gh-119333, gh-99633: Replace enter/exit events with "switched" #124776
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The PyContext struct is not intended to be public, and users of the API don't need anything more specific than PyObject. Also see pythongh-78943.
…back I believe that the value of a simpler API (and defense against poorly written callbacks) outweighs the cost of backing up and restoring the thread's exception state.
The exception is ignored so change the return type from `int` to `void` to discourage callbacks from raising an exception in the first place.
* Add definitions for "context", "current context", and "context
management protocol".
* Update related definitions to be consistent with the new
definitions.
* Restructure the documentation for the `contextvars.Context` class
to prepare for adding context manager support, and for consistency
with the definitions.
* Use `testcode` and `testoutput` to test the `Context.run` example.
* Expand the documentation for the `Py_CONTEXT_EVENT_ENTER` and
`Py_CONTEXT_EVENT_EXIT` events to clarify and to prepare for
planned changes.
This was referenced Sep 30, 2024
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…tchCallback I changed my mind about where to back up and restore the exception. Instead of backing up and restoring once outside the loop over the registered callbacks, the exception is backed up and restored each time a callback is called. This avoids concerns about what happens if a callback raises an exception but returns non-negative. This is also expected to be more efficient, not less, because the common case is no watchers at all, in which case there is no backup/restore overhead. The second most common case is one watcher, which is no less efficient than before. The only time this is more expensive is when there are two or more watchers, in which case the overhead of the watcher callbacks probably dwarfs the overhead of the extra backups anyways.
…ched" Users want to know when the current context switches to a different context object. Right now this happens when and only when a context is entered or exited, so the enter and exit events are synonymous with "switched". However, if the changes proposed for pythongh-99633 are implemented, the current context will also switch for reasons other than context enter or exit. Since users actually care about context switches and not enter or exit, replace the enter and exit events with a single switched event. The former exit event was emitted just before exiting the context. The new switched event is emitted after the context is exited to match the semantics users expect of an event with a past-tense name. If users need the ability to clean up before the switch takes effect, another event type can be added in the future. It is not added here because YAGNI. I skipped 0 in the enum as a matter of practice. Skipping 0 makes it easier to troubleshoot when code forgets to set zeroed memory, and it aligns with best practices for other tools (e.g., https://protobuf.dev/programming-guides/dos-donts/#unspecified-enum).
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Users want to know when the current context switches to a different context object. Right now this happens when and only when a context is entered or exited, so the enter and exit events are synonymous with "switched". However, if the changes proposed for gh-99633 are implemented, the current context will also switch for reasons other than context enter or exit. Since users actually care about context switches and not enter or exit, replace the enter and exit events with a single switched event.
The former exit event was emitted just before exiting the context. The new switched event is emitted after the context is exited to match the semantics users expect of an event with a past-tense name. If users need the ability to clean up before the switch takes effect, another event type can be added in the future. It is not added here because YAGNI.
This commit amends a feature that is new to v3.14 so I did not include a NEWS blurb (the existing blurb from #119335 should suffice).
This PR is marked as draft because it is based on commits from pull requests that have not yet been merged:
Once those are merged I'll rebase this and mark it as ready for review. Until then, only the tip commit is relevant to this PR.
cc @fried
contextvars.Context#99633📚 Documentation preview 📚: https://cpython-previews--124776.org.readthedocs.build/