python · larryhastings · May 16, 2023 · May 5, 2023 · May 5, 2023 · May 8, 2023
@@ -1,13 +1,25 @@
 PEP: 649
 Title: Deferred Evaluation Of Annotations Using Descriptors
 Author: Larry Hastings <larry@hastings.org>
-Status: Draft
+Discussions-To: https://discuss.python.org/t/pep-649-deferred-evaluation-of-annotations-tentatively-accepted/21331/
+Status: Accepted
 Type: Standards Track
 Topic: Typing
 Content-Type: text/x-rst
 Created: 11-Jan-2021
-Post-History: 11-Jan-2021, 11-Apr-2021, 19-Apr-2023
-
+Python-Version: 3.13
+Post-History: `11-Jan-2021 <https://mail.python.org/archives/list/python-dev@python.org/thread/5QMMCRF4HTRRNJV56CGHVI5GRHVBDGQO/>`__,
+              `12-Apr-2021 <https://mail.python.org/archives/list/python-dev@python.org/thread/QSASX6PZ3LIIFIANHQQFS752BJYFUFPY/>`__,
+              `18-Apr-2021 <https://mail.python.org/archives/list/python-dev@python.org/thread/WUZGTGE43T7XV3EUGT6AN2N52OD3U7AE/>`__,
+              `09-Aug-2021 <https://mail.python.org/archives/list/python-dev@python.org/thread/2MEOWHCVDLPABOBLYUGRXVOOOBYOLLU6/>`__,
+              `20-Oct-2021 <https://mail.python.org/archives/list/python-dev@python.org/thread/SZLWVYV2HPLU6AH7DOUD7DWFUGBJGQAY/>`__,
+              `20-Oct-2021 <https://discuss.python.org/t/type-annotations-pep-649-and-pep-563/11363>`__,
+              `17-Nov-2021 <https://mail.python.org/archives/list/python-dev@python.org/thread/VIZEBX5EYMSYIJNDBF6DMUMZOCWHARSO/>`__,
+              `15-Mar-2022 <https://discuss.python.org/t/finding-edge-cases-for-peps-484-563-and-649-type-annotations/14314>`__,
+              `23-Nov-2022 <https://discuss.python.org/t/pep-649-deferred-evaluation-of-annotations-tentatively-accepted/21331>`__,
+              `07-Feb-2023 <https://discuss.python.org/t/two-polls-on-how-to-revise-pep-649/23628>`__,
+              `11-Apr-2023 <https://discuss.python.org/t/a-massive-pep-649-update-with-some-major-course-corrections/25672>`__,
+Resolution: https://discuss.python.org/t/pep-649-deferred-evaluation-of-annotations-tentatively-accepted/21331/43

 ********
 Abstract
@@ -75,6 +87,7 @@ in a specific format.
 Format identifiers are always predefined integer values.
 The formats defined by this PEP are:

+
 * ``inspect.VALUE = 1``

  The default value.
@@ -528,6 +541,31 @@ resulting in this PEP.
 Implementation
 **************

+Observed semantics for annotations expressions
+==============================================
+
+For any object ``o`` that supports annotations,
+provided that all names evaluated in the annotations expressions
+are bound before ``o`` is defined and never subsequently rebound,
+``o.__annotations__`` will produce an identical annotations dict both
+when "stock" semantics are active and when this PEP is active.
+In particular, name resolution will be performed identically in
+both scenarios.
+
+When this PEP is active, the value of ``o.__annotations__``
+won't be calculated until the first time ``o.__annotations__``
+itself is evaluated.  All evaluation of the annotation expressions
+is delayed until this moment, which also means that
+
+* names referenced in the annotations expressions will use their
+  *current* value at this moment, and
+* if evaluating the annotations expressions raises an exception,
+  that exception will be raised at this moment.
+
+Once ``o.__annotations__`` is successfully calculated for the
+first time, this value is cached and will be returned by future
+requests for ``o.__annotations__``.
+
 __annotate__ and __annotations__
 ================================

@@ -556,9 +594,9 @@ code should prefer to interact with it only via the

 The callable stored in ``__annotate__`` must accept a
 single required positional argument called ``format``,
-which will always be an ``int``.  It must either return
-a dict (or subclass of dict) or raise
-``NotImplementedError()``.
+which will always be an ``int`` (or a subclass of ``int``).
+It must either return a dict (or subclass of dict) or
+raise ``NotImplementedError()``.

 Here's a formal definition of ``__annotate__``, as it will
 appear in the "Magic methods" section of the Python
@@ -573,25 +611,25 @@ Language Reference:
    annotations values should be provided.  Must be one of the
    following:

-    ``1`` (exported as ``inspect.VALUE``)
+    ``inspect.VALUE`` (equivalent to the ``int`` constant ``1``)

        Values are the result of evaluating the annotation expressions.

-    ``2`` (exported as ``inspect.SOURCE``)
+    ``inspect.FORWARDREF`` (equivalent to the ``int`` constant ``2``)
+
+        Values are real annotation values (as per ``inspect.VALUE`` format)
+        for defined values, and ``ForwardRef`` proxies for undefined values.
+        Real objects may be exposed to, or contain references to,
+        ``ForwardRef`` proxy objects.
+
+    ``inspect.SOURCE`` (equivalent to the ``int`` constant ``3``)

        Values are the text string of the annotation as it
        appears in the source code.  May only be approximate;
        whitespace may be normalized, and constant values may
        be optimized.  It's possible the exact values of these
        strings could change in future version of Python.

-    ``3`` (exported as ``inspect.FORWARDREF``)
-
-        Values are real annotation values (as per ``inspect.VALUE`` format)
-        for defined values, and ``ForwardRef`` proxies for undefined values.
-        Real objects may be exposed to, or contain references to,
-        ``ForwardRef`` proxy objects.
-
    If an ``__annotate__`` function doesn't support the requested
    format, it must raise ``NotImplementedError()``.
    ``__annotate__`` functions must always support ``1`` (``inspect.VALUE``)
@@ -713,25 +751,12 @@ makes some modifications to the annotations before it returns them.
 This PEP adds a new keyword-only parameter to these two functions,
 ``format``.  ``format`` specifies what format the values in the
 annotations dict should be returned in.
-``format`` accepts the following values, defined as attributes on the
-``inspect`` module::
-
-    VALUE = 1
-    FORWARDREF = 2
-    SOURCE = 3
+The ``format`` parameter on these two functions accepts the same values
+as the ``format`` parameter on the ``__annotate__`` magic method
+defined above; however, these ``format`` parameters also have a default
+value of ``inspect.VALUE``.

-The default value for the ``format`` parameter is ``1``,
-which is ``VALUE`` format.
-
-The defined ``format`` values are guaranteed to be contiguous,
-and the ``inspect`` module also publishes attributes representing
-the minimum and maximum supported ``format`` values::
-
-    FORMAT_MIN = VALUE
-    FORMAT_MAX = SOURCE
-
-
-Also, when either ``__annotations__`` or ``__annotate__`` is updated on an
+When either ``__annotations__`` or ``__annotate__`` is updated on an
 object, the other of those two attributes is now out-of-date and should also
 either be updated or deleted (set to ``None``, in the case of ``__annotate__``
 which cannot be deleted).  In general, the semantics established in the previous
@@ -1017,40 +1042,29 @@ the wrapped callable, which for simplicity must be named
        return ann


-Other modifications to existing objects
-=======================================
+Other modifications to the Python runtime
+=========================================

-This PEP adds two more attributes to existing Python objects:
-a ``__locals__`` attribute to function objects, and
-an optional ``__globals__`` attribute to class objects.
-
-In Python, the bytecode interpreter can reference both a
-"globals" and a "locals" dictionary.  However, the current
-function object can only be bound to a globals dictionary,
-via the ``__globals__`` attribute.  Traditionally the
-"locals" dictionary is only set when executing a class.
-This PEP needs to set the "locals" dictionary to the class dict
-when evaluating annotations defined inside a class namespace.
-So this PEP defines a new ``__locals__`` attribute on
-functions.  By default it is uninitialized, or rather is set
-to an internal value that indicates it hasn't been explicitly set.
-It can be set to either ``None`` or a dictionary.  If it's set to
-a dictionary, the interpreter will use that dictionary as
-the "locals" dictionary when running the function.
-
-In Python, function objects contain a reference to their own
-``__globals__``.  However, class objects aren't currently
-defined as doing so in Python.  The implementation of
-``__annotate__`` in CPython needs a reference to the module
-globals in order to bind the unbound code object.  So this PEP
-defines a new ``__globals__`` attribute on class objects,
-which stores a reference to the globals for the module where
-the class was defined.  Note that this attribute is optional,
-but was useful for the CPython implementation.
-
-(The class ``__globals__`` attribute does create a new reference
-cycle, between a class and its module.  However, any class that
-contains a method already participates in at least one such cycle.)
+This PEP does not dictate exactly how it should be
+implemented; that is left up to the language implementation
+maintainers.  However, the best implementation of this
+PEP may require adding additional information to existing
+Python objects, which is implicitly condoned by the acceptance
+of this PEP.
+
+For example, it may be necessary to add a
+``__globals__`` attribute to class objects, so that the
+``__annotate__`` function for that class can be lazily
+bound, only on demand.  Also, ``__annotate__`` functions
+defined on methods defined in a class may need to retain
+a reference to the class's ``__dict__``, in order to
+correctly evaluate names bound in that class.  It's expected
+that the CPython implementation of this PEP will include
+both those new attributes.
+
+All such new information added to existing Python objects
+should be done with "dunder" attributes, as they will of
+course be implementation details.


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