python · ncoghlan · May 21, 2024 · May 20, 2024 · May 20, 2024 · May 20, 2024
diff --git a/Doc/c-api/frame.rst b/Doc/c-api/frame.rst
@@ -121,17 +121,18 @@ See also :ref:`Reflection <reflection>`.
 .. c:function:: PyObject* PyFrame_GetLocals(PyFrameObject *frame)
 
    Get the *frame*'s :attr:`~frame.f_locals` attribute.
-   If the frame refers to a function or comprehension, this returns
-   a write-through proxy object that allows modifying the locals.
-   In all other cases (classes, modules) it returns the :class:`dict`
-   representing the frame locals directly.
+   If the frame refers to an :term:`optimized scope`, this returns a
+   write-through proxy object that allows modifying the locals.
+   In all other cases (classes, modules, :func:`exec`, :func:`eval`) it returns
+   the mapping representing the frame locals directly (as described for
+   :func:`locals`).
 
    Return a :term:`strong reference`.
 
    .. versionadded:: 3.11
 
    .. versionchanged:: 3.13
-      Return a proxy object for functions and comprehensions.
+      As part of :pep:`667`, return a proxy object for optimized scopes.
 
 
 .. c:function:: int PyFrame_GetLineNumber(PyFrameObject *frame)

diff --git a/Doc/glossary.rst b/Doc/glossary.rst
@@ -889,6 +889,15 @@ Glossary
       (methods).  Also the ultimate base class of any :term:`new-style
       class`.
 
+   optimized scope
+      A scope where target local variable names are reliably known to the
+      compiler when the code is compiled, allowing optimization of read and
+      write access to these names. The local namespaces for functions,
+      generators, coroutines, comprehensions, and generator expressions are
+      optimized in this fashion. Note: most interpreter optimizations are
+      applied to all scopes, only those relying on a known set of local
+      and nonlocal variable names are restricted to optimized scopes.
+
    package
       A Python :term:`module` which can contain submodules or recursively,
       subpackages.  Technically, a package is a Python module with a

diff --git a/Doc/library/code.rst b/Doc/library/code.rst
@@ -18,9 +18,9 @@ build applications which provide an interactive interpreter prompt.
    This class deals with parsing and interpreter state (the user's namespace); it
    does not deal with input buffering or prompting or input file naming (the
    filename is always passed in explicitly). The optional *locals* argument
-   specifies the dictionary in which code will be executed; it defaults to a newly
-   created dictionary with key ``'__name__'`` set to ``'__console__'`` and key
-   ``'__doc__'`` set to ``None``.
+   specifies a mapping to use as the namespace in which code will be executed;
+   it defaults to a newly created dictionary with key ``'__name__'`` set to
+   ``'__console__'`` and key ``'__doc__'`` set to ``None``.
 
 
 .. class:: InteractiveConsole(locals=None, filename="<console>", local_exit=False)

diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
@@ -543,18 +543,19 @@ are always available.  They are listed here in alphabetical order.
 
    The *expression* argument is parsed and evaluated as a Python expression
    (technically speaking, a condition list) using the *globals* and *locals*
-   dictionaries as global and local namespace.  If the *globals* dictionary is
+   mappings as global and local namespace.  If the *globals* dictionary is
    present and does not contain a value for the key ``__builtins__``, a
    reference to the dictionary of the built-in module :mod:`builtins` is
    inserted under that key before *expression* is parsed.  That way you can
    control what builtins are available to the executed code by inserting your
    own ``__builtins__`` dictionary into *globals* before passing it to
-   :func:`eval`.  If the *locals* dictionary is omitted it defaults to the
-   *globals* dictionary.  If both dictionaries are omitted, the expression is
+   :func:`eval`.  If the *locals* mapping is omitted it defaults to the
+   *globals* dictionary.  If both mappings are omitted, the expression is
    executed with the *globals* and *locals* in the environment where
-   :func:`eval` is called.  Note, *eval()* does not have access to the
+   :func:`eval` is called.  Note, *eval()* will only have access to the
    :term:`nested scopes <nested scope>` (non-locals) in the enclosing
-   environment.
+   environment if they are already referenced in the scope that is calling
+   :func:`eval` (e.g. via a :keyword:`nonlocal` statement).
 
    Example:
 
@@ -587,6 +588,11 @@ are always available.  They are listed here in alphabetical order.
 
       The *globals* and *locals* arguments can now be passed as keywords.
 
+   .. versionchanged:: 3.13
+
+      The semantics of the default *locals* namespace have been adjusted as
+      described for the :func:`locals` builtin.
+
 .. index:: pair: built-in function; exec
 
 .. function:: exec(source, /, globals=None, locals=None, *, closure=None)
@@ -612,9 +618,15 @@ are always available.  They are listed here in alphabetical order.
 
    .. note::
 
-      Most users should just pass a *globals* argument and never *locals*.
-      If exec gets two separate objects as *globals* and *locals*, the code
-      will be executed as if it were embedded in a class definition.
+      When ``exec`` gets two separate objects as *globals* and *locals*, the
+      code will be executed as if it were embedded in a class definition. This
+      means functions and classes defined in the executed code will not be able
+      to access variables assigned at the top level (as the "top level"
+      variables are treated as class variables in a class definition).
+      Passing a :class:`collections.ChainMap` instance as *globals* allows name
+      lookups to be chained across multiple mappings without triggering this
+      behaviour. Values assigned to top level names in the executed code can be
+      retrieved by passing an empty dictionary as the first entry in the chain.
 
    If the *globals* dictionary does not contain a value for the key
    ``__builtins__``, a reference to the dictionary of the built-in module
@@ -635,7 +647,7 @@ are always available.  They are listed here in alphabetical order.
    .. note::
 
       The built-in functions :func:`globals` and :func:`locals` return the current
-      global and local dictionary, respectively, which may be useful to pass around
+      global and local namespace, respectively, which may be useful to pass around
       for use as the second and third argument to :func:`exec`.
 
    .. note::
@@ -651,6 +663,11 @@ are always available.  They are listed here in alphabetical order.
 
       The *globals* and *locals* arguments can now be passed as keywords.
 
+   .. versionchanged:: 3.13
+
+      The semantics of the default *locals* namespace have been adjusted as
+      described for the :func:`locals` builtin.
+
 
 .. function:: filter(function, iterable)
 
@@ -1056,39 +1073,51 @@ are always available.  They are listed here in alphabetical order.
     variable names as the keys, and their currently bound references as the
     values.
 
-    At module scope, as well as when using ``exec()`` or ``eval()`` with a
-    single namespace, this function returns the same namespace as ``globals()``.
+    At module scope, as well as when using :func:`exec` or :func:`eval` with
+    a single namespace, this function returns the same namespace as
+    :func:`globals`.
 
     At class scope, it returns the namespace that will be passed to the
     metaclass constructor.
 
     When using ``exec()`` or ``eval()`` with separate local and global
-    namespaces, it returns the local namespace passed in to the function call.
+    arguments, it returns the local namespace passed in to the function call.
 
     In all of the above cases, each call to ``locals()`` in a given frame of
     execution will return the *same* mapping object. Changes made through
-    the mapping object returned from ``locals()`` will be visible as bound,
-    rebound, or deleted local variables, and binding, rebinding, or deleting
-    local variables will immediately affect the contents of the returned mapping
-    object.
-
-    At function scope (including for generators and coroutines), each call to
-    ``locals()`` instead returns a fresh dictionary containing the current
-    bindings of the function's local variables and any nonlocal cell references.
-    In this case, name binding changes made via the returned dict are *not*
-    written back to the corresponding local variables or nonlocal cell
-    references, and binding, rebinding, or deleting local variables and nonlocal
-    cell references does *not* affect the contents of previously returned
-    dictionaries.
+    the mapping object returned from ``locals()`` will be visible as assigned,
+    reassigned, or deleted local variables, and assigning, reassigning, or
+    deleting local variables will immediately affect the contents of the
+    returned mapping object.
+
+    In an :term:`optimized scope` (including functions, generators, and
+    coroutines), each call to ``locals()`` instead returns a fresh dictionary
+    containing the current bindings of the function's local variables and any
+    nonlocal cell references. In this case, name binding changes made via the
+    returned dict are *not* written back to the corresponding local variables
+    or nonlocal cell references, and assigning, reassigning, or deleting local
+    variables and nonlocal cell references does *not* affect the contents
+    of previously returned dictionaries.
+
+    Calling ``locals()`` as part of a comprehension in a function, generator, or
+    coroutine is equivalent to calling it in the containing scope, except that
+    the comprehension's initialised iteration variables will be included. In
+    other scopes, it behaves as if the comprehension were running as a nested
+    function.
+
+    Calling ``locals()`` as part of a generator expression is equivalent to
+    calling it in a nested generator function.
+
+   .. versionchanged:: 3.12
+      The behaviour of ``locals()`` in a comprehension has been updated as
+      described in :pep:`709`.
 
    .. versionchanged:: 3.13
-      In previous versions, the semantics of mutating the mapping object
-      returned from this function were formally undefined. In CPython
-      specifically, the mapping returned at function scope could be
-      implicitly refreshed by other operations, such as calling ``locals()``
-      again. Obtaining the legacy CPython behaviour now requires explicit
-      calls to update the initially returned dictionary with the results
-      of subsequent calls to ``locals()``.
+      As part of :pep:`667`, the semantics of mutating the mapping objects
+      returned from this function are now defined. The behavior in
+      :term:`optimized scopes <optimized scope>` is now as described above.
+      Aside from being defined, the behaviour in other scopes remains
+      unchanged from previous versions.
 
 
 .. function:: map(function, iterable, *iterables)
@@ -1975,14 +2004,18 @@ are always available.  They are listed here in alphabetical order.
    :attr:`~object.__dict__` attributes (for example, classes use a
    :class:`types.MappingProxyType` to prevent direct dictionary updates).
 
-   Without an argument, :func:`vars` acts like :func:`locals`.  Note, the
-   locals dictionary is only useful for reads since updates to the locals
-   dictionary are ignored.
+   Without an argument, :func:`vars` acts like :func:`locals`.
 
    A :exc:`TypeError` exception is raised if an object is specified but
    it doesn't have a :attr:`~object.__dict__` attribute (for example, if
    its class defines the :attr:`~object.__slots__` attribute).
 
+   .. versionchanged:: 3.13
+
+      The result of calling this function without an argument has been
+      updated as described for the :func:`locals` builtin.
+
+
 .. function:: zip(*iterables, strict=False)
 
    Iterate over several iterables in parallel, producing tuples with an item

@@ -123,6 +123,11 @@ The typical usage to inspect a crashed program is::
    0
    (Pdb)
 
+.. versionchanged:: 3.13
+   The implementation of :pep:`667` means that name assignments made via ``pdb``
+   will immediately affect the active scope, even when running inside an
+   :term:`optimized scope`.
+
 
 The module defines the following functions; each enters the debugger in a
 slightly different way:
@@ -579,18 +584,17 @@ can be overridden by the local file.
 
 .. pdbcommand:: interact
 
-   Start an interactive interpreter (using the :mod:`code` module) whose global
-   namespace contains all the (global and local) names found in the current
-   scope. Use ``exit()`` or ``quit()`` to exit the interpreter and return to
-   the debugger.
+   Start an interactive interpreter (using the :mod:`code` module) in a new
+   global namespace initialised from the local and global namespaces for the
+   current scope. Use ``exit()`` or ``quit()`` to exit the interpreter and
+   return to the debugger.
 
    .. note::
 
-      Because interact creates a new global namespace with the current global
-      and local namespace for execution, assignment to variables will not
-      affect the original namespaces.
-      However, modification to the mutable objects will be reflected in the
-      original namespaces.
+      As ``interact`` creates a new dedicated namespace for code execution,
+      assignments to variables will not affect the original namespaces.
+      However, modifications to any referenced mutable objects will be reflected
+      in the original namespaces as usual.
 
    .. versionadded:: 3.2
 

diff --git a/Doc/library/profile.rst b/Doc/library/profile.rst
@@ -234,7 +234,7 @@ functions:
 .. function:: runctx(command, globals, locals, filename=None, sort=-1)
 
    This function is similar to :func:`run`, with added arguments to supply the
-   globals and locals dictionaries for the *command* string. This routine
+   globals and locals mappings for the *command* string. This routine
    executes::
 
       exec(command, globals, locals)

diff --git a/Doc/library/traceback.rst b/Doc/library/traceback.rst
@@ -473,7 +473,7 @@ in a :ref:`traceback <traceback-objects>`.
    attribute accessed (which also happens when casting it to a :class:`tuple`).
    :attr:`~FrameSummary.line` may be directly provided, and will prevent line
    lookups happening at all. *locals* is an optional local variable
-   dictionary, and if supplied the variable representations are stored in the
+   mapping, and if supplied the variable representations are stored in the
    summary for later display.
 
    :class:`!FrameSummary` instances have the following attributes:

@@ -1349,7 +1349,7 @@ Special read-only attributes
    * - .. attribute:: frame.f_locals
      - The dictionary used by the frame to look up
        :ref:`local variables <naming>`.
-       If the frame refers to a function or comprehension,
+       If the frame refers to an :term:`optimized scope`,
        this may return a write-through proxy object.
 
        .. versionchanged:: 3.13

diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
@@ -94,10 +94,11 @@ Interpreter improvements:
   Performance improvements are modest -- we expect to be improving this
   over the next few releases.
 
-* :pep:`667`: :attr:`FrameType.f_locals <frame.f_locals>` when used in
-  a function now returns a write-through proxy to the frame's locals,
-  rather than a ``dict``. See the PEP for corresponding C API changes
-  and deprecations.
+* :pep:`667`: The :func:`locals` builtin now has
+  :ref:`defined semantics <whatsnew313-locals-semantics>` when mutating the
+  returned mapping. Python debuggers and similar tools may now more reliably
+  update local variables in optimized frames even during concurrent code
+  execution.
 
 New typing features:
 
@@ -247,6 +248,34 @@ Improved Error Messages
   through ``self.X`` from any function in its body. (Contributed by Irit Katriel
   in :gh:`115775`.)
 
+.. _whatsnew313-locals-semantics:
+
+Defined mutation semantics for ``locals()``
+-------------------------------------------
+
+Historically, the expected result of mutating the return value of :func:`locals`
+has been left to individual Python implementations to define.
+
+Through :pep:`667`, Python 3.13 standardises the historical behaviour of CPython
+for most code execution scopes, but changes
+:term:`optimized scopes <optimized scope>` (functions, generators, coroutines,
+comprehensions, and generator expressions) to explicitly return independent
+snapshots of the currently assigned local variables, including locally
+referenced nonlocal variables captured in closures.
+
+To ensure debuggers and similar tools can reliably update local variables in
+scopes affected by this change, :attr:`FrameType.f_locals <frame.f_locals>` now
+returns a write-through proxy to the frame's local and locally referenced
+nonlocal variables in these scopes, rather than returning an inconsistently
+updated shared  ``dict`` instance with undefined runtime semantics.
+
+See :pep:`667` for more details, including related C API changes and
+deprecations.
+
+(PEP and implementation contributed by Mark Shannon and Tian Gao in
+:gh:`74929`. Documentation updates provided by Guido van Rossum and
+Alyssa Coghlan.)
+
 Incremental Garbage Collection
 ------------------------------
 
@@ -2177,6 +2206,24 @@ Changes in the Python API
   returned by :meth:`zipfile.ZipFile.open` was changed from ``'r'`` to ``'rb'``.
   (Contributed by Serhiy Storchaka in :gh:`115961`.)
 
+* Calling :func:`locals` in an :term:`optimized scope` now produces an
+  independent snapshot on each call, and hence no longer implicitly updates
+  previously returned references. Obtaining the legacy CPython behaviour now
+  requires explicit calls to update the initially returned dictionary with the
+  results of subsequent calls to ``locals()``. (Changed as part of :pep:`667`.)
+
+* Calling :func:`locals` from a comprehension at module or class scope
+  (including via ``exec`` or ``eval``) once more behaves as if the comprehension
+  were running as an independent nested function (i.e. the local variables from
+  the containing scope are not included). In Python 3.12, this had changed
+  to include the local variables from the containing scope when implementing
+  :pep:`709`. (Changed as part of :pep:`667`.)
+
+* Accessing :attr:`FrameType.f_locals <frame.f_locals>` in an
+  :term:`optimized scope` now returns a write-through proxy rather than a
+  snapshot that gets updated at ill-specified times. If a snapshot is desired,
+  it must be created explicitly with ``dict`` or the proxy's ``.copy()`` method.
+  (Changed as part of :pep:`667`.)
 
 Changes in the C API
 --------------------

diff --git a/Lib/code.py b/Lib/code.py
@@ -25,10 +25,10 @@ class InteractiveInterpreter:
     def __init__(self, locals=None):
         """Constructor.
 
-        The optional 'locals' argument specifies the dictionary in
-        which code will be executed; it defaults to a newly created
-        dictionary with key "__name__" set to "__console__" and key
-        "__doc__" set to None.
+        The optional 'locals' argument specifies a mapping to use as the
+        namespace in which code will be executed; it defaults to a newly
+        created dictionary with key "__name__" set to "__console__" and
+        key "__doc__" set to None.
 
         """
         if locals is None:

@@ -392,9 +392,12 @@ def setup(self, f, tb):
             self.tb_lineno[tb.tb_frame] = lineno
             tb = tb.tb_next
         self.curframe = self.stack[self.curindex][0]
-        # The f_locals dictionary is updated from the actual frame
-        # locals whenever the .f_locals accessor is called, so we
-        # cache it here to ensure that modifications are not overwritten.
+        # The f_locals dictionary used to be updated from the actual frame
+        # locals whenever the .f_locals accessor was called, so it was
+        # cached here to ensure that modifications were not overwritten. While
+        # the caching is no longer required now that f_locals is a direct proxy
+        # on optimized frames, it's also harmless, so the code structure has
+        # been left unchanged.
         self.curframe_locals = self.curframe.f_locals
         self.set_convenience_variable(self.curframe, '_frame', self.curframe)