[Python-Dev] PEP 558: Defined semantics for locals()

Hi folks, It's been a long time coming, but I've finally made enough progress on the reference implementation that I think it's time to ask Nathaniel to pronounce on the current iteration of PEP 558 (Defined semantics for locals()). The rendered version is up at https://www.python.org/dev/peps/pep-0558/, and I've included the plain text version below. For those that are reading the PEP for the first time, the gist is: * standardise on Python 3.10 behaviour *except* that locals() at function scope returns a fresh snapshot every time instead of a reference to the frame level state cache * make the Python level frame.f_locals on optimised frames a write-through proxy that keeps both the real fast locals storage and the C level f_locals state cache up to date * add new C APIs that allow C code to explicitly request the semantics the client code actually wants ("behave like the Python locals() builtin", "always make a copy", "always provide a read-only view") * soft-deprecate the legacy PyEval_GetLocals() API (while ensuring it still works) * use the new features to significantly improve the performance of code execution tracing hooks implemented in Python For those that remember reading older versions of the PEP, the key changes relative to the last discourse thread (back in late 2019/early 2020) are: * incorporating the C API design improvements from the 2019/20 Discourse thread * incorporating Mark Shannon's feedback from earlier this year (most notably, changing the proxy design to only create a reference cycle from the frames back to the fast locals proxies that reference them if you store a reference to the proxy as a local variable on the frame) * trying (and failing) to remove the fast locals proxy dependency on the C level f_locals cache on optimised frame objects. Instead, the fast locals proxy more explicitly uses that dictionary as a state cache to speed up certain operations (e.g. dict equality comparisons, iteration, and rendering the proxy contents as a string), and to store keys that don't correspond to frame level variables, while also exposing a new ``sync_frame_cache()`` method to sync the state cache on the underlying frame with changes made via other mechanisms. Cheers, Nick. P.S. The PEP text in the email already incorporates the text review edits in https://github.com/python/peps/pull/2038/files that haven't been merged to the web version yet. Suggested clarifications to the text of the PEP that don't affect the overall design can be added as comments on that PR rather than being added to the mailing list thread. ===================== PEP: 558 Title: Defined semantics for locals() Author: Nick Coghlan <ncoghlan@gmail.com> BDFL-Delegate: Nathaniel J. Smith Discussions-To: <python-dev@python.org> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 08-Sep-2017 Python-Version: 3.11 Post-History: 2017-09-08, 2019-05-22, 2019-05-30, 2019-12-30, 2021-07-18 Abstract ======== The semantics of the ``locals()`` builtin have historically been underspecified and hence implementation dependent. This PEP proposes formally standardising on the behaviour of the CPython 3.10 reference implementation for most execution scopes, with some adjustments to the behaviour at function scope to make it more predictable and independent of the presence or absence of tracing functions. In addition, it proposes that the following functions be added to the stable Python C API/ABI:: PyObject * PyLocals_Get(); int PyLocals_GetReturnsCopy(); PyObject * PyLocals_GetCopy(); PyObject * PyLocals_GetView(); It also proposes the addition of several supporting functions and type definitions to the CPython C API. Rationale ========= While the precise semantics of the ``locals()`` builtin are nominally undefined, in practice, many Python programs depend on it behaving exactly as it behaves in CPython (at least when no tracing functions are installed). Other implementations such as PyPy are currently replicating that behaviour, up to and including replication of local variable mutation bugs that can arise when a trace hook is installed [1]_. While this PEP considers CPython's current behaviour when no trace hooks are installed to be largely acceptable, it considers the current behaviour when trace hooks are installed to be problematic, as it causes bugs like [1]_ *without* even reliably enabling the desired functionality of allowing debuggers like ``pdb`` to mutate local variables [3]_. Review of the initial PEP and the draft implementation then identified an opportunity for simplification of both the documentation and implementation of the function level ``locals()`` behaviour by updating it to return an independent snapshot of the function locals and closure variables on each call, rather than continuing to return the semi-dynamic intermittently updated shared copy that it has historically returned in CPython. Proposal ======== The expected semantics of the ``locals()`` builtin change based on the current execution scope. For this purpose, the defined scopes of execution are: * module scope: top-level module code, as well as any other code executed using ``exec()`` or ``eval()`` with a single namespace * class scope: code in the body of a ``class`` statement, as well as any other code executed using ``exec()`` or ``eval()`` with separate local and global namespaces * function scope: code in the body of a ``def`` or ``async def`` statement, or any other construct that creates an optimized code block in CPython (e.g. comprehensions, lambda functions) This PEP proposes elevating most of the current behaviour of the CPython reference implementation to become part of the language specification, *except* that each call to ``locals()`` at function scope will create a new dictionary object, rather than caching a common dict instance in the frame object that each invocation will update and return. This PEP also proposes to largely eliminate the concept of a separate "tracing" mode from the CPython reference implementation. In releases up to and including Python 3.10, the CPython interpreter behaves differently when a trace hook has been registered in one or more threads via an implementation dependent mechanism like ``sys.settrace`` ([4]_) in CPython's ``sys`` module or ``PyEval_SetTrace`` ([5]_) in CPython's C API. This PEP proposes changes to CPython's behaviour at function scope that make the ``locals()`` builtin semantics when a trace hook is registered identical to those used when no trace hook is registered, while also making the related frame API semantics clearer and easier for interactive debuggers to rely on. The proposed elimination of tracing mode affects the semantics of frame object references obtained through other means, such as via a traceback, or via the ``sys._getframe()`` API, as the write-through semantics needed for trace hook support are always provided by the ``f_locals`` attribute on frame objects, rather than being runtime state dependent. New ``locals()`` documentation ------------------------------ The heart of this proposal is to revise the documentation for the ``locals()`` builtin to read as follows: Return a mapping object representing the current local symbol table, with 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 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. 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. There would also be a versionchanged note for the release making this change: In prior versions, the semantics of mutating the mapping object returned from ``locals()`` were formally undefined. In CPython specifically, the mapping returned at function scope could be implicitly refreshed by other operations, such as calling ``locals()`` again, or the interpreter implicitly invoking a Python level trace function. Obtaining the legacy CPython behaviour now requires explicit calls to update the initially returned dictionary with the results of subsequent calls to ``locals()``. For reference, the current documentation of this builtin reads as follows: Update and return a dictionary representing the current local symbol table. Free variables are returned by locals() when it is called in function blocks, but not in class blocks. Note: The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter. (In other words: the status quo is that the semantics and behaviour of ``locals()`` are formally implementation defined, whereas the proposed state after this PEP is that the only implementation defined behaviour will be that associated with whether or not the implementation emulates the CPython frame API, with the behaviour in all other cases being defined by the language and library references) Module scope ------------ At module scope, as well as when using ``exec()`` or ``eval()`` with a single namespace, ``locals()`` must return the same object as ``globals()``, which must be the actual execution namespace (available as ``inspect.currentframe().f_locals`` in implementations that provide access to frame objects). Variable assignments during subsequent code execution in the same scope must dynamically change the contents of the returned mapping, and changes to the returned mapping must change the values bound to local variable names in the execution environment. To capture this expectation as part of the language specification, the following paragraph will be added to the documentation for ``locals()``: At module scope, as well as when using ``exec()`` or ``eval()`` with a single namespace, this function returns the same namespace as ``globals()``. This part of the proposal does not require any changes to the reference implementation - it is standardisation of the current behaviour. Class scope ----------- At class scope, as well as when using ``exec()`` or ``eval()`` with separate global and local namespaces, ``locals()`` must return the specified local namespace (which may be supplied by the metaclass ``__prepare__`` method in the case of classes). As for module scope, this must be a direct reference to the actual execution namespace (available as ``inspect.currentframe().f_locals`` in implementations that provide access to frame objects). Variable assignments during subsequent code execution in the same scope must change the contents of the returned mapping, and changes to the returned mapping must change the values bound to local variable names in the execution environment. The mapping returned by ``locals()`` will *not* be used as the actual class namespace underlying the defined class (the class creation process will copy the contents to a fresh dictionary that is only accessible by going through the class machinery). For nested classes defined inside a function, any nonlocal cells referenced from the class scope are *not* included in the ``locals()`` mapping. To capture this expectation as part of the language specification, the following two paragraphs will be added to the documentation for ``locals()``: When using ``exec()`` or ``eval()`` with separate local and global namespaces, [this function] returns the given local namespace. At class scope, it returns the namespace that will be passed to the metaclass constructor. This part of the proposal does not require any changes to the reference implementation - it is standardisation of the current behaviour. Function scope -------------- At function scope, interpreter implementations are granted significant freedom to optimise local variable access, and hence are NOT required to permit arbitrary modification of local and nonlocal variable bindings through the mapping returned from ``locals()``. Historically, this leniency has been described in the language specification with the words "The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter." This PEP proposes to change that text to instead say: 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. This part of the proposal *does* require changes to the CPython reference implementation, as CPython currently returns a shared mapping object that may be implicitly refreshed by additional calls to ``locals()``, and the "write back" strategy currently used to support namespace changes from trace functions also doesn't comply with it (and causes the quirky behavioural problems mentioned in the Rationale). CPython Implementation Changes ============================== Summary of proposed implementation-specific changes --------------------------------------------------- * Changes are made as necessary to provide the updated Python level semantics * Two new functions are added to the stable ABI to replicate the updated behaviour of the Python ``locals()`` builtin:: PyObject * PyLocals_Get(); int PyLocals_GetReturnsCopy(); * One new function is added to the stable ABI to efficiently get a snapshot of the local namespace in the running frame:: PyObject * PyLocals_GetCopy(); * One new function is added to the stable ABI to get a read-only view of the local namespace in the running frame:: PyObject * PyLocals_GetView(); * Corresponding frame accessor functions for these new public APIs are added to the CPython frame C API * On optimised frames, the Python level ``f_locals`` API will become a direct read/write proxy for the frame's local and closure variable storage, but will use the C level ``f_locals`` struct field to hold a value cache that also allows for storage of arbitrary additional keys. Additional details on the expected behaviour of that fast locals proxy are given below. * No C API function is added to get access to a mutable mapping for the local namespace. Instead, ``PyObject_GetAttrString(frame, "f_locals")`` is used, the same API as is used in Python code. * ``PyEval_GetLocals()`` remains supported and does not emit a programmatic warning, but will be deprecated in the documentation in favour of the new APIs * ``PyFrame_FastToLocals()`` and ``PyFrame_FastToLocalsWithError()`` remain supported and do not emit a programmatic warning, but will be deprecated in the documentation in favour of the new APIs * ``PyFrame_LocalsToFast()`` always raises ``RuntimeError()``, indicating that ``PyObject_GetAttrString(frame, "f_locals")`` should be used to obtain a mutable read/write mapping for the local variables. * The trace hook implementation will no longer call ``PyFrame_FastToLocals()`` implicitly. The version porting guide will recommend migrating to ``PyFrame_GetLocalsView()`` for read-only access and ``PyObject_GetAttrString(frame, "f_locals")`` for read/write access. Providing the updated Python level semantics -------------------------------------------- The implementation of the ``locals()`` builtin is modified to return a distinct copy of the local namespace rather than a direct reference to the internal dynamically updated snapshot returned by ``PyEval_GetLocals()``. Resolving the issues with tracing mode behaviour ------------------------------------------------ The current cause of CPython's tracing mode quirks (both the side effects from simply installing a tracing function and the fact that writing values back to function locals only works for the specific function being traced) is the way that locals mutation support for trace hooks is currently implemented: the ``PyFrame_LocalsToFast`` function. When a trace function is installed, CPython currently does the following for function frames (those where the code object uses "fast locals" semantics): 1. Calls ``PyFrame_FastToLocals`` to update the dynamic snapshot 2. Calls the trace hook (with tracing of the hook itself disabled) 3. Calls ``PyFrame_LocalsToFast`` to capture any changes made to the dynamic snapshot This approach is problematic for a few different reasons: * Even if the trace function doesn't mutate the snapshot, the final step resets any cell references back to the state they were in before the trace function was called (this is the root cause of the bug report in [1]_) * If the trace function *does* mutate the snapshot, but then does something that causes the snapshot to be refreshed, those changes are lost (this is one aspect of the bug report in [3]_) * If the trace function attempts to mutate the local variables of a frame other than the one being traced (e.g. ``frame.f_back.f_locals``), those changes will almost certainly be lost (this is another aspect of the bug report in [3]_) * If a ``locals()`` reference is passed to another function, and *that* function mutates the snapshot namespace, then those changes *may* be written back to the execution frame *if* a trace hook is installed The proposed resolution to this problem is to take advantage of the fact that whereas functions typically access their *own* namespace using the language defined ``locals()`` builtin, trace functions necessarily use the implementation dependent ``frame.f_locals`` interface, as a frame reference is what gets passed to hook implementations. Instead of being a direct reference to the internal dynamic snapshot used to populate the independent snapshots returned by ``locals()``, the Python level ``frame.f_locals`` will be updated to instead return a dedicated proxy type that has two internal attributes not exposed as part of the Python runtime API: * *frame*: the underlying frame that the snapshot is for * *fast_refs*: a mapping from variable names to either fast local storage offsets (for local variables) or to closure cells (for closure variables). This mapping is lazily initialized on the first read or write access through the proxy, rather than being eagerly populated as soon as the proxy is created. The C level ``f_locals`` attribute on the frame object is treated as a cache by the fast locals proxy, as some operations (such as equality comparisons) require a regular dictionary mapping from names to their respective values. Fast local variables and cell variables are stored in the cache if they are currently bound to a value. Arbitrary additional attributes may also be stored in the cache. It *is* possible for the cache to get out of sync with the actual frame state (e.g. as code executes binding and unbinding operations, or if changes are made directly to the cache dict). A dedicated ``sync_frame_cache()`` method is provided that runs ``PyFrame_FastToLocalsWithError()`` to ensure the cache is consistent with the current frame state. ``__getitem__`` operations on the proxy will populate the ``fast_refs`` mapping (if it is not already populated), and then either return the relevant value (if the key is found in either the ``fast_refs`` mapping or the ``f_locals`` dynamic snapshot stored on the frame), or else raise ``KeyError``. Variables that are defined but not currently bound raise ``KeyError`` (just as they're omitted from the result of ``locals()``). As the frame storage is always accessed directly, the proxy will automatically pick up name binding operations that take place as the function executes. The cache dictionary is implicitly updated when individual variables are read from the frame state (including for containment checks, which need to check if the name is currently bound or unbound). Similarly, ``__setitem__`` and ``__delitem__`` operations on the proxy will directly affect the corresponding fast local or cell reference on the underlying frame, ensuring that changes are immediately visible to the running Python code, rather than needing to be written back to the runtime storage at some later time. Such changes are also immediately written to the ``f_locals`` cache to reduce the opportunities for the cache to get out of sync with the frame state. Keys that are not defined as local or closure variables on the underlying frame are still written to the ``f_locals`` cache on optimised frames. This allows utilities like ``pdb`` (which writes ``__return__`` and ``__exception__`` values into the frame ``f_locals`` mapping) to continue working as they always have. These additional keys that do not correspond to a local or closure variable on the frame will be left alone by future cache sync operations. Other ``Mapping`` and ``MutableMapping`` methods will behave as expected for a mapping with these essential method semantics, with the exception that only intrinsically ``O(n)`` operations (e.g. copying, rendering as a string) and operations that operate on a single key (e.g. getting, setting, deleting, or popping) will implicitly refresh the value cache. Other operations (e.g. length checks, equality checks, iteration) may use the value cache without first ensuring that it is up to date (as ensuring the cache is up to date is itself an ``O(n)`` operation). An additional benefit of storing only the variable value cache on the frame (rather than storing an instance of the proxy type), is that it avoids creating a reference cycle from the frame back to itself, so the frame will only be kept alive if another object retains a reference to a proxy instance. Changes to the stable C API/ABI ------------------------------- Unlike Python code, extension module functions that call in to the Python C API can be called from any kind of Python scope. This means it isn't obvious from the context whether ``locals()`` will return a snapshot or not, as it depends on the scope of the calling Python code, not the C code itself. This means it is desirable to offer C APIs that give predictable, scope independent, behaviour. However, it is also desirable to allow C code to exactly mimic the behaviour of Python code at the same scope. To enable mimicking the behaviour of Python code, the stable C ABI would gain the following new functions:: PyObject * PyLocals_Get(); int PyLocals_GetReturnsCopy(); ``PyLocals_Get()`` is directly equivalent to the Python ``locals()`` builtin. It returns a new reference to the local namespace mapping for the active Python frame at module and class scope, and when using ``exec()`` or ``eval()``. It returns a shallow copy of the active namespace at function/coroutine/generator scope. ``PyLocals_GetReturnsCopy()`` returns zero if ``PyLocals_Get()`` returns a direct reference to the local namespace mapping, and a non-zero value if it returns a shallow copy. This allows extension module code to determine the potential impact of mutating the mapping returned by ``PyLocals_Get()`` without needing access to the details of the running frame object. To allow extension module code to behave consistently regardless of the active Python scope, the stable C ABI would gain the following new functions:: PyObject * PyLocals_GetCopy(); PyObject * PyLocals_GetView(); ``PyLocals_GetCopy()`` returns a new dict instance populated from the current locals namespace. Roughly equivalent to ``dict(locals())`` in Python code, but avoids the double-copy in the case where ``locals()`` already returns a shallow copy. ``PyLocals_GetView()`` returns a new read-only mapping proxy instance for the current locals namespace. This view immediately reflects all local variable changes, independently of whether the running frame is optimised or not. The existing ``PyEval_GetLocals()`` API will retain its existing behaviour in CPython (mutable locals at class and module scope, shared dynamic snapshot otherwise). However, its documentation will be updated to note that the conditions under which the shared dynamic snapshot get updated have changed. The ``PyEval_GetLocals()`` documentation will also be updated to recommend replacing usage of this API with whichever of the new APIs is most appropriate for the use case: * Use ``PyLocals_GetView()`` for read-only access to the current locals namespace. * Use ``PyLocals_GetCopy()`` for a regular mutable dict that contains a copy of the current locals namespace, but has no ongoing connection to the active frame. * Use ``PyLocals_Get()`` to exactly match the semantics of the Python level ``locals()`` builtin. * Query ``PyLocals_GetReturnsCopy()`` explicitly to implement custom handling (e.g. raising a meaningful exception) for scopes where ``PyLocals_Get()`` would return a shallow copy rather than granting read/write access to the locals namespace. * Use implementation specific APIs (e.g. ``PyObject_GetAttrString(frame, "f_locals")``) if read/write access to the frame is required and ``PyLocals_GetReturnsCopy()`` is true. Changes to the public CPython C API ----------------------------------- The existing ``PyEval_GetLocals()`` API returns a borrowed reference, which means it cannot be updated to return the new shallow copies at function scope. Instead, it will continue to return a borrowed reference to an internal dynamic snapshot stored on the frame object. This shared mapping will behave similarly to the existing shared mapping in Python 3.10 and earlier, but the exact conditions under which it gets refreshed will be different. Specifically, it will be updated only in the following circumstance: * any call to ``PyEval_GetLocals()``, ``PyLocals_Get()``, ``PyLocals_GetCopy()``, or the Python ``locals()`` builtin while the frame is running * any call to ``PyFrame_GetLocals()``, ``PyFrame_GetLocalsCopy()``, ``_PyFrame_BorrowLocals()``, ``PyFrame_FastToLocals()``, or ``PyFrame_FastToLocalsWithError()`` for the frame * retrieving the ``f_locals`` attribute from a Python level frame object * any call to the ``sync_frame_cache()`` method on a fast locals proxy referencing that frame * any operation on a fast locals proxy object that requires the shared mapping to be up to date on the underlying frame. In the initial reference implementation, those operations are those that are intrinsically ``O(n)`` operations (``flp.copy()`` and rendering as a string), as well as those that refresh the cache entries for individual keys. Accessing the frame "view" APIs will *not* implicitly update the shared dynamic snapshot, and the CPython trace hook handling will no longer implicitly update it either. (Note: even though ``PyEval_GetLocals()`` is part of the stable C API/ABI, the specifics of when the namespace it returns gets refreshed are still an interpreter implementation detail) The additions to the public CPython C API are the frame level enhancements needed to support the stable C API/ABI updates:: PyObject * PyFrame_GetLocals(frame); int PyFrame_GetLocalsReturnsCopy(frame); PyObject * PyFrame_GetLocalsCopy(frame); PyObject * PyFrame_GetLocalsView(frame); PyObject * _PyFrame_BorrowLocals(frame); ``PyFrame_GetLocals(frame)`` is the underlying API for ``PyLocals_Get()``. ``PyFrame_GetLocalsReturnsCopy(frame)`` is the underlying API for ``PyLocals_GetReturnsCopy()``. ``PyFrame_GetLocalsCopy(frame)`` is the underlying API for ``PyLocals_GetCopy()``. ``PyFrame_GetLocalsView(frame)`` is the underlying API for ``PyLocals_GetView()``. ``_PyFrame_BorrowLocals(frame)`` is the underlying API for ``PyEval_GetLocals()``. The underscore prefix is intended to discourage use and to indicate that code using it is unlikely to be portable across implementations. However, it is documented and visible to the linker in order to avoid having to access the internals of the frame struct from the ``PyEval_GetLocals()`` implementation. The ``PyFrame_LocalsToFast()`` function will be changed to always emit ``RuntimeError``, explaining that it is no longer a supported operation, and affected code should be updated to use ``PyObject_GetAttrString(frame, "f_locals")`` to obtain a read/write proxy instead. In addition to the above documented interfaces, the draft reference implementation also exposes the following undocumented interfaces:: PyTypeObject _PyFastLocalsProxy_Type; #define _PyFastLocalsProxy_CheckExact(self) \ (Py_TYPE(self) == &_PyFastLocalsProxy_Type) This type is what the reference implementation actually returns from ``PyObject_GetAttrString(frame, "f_locals")`` for optimized frames (i.e. when ``PyFrame_GetLocalsReturnsCopy()`` returns true). Reducing the runtime overhead of trace hooks -------------------------------------------- As noted in [9]_, the implicit call to ``PyFrame_FastToLocals()`` in the Python trace hook support isn't free, and could be rendered unnecessary if the frame proxy read values directly from the frame instead of getting them from the mapping. As the new frame locals proxy type doesn't require separate data refresh steps, this PEP incorporates Victor Stinner's proposal to no longer implicitly call ``PyFrame_FastToLocalsWithError()`` before calling trace hooks implemented in Python. Code using the new frame view APIs will have the dynamic locals snapshot implicitly refreshed when accessing methods that need it, while code using the ``PyEval_GetLocals()`` API will implicitly refresh it when making that call. The PEP necessarily also drops the implicit call to ``PyFrame_LocalsToFast()`` when returning from a trace hook, as that API now always raises an exception. Design Discussion ================= Changing ``locals()`` to return independent snapshots at function scope ----------------------------------------------------------------------- The ``locals()`` builtin is a required part of the language, and in the reference implementation it has historically returned a mutable mapping with the following characteristics: * each call to ``locals()`` returns the *same* mapping object * for namespaces where ``locals()`` returns a reference to something other than the actual local execution namespace, each call to ``locals()`` updates the mapping object with the current state of the local variables and any referenced nonlocal cells * changes to the returned mapping *usually* aren't written back to the local variable bindings or the nonlocal cell references, but write backs can be triggered by doing one of the following: * installing a Python level trace hook (write backs then happen whenever the trace hook is called) * running a function level wildcard import (requires bytecode injection in Py3) * running an ``exec`` statement in the function's scope (Py2 only, since ``exec`` became an ordinary builtin in Python 3) Originally this PEP proposed to retain the first two of these properties, while changing the third in order to address the outright behaviour bugs that it can cause. In [7]_ Nathaniel Smith made a persuasive case that we could make the behaviour of ``locals()`` at function scope substantially less confusing by retaining only the second property and having each call to ``locals()`` at function scope return an *independent* snapshot of the local variables and closure references rather than updating an implicitly shared snapshot. As this revised design also made the implementation markedly easier to follow, the PEP was updated to propose this change in behaviour, rather than retaining the historical shared snapshot. Keeping ``locals()`` as a snapshot at function scope ---------------------------------------------------- As discussed in [7]_, it would theoretically be possible to change the semantics of the ``locals()`` builtin to return the write-through proxy at function scope, rather than switching it to return independent snapshots. This PEP doesn't (and won't) propose this as it's a backwards incompatible change in practice, even though code that relies on the current behaviour is technically operating in an undefined area of the language specification. Consider the following code snippet:: def example(): x = 1 locals()["x"] = 2 print(x) Even with a trace hook installed, that function will consistently print ``1`` on the current reference interpreter implementation:: >>> example() 1 >>> import sys >>> def basic_hook(*args): ... return basic_hook ... >>> sys.settrace(basic_hook) >>> example() 1 Similarly, ``locals()`` can be passed to the ``exec()`` and ``eval()`` builtins at function scope (either explicitly or implicitly) without risking unexpected rebinding of local variables or closure references. Provoking the reference interpreter into incorrectly mutating the local variable state requires a more complex setup where a nested function closes over a variable being rebound in the outer function, and due to the use of either threads, generators, or coroutines, it's possible for a trace function to start running for the nested function before the rebinding operation in the outer function, but finish running after the rebinding operation has taken place (in which case the rebinding will be reverted, which is the bug reported in [1]_). In addition to preserving the de facto semantics which have been in place since PEP 227 introduced nested scopes in Python 2.1, the other benefit of restricting the write-through proxy support to the implementation-defined frame object API is that it means that only interpreter implementations which emulate the full frame API need to offer the write-through capability at all, and that JIT-compiled implementations only need to enable it when a frame introspection API is invoked, or a trace hook is installed, not whenever ``locals()`` is accessed at function scope. Returning snapshots from ``locals()`` at function scope also means that static analysis for function level code will be more reliable, as only access to the frame machinery will allow rebinding of local and nonlocal variable references in a way that is hidden from static analysis. What happens with the default args for ``eval()`` and ``exec()``? ----------------------------------------------------------------- These are formally defined as inheriting ``globals()`` and ``locals()`` from the calling scope by default. There isn't any need for the PEP to change these defaults, so it doesn't, and ``exec()`` and ``eval()`` will start running in a shallow copy of the local namespace when that is what ``locals()`` returns. This behaviour will have potential performance implications, especially for functions with large numbers of local variables (e.g. if these functions are called in a loop, calling ``globals()`` and ``locals()`` once before the loop and then passing the namespace into the function explicitly will give the same semantics and performance characteristics as the status quo, whereas relying on the implicit default would create a new shallow copy of the local namespace on each iteration). (Note: the reference implementation draft PR has updated the ``locals()`` and ``vars()``, ``eval()``, and ``exec()`` builtins to use ``PyLocals_Get()``. The ``dir()`` builtin still uses ``PyEval_GetLocals()``, since it's only using it to make a list from the keys). Changing the frame API semantics in regular operation ----------------------------------------------------- Earlier versions of this PEP proposed having the semantics of the frame ``f_locals`` attribute depend on whether or not a tracing hook was currently installed - only providing the write-through proxy behaviour when a tracing hook was active, and otherwise behaving the same as the historical ``locals()`` builtin. That was adopted as the original design proposal for a couple of key reasons, one pragmatic and one more philosophical: * Object allocations and method wrappers aren't free, and tracing functions aren't the only operations that access frame locals from outside the function. Restricting the changes to tracing mode meant that the additional memory and execution time overhead of these changes would be as close to zero in regular operation as we can possibly make them. * "Don't change what isn't broken": the current tracing mode problems are caused by a requirement that's specific to tracing mode (support for external rebinding of function local variable references), so it made sense to also restrict any related fixes to tracing mode However, actually attempting to implement and document that dynamic approach highlighted the fact that it makes for a really subtle runtime state dependent behaviour distinction in how ``frame.f_locals`` works, and creates several new edge cases around how ``f_locals`` behaves as trace functions are added and removed. Accordingly, the design was switched to the current one, where ``frame.f_locals`` is always a write-through proxy, and ``locals()`` is always a snapshot, which is both simpler to implement and easier to explain. Regardless of how the CPython reference implementation chooses to handle this, optimising compilers and interpreters also remain free to impose additional restrictions on debuggers, such as making local variable mutation through frame objects an opt-in behaviour that may disable some optimisations (just as the emulation of CPython's frame API is already an opt-in flag in some Python implementations). Continuing to support storing additional data on optimised frames ----------------------------------------------------------------- One of the draft iterations of this PEP proposed removing the ability to store additional data on optimised frames by writing to ``frame.f_locals`` keys that didn't correspond to local or closure variable names on the underlying frame. While this idea offered some attractive simplification of the fast locals proxy implementation, ``pdb`` stores ``__return__`` and ``__exception__`` values on arbitrary frames, so the standard library test suite fails if that functionality no longer works. Accordingly, the ability to store arbitrary keys was retained, at the expense of certain operations on proxy objects currently either being slower than desired (as they need to update the dynamic snapshot in order to provide correct behaviour), or else assuming that the cache is currently up to date (and hence potentially giving an incorrect answer if the frame state has changed in a way that doesn't automatically update the cache contents). It is expected that the exact details of the interaction between the fast locals proxy and the ``f_locals`` value cache on the underlying frame will evolve over time as opportunities for improvement are identified. Historical semantics at function scope -------------------------------------- The current semantics of mutating ``locals()`` and ``frame.f_locals`` in CPython are rather quirky due to historical implementation details: * actual execution uses the fast locals array for local variable bindings and cell references for nonlocal variables * there's a ``PyFrame_FastToLocals`` operation that populates the frame's ``f_locals`` attribute based on the current state of the fast locals array and any referenced cells. This exists for three reasons: * allowing trace functions to read the state of local variables * allowing traceback processors to read the state of local variables * allowing ``locals()`` to read the state of local variables * a direct reference to ``frame.f_locals`` is returned from ``locals()``, so if you hand out multiple concurrent references, then all those references will be to the exact same dictionary * the two common calls to the reverse operation, ``PyFrame_LocalsToFast``, were removed in the migration to Python 3: ``exec`` is no longer a statement (and hence can no longer affect function local namespaces), and the compiler now disallows the use of ``from module import *`` operations at function scope * however, two obscure calling paths remain: ``PyFrame_LocalsToFast`` is called as part of returning from a trace function (which allows debuggers to make changes to the local variable state), and you can also still inject the ``IMPORT_STAR`` opcode when creating a function directly from a code object rather than via the compiler This proposal deliberately *doesn't* formalise these semantics as is, since they only make sense in terms of the historical evolution of the language and the reference implementation, rather than being deliberately designed. Proposing several additions to the stable C API/ABI --------------------------------------------------- Historically, the CPython C API (and subsequently, the stable ABI) has exposed only a single API function related to the Python ``locals`` builtin: ``PyEval_GetLocals()``. However, as it returns a borrowed reference, it is not possible to adapt that interface directly to supporting the new ``locals()`` semantics proposed in this PEP. An earlier iteration of this PEP proposed a minimalist adaptation to the new semantics: one C API function that behaved like the Python ``locals()`` builtin, and another that behaved like the ``frame.f_locals`` descriptor (creating and returning the write-through proxy if necessary). The feedback [8]_ on that version of the C API was that it was too heavily based on how the Python level semantics were implemented, and didn't account for the behaviours that authors of C extensions were likely to *need*. The broader API now being proposed came from grouping the potential reasons for wanting to access the Python ``locals()`` namespace from an extension module into the following cases: * needing to exactly replicate the semantics of the Python level ``locals()`` operation. This is the ``PyLocals_Get()`` API. * needing to behave differently depending on whether writes to the result of ``PyLocals_Get()`` will be visible to Python code or not. This is handled by the ``PyLocals_GetReturnsCopy()`` query API. * always wanting a mutable namespace that has been pre-populated from the current Python ``locals()`` namespace, but *not* wanting any changes to be visible to Python code. This is the ``PyLocals_GetCopy()`` API. * always wanting a read-only view of the current locals namespace, without incurring the runtime overhead of making a full copy each time. This is the ``PyLocals_GetView()`` API. Historically, these kinds of checks and operations would only have been possible if a Python implementation emulated the full CPython frame API. With the proposed API, extension modules can instead ask more clearly for the semantics that they actually need, giving Python implementations more flexibility in how they provide those capabilities. Implementation ============== The reference implementation update is in development as a draft pull request on GitHub ([6]_). Acknowledgements ================ Thanks to Nathaniel J. Smith for proposing the write-through proxy idea in [1]_ and pointing out some critical design flaws in earlier iterations of the PEP that attempted to avoid introducing such a proxy. Thanks to Steve Dower and Petr Viktorin for asking that more attention be paid to the developer experience of the proposed C API additions [8]_. Thanks to Mark Shannon for pushing for further simplification of the C level API and semantics (and restarting discussion on the PEP in early 2021 after a few years of inactivity). References ========== .. [1] Broken local variable assignment given threads + trace hook + closure (https://bugs.python.org/issue30744) .. [2] Clarify the required behaviour of ``locals()`` (https://bugs.python.org/issue17960) .. [3] Updating function local variables from pdb is unreliable (https://bugs.python.org/issue9633) .. [4] CPython's Python API for installing trace hooks (https://docs.python.org/dev/library/sys.html#sys.settrace) .. [5] CPython's C API for installing trace hooks (https://docs.python.org/3/c-api/init.html#c.PyEval_SetTrace) .. [6] PEP 558 reference implementation (https://github.com/python/cpython/pull/3640/files) .. [7] Nathaniel's review of possible function level semantics for locals() (https://mail.python.org/pipermail/python-dev/2019-May/157738.html) .. [8] Discussion of more intentionally designed C API enhancements (https://discuss.python.org/t/pep-558-defined-semantics-for-locals/2936/3) .. [9] Disable automatic update of frame locals during tracing (https://bugs.python.org/issue42197) Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia