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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
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JelleZijlstra and AlexWaygood authored Sep 24, 2024
1 parent 33ee31a commit 697e13d
Showing 1 changed file with 30 additions and 23 deletions.
53 changes: 30 additions & 23 deletions Doc/whatsnew/3.14.rst
Original file line number Diff line number Diff line change
Expand Up @@ -87,47 +87,54 @@ 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'}
This example shows how these formats behave:

.. doctest::

>>> from annotationlib import get_annotations, Format
>>> def func(arg: Undefined):
... pass
>>> get_annotations(func, format=Format.VALUE)
Traceback (most recent call last):
...
NameError: name 'Undefined' is not defined
>>> get_annotations(func, format=Format.FORWARDREF)
{'arg': ForwardRef('Undefined')}
>>> 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
If you define 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.
writing annotations the same way you did with 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.
remove that import. However, if you rely on third-party libraries that read annotations,
those libraries may need changes to support unquoted annotations before they
work as expected.

Implications if you read ``__annotations__``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Implications for readers of ``__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`
annotations. For example, you may want to use :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
The changes in Python 3.14 are designed to rework how ``__annotations__``
works at runtime while minimizing breakage 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,
if you rely on undocumented details of the annotation behavior or on private
functions in the standard library, there are many ways in which 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``
Expand All @@ -137,9 +144,9 @@ 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.
Python without deferred evaluation of annotations, reaches its end of life.
In Python 3.14, the behavior of code using ``from __future__ import annotations``
does not change.
is unchanged.


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