GH-91052: Add C API for watching dictionaries (GH-31787)

Doc/c-api/dict.rst

@@ -238,3 +238,54 @@ Dictionary Objects
           for key, value in seq2:
               if override or key not in a:
                   a[key] = value
+
+.. c:function:: int PyDict_AddWatcher(PyDict_WatchCallback callback)
+
+   Register *callback* as a dictionary watcher. Return a non-negative integer
+   id which must be passed to future calls to :c:func:`PyDict_Watch`. In case
+   of error (e.g. no more watcher IDs available), return ``-1`` and set an
+   exception.
+
+.. c:function:: int PyDict_ClearWatcher(int watcher_id)
+
+   Clear watcher identified by *watcher_id* previously returned from
+   :c:func:`PyDict_AddWatcher`. Return ``0`` on success, ``-1`` on error (e.g.
+   if the given *watcher_id* was never registered.)
+
+.. c:function:: int PyDict_Watch(int watcher_id, PyObject *dict)
+
+   Mark dictionary *dict* as watched. The callback granted *watcher_id* by
+   :c:func:`PyDict_AddWatcher` will be called when *dict* is modified or
+   deallocated.
+
+.. c:type:: PyDict_WatchEvent
+
+   Enumeration of possible dictionary watcher events: ``PyDict_EVENT_ADDED``,
+   ``PyDict_EVENT_MODIFIED``, ``PyDict_EVENT_DELETED``, ``PyDict_EVENT_CLONED``,
+   ``PyDict_EVENT_CLEARED``, or ``PyDict_EVENT_DEALLOCATED``.
+
+.. c:type:: int (*PyDict_WatchCallback)(PyDict_WatchEvent event, PyObject *dict, PyObject *key, PyObject *new_value)
+
+   Type of a dict watcher callback function.
+
+   If *event* is ``PyDict_EVENT_CLEARED`` or ``PyDict_EVENT_DEALLOCATED``, both
+   *key* and *new_value* will be ``NULL``. If *event* is ``PyDict_EVENT_ADDED``
+   or ``PyDict_EVENT_MODIFIED``, *new_value* will be the new value for *key*.
+   If *event* is ``PyDict_EVENT_DELETED``, *key* is being deleted from the
+   dictionary and *new_value* will be ``NULL``.
+
+   ``PyDict_EVENT_CLONED`` occurs when *dict* was previously empty and another
+   dict is merged into it. To maintain efficiency of this operation, per-key
+   ``PyDict_EVENT_ADDED`` events are not issued in this case; instead a
+   single ``PyDict_EVENT_CLONED`` is issued, and *key* will be the source
+   dictionary.
+
+   The callback may inspect but must not modify *dict*; doing so could have
+   unpredictable effects, including infinite recursion.
+
+   Callbacks occur before the notified modification to *dict* takes place, so
+   the prior state of *dict* can be inspected.
+
+   If the callback returns with an exception set, it must return ``-1``; this
+   exception will be printed as an unraisable exception using
+   :c:func:`PyErr_WriteUnraisable`. Otherwise it should return ``0``.