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PEP 558 -- Defined semantics for locals()

PEP:558
Title:Defined semantics for locals()
Author:Nick Coghlan <ncoghlan at gmail.com>
BDFL-Delegate:Nathaniel J. Smith
Discussions-To:https://discuss.python.org/t/pep-558-defined-semantics-for-locals/2936
Status:Draft
Type:Standards Track
Created:2017-09-08
Python-Version:3.9
Post-History:2017-09-08, 2019-05-22, 2019-05-30, 2019-12-30

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 3.8 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.

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 snapshot 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)

We also allow interpreters to define two "modes" of execution, with only the first mode being considered part of the language specification itself:

  • regular operation: the way the interpreter behaves by default
  • tracing mode: the way the interpreter behaves 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

For regular operation, 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.

For tracing mode, this PEP proposes changes to CPython's behaviour at function scope that make the locals() builtin semantics identical to those used in regular operation, while also making the related frame API semantics clearer and easier for interactive debuggers to rely on.

The proposed tracing mode changes also affect the semantics of frame object references obtained through other means, such as via a traceback, or via the sys._getframe() API.

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 snapshot of the function's local variables and any nonlocal cell references. In this case, changes made via the snapshot 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 created snapshots.

There would also be a versionchanged note for Python 3.9:

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 originally returned snapshot from a freshly updated one.

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.

The semantics at module scope are required to be the same in both tracing mode (if provided by the implementation) and in regular operation.

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.

The semantics at class scope are required to be the same in both tracing mode (if provided by the implementation) and in regular operation.

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 snapshot of the function's local variables and any nonlocal cell references. In this case, changes made via the snapshot 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 created snapshots.

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

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(), frame.f_locals will be updated to instead return a dedicated proxy type (implemented as a private subclass of the existing types.MappingProxyType) that has two internal attributes not exposed as part of the Python runtime API:

  • mapping: an implicitly updated snapshot of the function local variables and closure references, as well as any arbitrary items that have been set via the mapping API, even if they don't have storage allocated for them on the underlying frame
  • frame: the underlying frame that the snapshot is for

For backwards compatibility, the stored snapshot will continue to be made available through the public PyEval_GetLocals() C API.

__getitem__ operations on the proxy will read directly from the stored snapshot.

The stored snapshot is implicitly updated when the f_locals attribute is retrieved from the frame object, as well as individual keys being updated by mutating operations on the proxy itself. This means that if a reference to the proxy is obtained from within the function, the proxy won't implicitly pick up name binding operations that take place as the function executes - the f_locals attribute on the frame will need to be accessed again in order to trigger a refresh.

__setitem__ and __delitem__ operations on the proxy will affect not only the dynamic snapshot, but also the corresponding fast local or cell reference on the underlying frame.

After a frame has finished executing, cell references can still be updated via the proxy, but the link back to the underlying frame is explicitly broken to avoid creating a persistent reference cycle that unexpectedly keeps frames alive.

Other MutableMapping methods will behave as expected for a mapping with these essential method semantics.

Making the behaviour at function scope less surprising

The locals() builtin will be made aware of the new fast locals proxy type, and when it detects it on a frame, will return a fresh snapshot of the local namespace (i.e. the equivalent of dict(frame.f_locals)) rather than returning the proxy directly.

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 dynamic snapshots at function scope. Instead, it will return a borrowed reference to the internal mapping maintained by the fast locals proxy. This shared mapping will behave similarly to the existing shared mapping in Python 3.8 and earlier, but the exact conditions under which it gets refreshed will be different. Specifically:

  • accessing the Python level f_locals frame attribute
  • any call to PyFrame_GetPyLocals() or PyFrame_GetLocalsAttribute() for the frame
  • any call to PyEval_GetLocals(), PyEval_GetPyLocals() or the Python locals() builtin while the frame is running

A new PyFrame_GetPyLocals(frame) API will be provided such that PyFrame_GetPyLocals(PyEval_GetFrame()) directly matches the semantics of the Python locals() builtin, returning a shallow copy of the internal mapping at function scope, rather than a direct reference to it.

A new PyEval_GetPyLocals() API will be provided as a convenience wrapper for the above operation that is suitable for inclusion in the stable ABI.

A new PyFrame_GetLocalsAttribute(frame) API will be provided as the C level equivalent of accessing pyframe.f_locals in Python. Like the Python level descriptor, the new API will implicitly create the write-through proxy object for function level frames if it doesn't already exist, and update the stored mapping to ensure it reflects the current state of the function local variables and closure references.

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 PyFrame_GetPyLocals(frame) or PyFrame_GetLocalsAttribute(frame) instead.

Additions to the stable ABI

The new PyEval_GetPyLocals() API will be added to the stable ABI. The other new C API functions will be part of the CPython specific API only.

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 mutation of local and nonlocal variables in a way that's 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.

However, usage of the C level PyEval_GetLocals() API in the CPython reference implementation will need to be reviewed to determine which cases need to be changed to use the new PyEval_GetPyLocals() API instead.

These changes will also have potential performance implications, especially for functions with large numbers of local variables (e.g. if these functions are called in a loop, calling 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 snapshot on each iteration).

(Note: the reference implementation draft PR has updated the locals() and vars() builtins to use PyEval_GetPyLocals(), but has not yet updated the default local namespace arguments for eval() and exec()).

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).

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.

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.

References

[1](1, 2, 3, 4, 5) 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](1, 2, 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](1, 2) Nathaniel's review of possible function level semantics for locals() (https://mail.python.org/pipermail/python-dev/2019-May/157738.html)
Source: https://github.com/python/peps/blob/master/pep-0558.rst