it is unregistered and returned.
Otherwise :func:`register_callback` returns ``None``.
+ .. audit-event:: sys.monitoring.register_callback func sys.monitoring.register_callback
+
Functions can be unregistered by calling
``sys.monitoring.register_callback(tool_id, event, None)``.
and local events by your code then the callback needs to be written to handle
either trigger.
-Registering or unregistering a callback function will generate a :func:`sys.audit` event.
-
Callback function arguments
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