pythongh-119180: Add discussion of annotations to the 3.14 What's New

Doc/whatsnew/3.14.rst

@@ -70,6 +70,78 @@ Summary -- Release highlights
 New Features
 ============
 
+.. _whatsnew-314-pep649:
+
+PEP 649: Deferred Evaluation of Annotations
+-------------------------------------------
+
+The :term:`annotations <annotation>` on functions, classes, and modules are no
+longer evaluated eagerly. Instead, annotations are stored in special-purpose
+:term:`annotation functions <annotation function>` and evaluated only when
+necessary. This is specified in :pep:`649` and :pep:`749`.
+
+The new :mod:`annotationlib` module provides tools for inspecting deferred
+annotations. Annotations may be evaluated in the :attr:`~annotationlib.Format.VALUE`
+format (which evaluates annotations to runtime values, similar to the behavior in
+earlier Python versions), the :attr:`~annotationlib.Format.FORWARDREF` format
+(which replaces undefined names with special markers), and the
+:attr:`~annotationlib.Format.SOURCE` format (which returns annotations as strings).
+
+This example shows how these formats behave::
+
+   from annotationlib import get_annotations, Format
+
+   def func(arg: Undefined):
+       pass
+
+   print(get_annotations(func, format=Format.VALUE))  # raises NameError
+   print(get_annotations(func, format=Format.FORWARDREF))  # {'arg': ForwardRef('Undefined')}
+   print(get_annotations(func, format=Format.SOURCE))  # {'arg': 'Undefined'}
+
+Implications if you write annotations in your code
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you write annotations in your code (for example, for use with a static type
+checker), then this change probably does not affect you: you can keep
+writing annotations the same way you did in previous versions of Python.
+
+You will likely be able to remove quoted strings in annotations, which are frequently
+used for forward references. Similarly, if you use ``from __future__ import annotations``
+to avoid having to write strings in annotations, you may well be able to
+remove it. However, if you rely on third-party libraries that read annotations,
+those libraries may need changes to support unquoted annotations.
+
+Implications if you read ``__annotations__``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If your code reads the ``__annotations__`` attribute on objects, you may want
+to make changes in order to support code that relies on deferred evaluation of
+annotations. For example, you may want to use the :func:`annotationlib.get_annotations`
+with the :attr:`~annotationlib.Format.FORWARDREF` format, as the :mod:`dataclasses`
+module now does.
+
+Related changes
+^^^^^^^^^^^^^^^
+
+The changes in Python 3.14 are designed to minimize changes to code that contains
+annotations in source code and to code that reads ``__annotations__``. However,
+there are many changes to the supporting code, and if you rely on undocumented
+details of the annotation behavior or on private functions in the standard library,
+your code may not work in Python 3.14. To safeguard your code against future changes,
+use only the documented functionality of the :mod:`annotationlib` module.
+
+``from __future__ import annotations``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In Python 3.7, :pep:`563` introduced the ``from __future__ import annotations``
+directive, which turns all annotations into strings. This directive is now
+considered deprecated and it is expected to be removed in a future version of Python.
+However, this removal will not happen until after Python 3.13, the last version of
+Python without deferred evaluation of annotations, loses support.
+In Python 3.14, the behavior of code using ``from __future__ import annotations``
+does not change.
+
+
 Improved Error Messages
 -----------------------
 
@@ -109,7 +181,9 @@ Other Language Changes
 New Modules
 ===========
 
-* None yet.
+* :mod:`annotationlib`: For introspecting :term:`annotations <annotation>`.
+  See :pep:`749` for more details.
+  (Contributed by Jelle Zijlstra in :gh:`119180`.)
 
 
 Improved Modules