aio-libs / multidict / 35975771385

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Keep the implicit None default of setdefault() in the module state (#1553)

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## What do these changes do?

`setdefault()` called without an explicit default built the implicit
`None` on every call, then carried a flag to release it again, although
only the call that actually inserts consumes it. `md_set_default()` now
reads a `NULL` *value* as `None`, so the method simply forwards what it
parsed and the flag, its branch and the `Py_CLEAR()` all go away.

The constant lives in the module state rather than coming from
`Py_GetConstant()` at the point of use, because the substitution happens
just before the critical section is taken. On 3.13+ either would do; on
3.10 to 3.12 the `pythoncapi_compat` shim initialises its table lazily
and calls `PyLong_FromLong()`, so a first call landing under the lock
could collect.

Instruction counts against `cd92a6c`, callgrind, 200k calls:

| case | master | this branch | |
| --- | --- | --- | --- |
| `d.setdefault(key)`, present key | 491,825,979 | 486,878,820 | -1.01%
|
| `d.setdefault(key, default)`, present key | 494,626,107 | 493,681,201
| -0.19% |
| `d.setdefault(key)`, inserting | 290,700,236 | 290,812,020 | +0.04% |

The inserting call is unchanged, as expected: it still stores the same
constant, just one that was not built for the occasion.

Only the C extension is touched; the pure-Python implementation is
unchanged.

## Are there changes in behavior for the user?

Not at the Python level; `setdefault()` returns and stores exactly what
it did before.

The public C API gains a little: `MultiDict_SetDefault()` forwards
straight to `md_set_default()`, so it now accepts `NULL` for
*default_value* and reads it as `None`, where the documentation
previously called the argument required. `docs/capi.rst` says so, and
both `_testcapi.c` and `_testcyapi.pyx` exercise it.

`MultiDict_CAPI_VERSION` stays at 1 deliberately. It exists to stop a
client built against a newer header fr... (continued)

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