gh-124872: Replace enter/exit events with "switched" (#125532)

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.

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).

Co-authored-by: Richard Hansen <rhansen@rhansen.org>
Co-authored-by: Victor Stinner <vstinner@python.org>

1 files changed, 4 insertions, 13 deletions

diff --git a/Include/cpython/context.h b/Include/cpython/context.h
index 3c9be78..3a7a4b4 100644
--- a/Include/cpython/context.h
+++ b/Include/cpython/context.h
@@ -29,20 +29,11 @@ PyAPI_FUNC(int) PyContext_Exit(PyObject *);
 
 typedef enum {
     /*
-     * A context has been entered, causing the "current context" to switch to
-     * it.  The object passed to the watch callback is the now-current
-     * contextvars.Context object.  Each enter event will eventually have a
-     * corresponding exit event for the same context object after any
-     * subsequently entered contexts have themselves been exited.
+     * The current context has switched to a different context.  The object
+     * passed to the watch callback is the now-current contextvars.Context
+     * object, or None if no context is current.
      */
-    Py_CONTEXT_EVENT_ENTER,
-    /*
-     * A context is about to be exited, which will cause the "current context"
-     * to switch back to what it was before the context was entered.  The
-     * object passed to the watch callback is the still-current
-     * contextvars.Context object.
-     */
-    Py_CONTEXT_EVENT_EXIT,
+    Py_CONTEXT_SWITCHED = 1,
 } PyContextEvent;
 
 /*