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gh-127124: Pass optional state to context watcher callback - #127140

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rhansen:gh-127124-context-watcher-opaque-state
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gh-127124: Pass optional state to context watcher callback#127140
rhansen wants to merge 3 commits into
python:mainfrom
rhansen:gh-127124-context-watcher-opaque-state

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@rhansen

@rhansenrhansen commented Nov 22, 2024

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This enables users to associate state with the callback without relying on globals.

Also:

  • Refactor the tests for improved readability and extensibility, and to cover the new state object.

  • Drop the pointer from the PyContext_WatchCallback typedef. This de-obfuscates the fact that pointers are involved, and makes it possible to forward-declare functions to improve readability:

    staticPyContext_WatchCallbackmy_callback;
    intmy_callback(PyObject*cbarg, PyContextEventevent, PyObject*obj)
    {
    ...
    }

This will conflict with #124741; if one is merged I'll rebase the other.


📚 Documentation preview 📚: https://cpython-previews--127140.org.readthedocs.build/

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Needs a NEWS entry (even though it's a change to a new feature, we want to show that we changed it in the a3 changelog)

@rhansen
rhansenforce-pushed the gh-127124-context-watcher-opaque-state branch from f6ffc28 to 3130a2fCompareNovember 24, 2024 08:37
@rhansenrhansen changed the title gh-127124: Add opaque pointer to context watcher callbackgh-127124: Pass optional state to context watcher callbackNov 24, 2024

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I'll open a separate PR for the callable idea I mentioned in #127124 (comment) so that we can compare the two and decide which we like better.

Edit: Done; see #127247.

PyContext_WatchCallback context_watchers[CONTEXT_MAX_WATCHERS];
struct {
PyContext_WatchCallback *callback;
PyObject *arg;

@rhansenrhansenNov 24, 2024

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Does this need a Py_VISIT somewhere? I didn't see any for the other fields in this struct, but maybe the GC does something special with this struct?

This enables users to associate state with the callback without
relying on globals.
Also:
* Refactor the tests for improved readability and extensibility, and
to cover the new state object.
* Drop the pointer from the `PyContext_WatchCallback` typedef. This
de-obfuscates the fact that pointers are involved, and makes it
possible to forward-declare functions to improve readability:
static PyContext_WatchCallback my_callback;
int
my_callback(PyObject *cbarg, PyContextEvent event, PyObject *obj)
{
...
}
@rhansen
rhansenforce-pushed the gh-127124-context-watcher-opaque-state branch from 3130a2f to c9e95d4CompareNovember 24, 2024 09:16
@ZeroIntensity

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FWIW, no need to force push changes--it makes reviewing harder. It's fine to just commit normally, and then everything gets squashed.

@rhansen

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Force pushing is the only way to update the commit message.

@ZeroIntensity

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Yes, but it makes reviewing more difficult. FWIW, nobody really cares about the commit messages in the PR itself, the end message that gets squashed is really what matters. (There's a section in the devguide about this, IIRC.)

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This PR is stale because it has been open for 30 days with no activity.

@github-actionsgithub-actionsBot added the stale Stale PR or inactive for long period of time. label Apr 19, 2026
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@rhansen@ZeroIntensity