is meant for tools that need to show a human-readable
+format that is close to the original source code. However, we cannot retrieve
+the original source in ``__annotate__`` functions, and in some cases, we have
+``__annotate__`` functions in Python code that do not have access to the original
+code. For example, this applies to :py:func:`dataclasses.make_dataclass`
+and the call-based syntax for :py:class:`typing.TypedDict`.
+
+This makes the name ``SOURCE`` a bit of a misnomer. The goal of the format
+should indeed be to recreate the source, but the name is likely to mislead
+users in practice. A more neutral name would emphasize that the format returns
+an annotation dictionary with only strings. We suggest ``STRING``.
+
+Specification
+-------------
+
+The ``SOURCE`` format is renamed to ``STRING``. To reiterate the changes in this
+PEP, the four supported formats are now:
+
+- ``VALUE``: the default format, which evaluates the annotations and returns the
+  resulting values.
+- ``VALUE_WITH_FAKE_GLOBALS``: for internal use; should be handled like ``VALUE``
+  by annotate functions that support execution with fake globals.
+- ``FORWARDREF``: replaces undefined names with ``ForwardRef`` objects.
+- ``STRING``: returns strings, attempts to recreate code close to the original source.
+
 Miscellaneous implementation details
 ====================================