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PEP 558: Fix mention of old locals() semantics #1266

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Jan 1, 2020
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PEP 558: Fix mention of old locals() semantics #1266

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PEP 558: Fix mention of old locals() semantics
ncoghlan Jan 1, 2020
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Add missing full stop
ncoghlan Jan 1, 2020
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13 changes: 8 additions & 5 deletions pep-0558.rst
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Expand Up @@ -289,18 +289,21 @@ 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 dynamic snapshot 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 either the Python or public C API:
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.

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