Thanks, Nick! This looks wonderful.
I do have a nitpick, below:
On 18. 07. 21 7:59, Nick Coghlan wrote:
[...]>
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.
Since this goes in the stable ABI, I'm thinking about how extensible
this will be in the future.
The proposal assumes that in the future, ``PyLocals_Get``, and thus
``locals()``, will never gain another kind of return value, however
unlikely that is.
AFAICS, code that uses this will usually check for a single special case
and fall back (or error) for the other(s), so I think it'd be reasonable
to make this an "enum" with two values. e.g.:
int PyLocals_GetReturnBehavior(); # better name?
#define PyLocals_DIRECT_REFERENCE 0
#define PyLocals_SHALLOW_COPY 1
Other values may be added in future versions of Python, if/when the
Python ``locals()`` builtin is changed to return a different kind of value.
(and same for PyFrame_GetLocalsReturnsCopy)
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