mirror of https://github.com/python/cpython
gh-74929: PEP 667 general docs update (gh-119201)
* expand on What's New entry for PEP 667 (including porting notes) * define 'optimized scope' as a glossary term * cover comprehensions and generator expressions in locals() docs * review all mentions of "locals" in documentation (updating if needed) * review all mentions of "f_locals" in documentation (updating if needed)
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Alyssa Coghlan
GitHub
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@ -121,17 +121,18 @@ See also :ref:`Reflection <reflection>`.
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.. c:function:: PyObject* PyFrame_GetLocals(PyFrameObject *frame)
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Get the *frame*'s :attr:`~frame.f_locals` attribute.
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If the frame refers to a function or comprehension, this returns
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a write-through proxy object that allows modifying the locals.
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In all other cases (classes, modules) it returns the :class:`dict`
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representing the frame locals directly.
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If the frame refers to an :term:`optimized scope`, this returns a
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write-through proxy object that allows modifying the locals.
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In all other cases (classes, modules, :func:`exec`, :func:`eval`) it returns
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the mapping representing the frame locals directly (as described for
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:func:`locals`).
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Return a :term:`strong reference`.
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.. versionadded:: 3.11
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.. versionchanged:: 3.13
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Return a proxy object for functions and comprehensions.
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As part of :pep:`667`, return a proxy object for optimized scopes.
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.. c:function:: int PyFrame_GetLineNumber(PyFrameObject *frame)
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@ -889,6 +889,15 @@ Glossary
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(methods). Also the ultimate base class of any :term:`new-style
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class`.
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optimized scope
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A scope where target local variable names are reliably known to the
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compiler when the code is compiled, allowing optimization of read and
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write access to these names. The local namespaces for functions,
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generators, coroutines, comprehensions, and generator expressions are
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optimized in this fashion. Note: most interpreter optimizations are
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applied to all scopes, only those relying on a known set of local
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and nonlocal variable names are restricted to optimized scopes.
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package
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A Python :term:`module` which can contain submodules or recursively,
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subpackages. Technically, a package is a Python module with a
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@ -18,9 +18,9 @@ build applications which provide an interactive interpreter prompt.
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This class deals with parsing and interpreter state (the user's namespace); it
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does not deal with input buffering or prompting or input file naming (the
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filename is always passed in explicitly). The optional *locals* argument
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specifies the dictionary in which code will be executed; it defaults to a newly
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created dictionary with key ``'__name__'`` set to ``'__console__'`` and key
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``'__doc__'`` set to ``None``.
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specifies a mapping to use as the namespace in which code will be executed;
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it defaults to a newly created dictionary with key ``'__name__'`` set to
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``'__console__'`` and key ``'__doc__'`` set to ``None``.
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.. class:: InteractiveConsole(locals=None, filename="<console>", local_exit=False)
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@ -543,18 +543,19 @@ are always available. They are listed here in alphabetical order.
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The *expression* argument is parsed and evaluated as a Python expression
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(technically speaking, a condition list) using the *globals* and *locals*
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dictionaries as global and local namespace. If the *globals* dictionary is
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mappings as global and local namespace. If the *globals* dictionary is
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present and does not contain a value for the key ``__builtins__``, a
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reference to the dictionary of the built-in module :mod:`builtins` is
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inserted under that key before *expression* is parsed. That way you can
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control what builtins are available to the executed code by inserting your
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own ``__builtins__`` dictionary into *globals* before passing it to
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:func:`eval`. If the *locals* dictionary is omitted it defaults to the
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*globals* dictionary. If both dictionaries are omitted, the expression is
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:func:`eval`. If the *locals* mapping is omitted it defaults to the
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*globals* dictionary. If both mappings are omitted, the expression is
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executed with the *globals* and *locals* in the environment where
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:func:`eval` is called. Note, *eval()* does not have access to the
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:func:`eval` is called. Note, *eval()* will only have access to the
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:term:`nested scopes <nested scope>` (non-locals) in the enclosing
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environment.
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environment if they are already referenced in the scope that is calling
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:func:`eval` (e.g. via a :keyword:`nonlocal` statement).
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Example:
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@ -587,6 +588,11 @@ are always available. They are listed here in alphabetical order.
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The *globals* and *locals* arguments can now be passed as keywords.
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.. versionchanged:: 3.13
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The semantics of the default *locals* namespace have been adjusted as
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described for the :func:`locals` builtin.
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.. index:: pair: built-in function; exec
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.. function:: exec(source, /, globals=None, locals=None, *, closure=None)
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@ -612,9 +618,15 @@ are always available. They are listed here in alphabetical order.
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.. note::
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Most users should just pass a *globals* argument and never *locals*.
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If exec gets two separate objects as *globals* and *locals*, the code
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will be executed as if it were embedded in a class definition.
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When ``exec`` gets two separate objects as *globals* and *locals*, the
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code will be executed as if it were embedded in a class definition. This
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means functions and classes defined in the executed code will not be able
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to access variables assigned at the top level (as the "top level"
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variables are treated as class variables in a class definition).
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Passing a :class:`collections.ChainMap` instance as *globals* allows name
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lookups to be chained across multiple mappings without triggering this
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behaviour. Values assigned to top level names in the executed code can be
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retrieved by passing an empty dictionary as the first entry in the chain.
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If the *globals* dictionary does not contain a value for the key
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``__builtins__``, a reference to the dictionary of the built-in module
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@ -635,7 +647,7 @@ are always available. They are listed here in alphabetical order.
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.. note::
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The built-in functions :func:`globals` and :func:`locals` return the current
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global and local dictionary, respectively, which may be useful to pass around
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global and local namespace, respectively, which may be useful to pass around
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for use as the second and third argument to :func:`exec`.
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.. note::
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@ -651,6 +663,11 @@ are always available. They are listed here in alphabetical order.
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The *globals* and *locals* arguments can now be passed as keywords.
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.. versionchanged:: 3.13
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The semantics of the default *locals* namespace have been adjusted as
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described for the :func:`locals` builtin.
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.. function:: filter(function, iterable)
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@ -1056,39 +1073,51 @@ are always available. They are listed here in alphabetical order.
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variable names as the keys, and their currently bound references as the
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values.
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At module scope, as well as when using ``exec()`` or ``eval()`` with a
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single namespace, this function returns the same namespace as ``globals()``.
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At module scope, as well as when using :func:`exec` or :func:`eval` with
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a single namespace, this function returns the same namespace as
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:func:`globals`.
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At class scope, it returns the namespace that will be passed to the
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metaclass constructor.
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When using ``exec()`` or ``eval()`` with separate local and global
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namespaces, it returns the local namespace passed in to the function call.
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arguments, it returns the local namespace passed in to the function call.
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In all of the above cases, each call to ``locals()`` in a given frame of
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execution will return the *same* mapping object. Changes made through
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the mapping object returned from ``locals()`` will be visible as bound,
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rebound, or deleted local variables, and binding, rebinding, or deleting
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local variables will immediately affect the contents of the returned mapping
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object.
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the mapping object returned from ``locals()`` will be visible as assigned,
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reassigned, or deleted local variables, and assigning, reassigning, or
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deleting local variables will immediately affect the contents of the
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returned mapping object.
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At function scope (including for generators and coroutines), each call to
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``locals()`` instead returns a fresh dictionary containing the current
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bindings of the function's local variables and any nonlocal cell references.
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In this case, name binding changes made via the returned dict are *not*
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written back to the corresponding local variables or nonlocal cell
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references, and binding, rebinding, or deleting local variables and nonlocal
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cell references does *not* affect the contents of previously returned
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dictionaries.
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In an :term:`optimized scope` (including functions, generators, and
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coroutines), each call to ``locals()`` instead returns a fresh dictionary
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containing the current bindings of the function's local variables and any
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nonlocal cell references. In this case, name binding changes made via the
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returned dict are *not* written back to the corresponding local variables
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or nonlocal cell references, and assigning, reassigning, or deleting local
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variables and nonlocal cell references does *not* affect the contents
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of previously returned dictionaries.
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Calling ``locals()`` as part of a comprehension in a function, generator, or
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coroutine is equivalent to calling it in the containing scope, except that
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the comprehension's initialised iteration variables will be included. In
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other scopes, it behaves as if the comprehension were running as a nested
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function.
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Calling ``locals()`` as part of a generator expression is equivalent to
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calling it in a nested generator function.
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.. versionchanged:: 3.12
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The behaviour of ``locals()`` in a comprehension has been updated as
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described in :pep:`709`.
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.. versionchanged:: 3.13
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In previous versions, the semantics of mutating the mapping object
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returned from this function were formally undefined. In CPython
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specifically, the mapping returned at function scope could be
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implicitly refreshed by other operations, such as calling ``locals()``
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again. Obtaining the legacy CPython behaviour now requires explicit
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calls to update the initially returned dictionary with the results
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of subsequent calls to ``locals()``.
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As part of :pep:`667`, the semantics of mutating the mapping objects
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returned from this function are now defined. The behavior in
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:term:`optimized scopes <optimized scope>` is now as described above.
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Aside from being defined, the behaviour in other scopes remains
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unchanged from previous versions.
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.. function:: map(function, iterable, *iterables)
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@ -1975,14 +2004,18 @@ are always available. They are listed here in alphabetical order.
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:attr:`~object.__dict__` attributes (for example, classes use a
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:class:`types.MappingProxyType` to prevent direct dictionary updates).
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Without an argument, :func:`vars` acts like :func:`locals`. Note, the
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locals dictionary is only useful for reads since updates to the locals
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dictionary are ignored.
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Without an argument, :func:`vars` acts like :func:`locals`.
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A :exc:`TypeError` exception is raised if an object is specified but
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it doesn't have a :attr:`~object.__dict__` attribute (for example, if
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its class defines the :attr:`~object.__slots__` attribute).
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.. versionchanged:: 3.13
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The result of calling this function without an argument has been
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updated as described for the :func:`locals` builtin.
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.. function:: zip(*iterables, strict=False)
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Iterate over several iterables in parallel, producing tuples with an item
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@ -123,6 +123,11 @@ The typical usage to inspect a crashed program is::
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0
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(Pdb)
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.. versionchanged:: 3.13
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The implementation of :pep:`667` means that name assignments made via ``pdb``
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will immediately affect the active scope, even when running inside an
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:term:`optimized scope`.
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The module defines the following functions; each enters the debugger in a
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slightly different way:
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@ -579,18 +584,17 @@ can be overridden by the local file.
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.. pdbcommand:: interact
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Start an interactive interpreter (using the :mod:`code` module) whose global
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namespace contains all the (global and local) names found in the current
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scope. Use ``exit()`` or ``quit()`` to exit the interpreter and return to
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the debugger.
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Start an interactive interpreter (using the :mod:`code` module) in a new
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global namespace initialised from the local and global namespaces for the
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current scope. Use ``exit()`` or ``quit()`` to exit the interpreter and
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return to the debugger.
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.. note::
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Because interact creates a new global namespace with the current global
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and local namespace for execution, assignment to variables will not
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affect the original namespaces.
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However, modification to the mutable objects will be reflected in the
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original namespaces.
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As ``interact`` creates a new dedicated namespace for code execution,
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assignments to variables will not affect the original namespaces.
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However, modifications to any referenced mutable objects will be reflected
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in the original namespaces as usual.
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.. versionadded:: 3.2
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@ -234,7 +234,7 @@ functions:
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.. function:: runctx(command, globals, locals, filename=None, sort=-1)
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This function is similar to :func:`run`, with added arguments to supply the
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globals and locals dictionaries for the *command* string. This routine
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globals and locals mappings for the *command* string. This routine
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executes::
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exec(command, globals, locals)
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@ -473,7 +473,7 @@ in a :ref:`traceback <traceback-objects>`.
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attribute accessed (which also happens when casting it to a :class:`tuple`).
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:attr:`~FrameSummary.line` may be directly provided, and will prevent line
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lookups happening at all. *locals* is an optional local variable
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dictionary, and if supplied the variable representations are stored in the
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mapping, and if supplied the variable representations are stored in the
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summary for later display.
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:class:`!FrameSummary` instances have the following attributes:
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@ -1349,7 +1349,7 @@ Special read-only attributes
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* - .. attribute:: frame.f_locals
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- The dictionary used by the frame to look up
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:ref:`local variables <naming>`.
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If the frame refers to a function or comprehension,
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If the frame refers to an :term:`optimized scope`,
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this may return a write-through proxy object.
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.. versionchanged:: 3.13
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@ -94,10 +94,11 @@ Interpreter improvements:
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Performance improvements are modest -- we expect to be improving this
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over the next few releases.
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* :pep:`667`: :attr:`FrameType.f_locals <frame.f_locals>` when used in
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a function now returns a write-through proxy to the frame's locals,
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rather than a ``dict``. See the PEP for corresponding C API changes
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and deprecations.
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* :pep:`667`: The :func:`locals` builtin now has
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:ref:`defined semantics <whatsnew313-locals-semantics>` when mutating the
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returned mapping. Python debuggers and similar tools may now more reliably
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update local variables in optimized frames even during concurrent code
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execution.
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New typing features:
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@ -247,6 +248,34 @@ Improved Error Messages
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through ``self.X`` from any function in its body. (Contributed by Irit Katriel
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in :gh:`115775`.)
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.. _whatsnew313-locals-semantics:
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Defined mutation semantics for ``locals()``
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-------------------------------------------
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Historically, the expected result of mutating the return value of :func:`locals`
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has been left to individual Python implementations to define.
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Through :pep:`667`, Python 3.13 standardises the historical behaviour of CPython
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for most code execution scopes, but changes
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:term:`optimized scopes <optimized scope>` (functions, generators, coroutines,
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comprehensions, and generator expressions) to explicitly return independent
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snapshots of the currently assigned local variables, including locally
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referenced nonlocal variables captured in closures.
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To ensure debuggers and similar tools can reliably update local variables in
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scopes affected by this change, :attr:`FrameType.f_locals <frame.f_locals>` now
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returns a write-through proxy to the frame's local and locally referenced
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nonlocal variables in these scopes, rather than returning an inconsistently
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updated shared ``dict`` instance with undefined runtime semantics.
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See :pep:`667` for more details, including related C API changes and
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deprecations.
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(PEP and implementation contributed by Mark Shannon and Tian Gao in
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:gh:`74929`. Documentation updates provided by Guido van Rossum and
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Alyssa Coghlan.)
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Incremental Garbage Collection
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------------------------------
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@ -2177,6 +2206,24 @@ Changes in the Python API
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returned by :meth:`zipfile.ZipFile.open` was changed from ``'r'`` to ``'rb'``.
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(Contributed by Serhiy Storchaka in :gh:`115961`.)
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* Calling :func:`locals` in an :term:`optimized scope` now produces an
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independent snapshot on each call, and hence no longer implicitly updates
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previously returned references. Obtaining the legacy CPython behaviour now
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requires explicit calls to update the initially returned dictionary with the
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results of subsequent calls to ``locals()``. (Changed as part of :pep:`667`.)
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* Calling :func:`locals` from a comprehension at module or class scope
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(including via ``exec`` or ``eval``) once more behaves as if the comprehension
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were running as an independent nested function (i.e. the local variables from
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the containing scope are not included). In Python 3.12, this had changed
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to include the local variables from the containing scope when implementing
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:pep:`709`. (Changed as part of :pep:`667`.)
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* Accessing :attr:`FrameType.f_locals <frame.f_locals>` in an
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:term:`optimized scope` now returns a write-through proxy rather than a
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snapshot that gets updated at ill-specified times. If a snapshot is desired,
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it must be created explicitly with ``dict`` or the proxy's ``.copy()`` method.
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(Changed as part of :pep:`667`.)
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Changes in the C API
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--------------------
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@ -25,10 +25,10 @@ class InteractiveInterpreter:
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def __init__(self, locals=None):
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"""Constructor.
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The optional 'locals' argument specifies the dictionary in
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which code will be executed; it defaults to a newly created
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dictionary with key "__name__" set to "__console__" and key
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"__doc__" set to None.
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The optional 'locals' argument specifies a mapping to use as the
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namespace in which code will be executed; it defaults to a newly
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created dictionary with key "__name__" set to "__console__" and
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key "__doc__" set to None.
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"""
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if locals is None:
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@ -392,9 +392,12 @@ class Pdb(bdb.Bdb, cmd.Cmd):
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self.tb_lineno[tb.tb_frame] = lineno
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tb = tb.tb_next
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self.curframe = self.stack[self.curindex][0]
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# The f_locals dictionary is updated from the actual frame
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# locals whenever the .f_locals accessor is called, so we
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# cache it here to ensure that modifications are not overwritten.
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# The f_locals dictionary used to be updated from the actual frame
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# locals whenever the .f_locals accessor was called, so it was
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# cached here to ensure that modifications were not overwritten. While
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# the caching is no longer required now that f_locals is a direct proxy
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# on optimized frames, it's also harmless, so the code structure has
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# been left unchanged.
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self.curframe_locals = self.curframe.f_locals
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self.set_convenience_variable(self.curframe, '_frame', self.curframe)
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Reference in New Issue