Open
Description
Feature or enhancement
Proposal:
I would like to change the Context watcher C API (new to v3.14, see #119333) to pass an opaque pointer to the callback. Specifically, change:
cpython/Include/cpython/context.h
Lines 39 to 55 in e8bb053
to:
/*
* Context object watcher callback function. arg is the same pointer passed to
* PyContext_AddWatcher when the callback was registered. The object passed to
* the callback is event-specific; see PyContextEvent for details.
*
* if the callback returns with an exception set, it must return -1. Otherwise
* it should return 0
*/
typedef int PyContext_WatchCallback(
void *arg, PyContextEvent event, PyObject *obj);
/*
* Register a per-interpreter callback that will be invoked for context events.
* arg is an optional opaque pointer that is passed back to the callback; it
* can be used to manage state if desired.
*
* Returns a handle that may be passed to PyContext_ClearWatcher on success,
* or -1 and sets and error if no more handles are available.
*/
PyAPI_FUNC(int) PyContext_AddWatcher(
PyContext_WatchCallback *callback, void *arg);
The original idea was for the callback to get any required state from the current context, but:
- The config/state is not guaranteed to be available in the context. For example, the Python code can do
ctx = contextvars.Context()
instead ofctx = contextvars.copy_context()
. Or it can docopy_context()
, but early during initialization and squirrel the context away for later use. The latter seems plausible for 3rd party libraries (where the user might not have much control). - The same watcher callback cannot be registered multiple times simultaneously, each with its own config/state. This came up while I was refactoring
TestContextObjectWatchers
to add some more tests for a change I’m working on.
I’m not sure how likely either is to come up in normal use, but I think it is worth changing the design before the 3.14 release cements it.
Has this already been discussed elsewhere?
I have already discussed this feature proposal on Discourse
Links to previous discussion of this feature:
https://discuss.python.org/t/v3-14a1-design-limitations-of-pycontext-addwatcher/68177/4
cc @fried