Allow limited PEP 523 plugins. (#2822)

pep-0669.rst

@@ -291,42 +291,35 @@ Backwards Compatibility
 
 This PEP is mostly backwards compatible.
 
-This PEP is incompatible with :pep:`523` as the behavior would be undefined,
-since the VM has no control over the behavior of :pep:`523` plugins.
+There are some compatibility issues with :pep:`523`, as the behavior
+of :pep:`523` plugins is outside of the VM's control.
+It is up to :pep:`523` plugins to ensure that they respect the semantics
+of this PEP. Simple plugins that do not change the state of the VM, and
+defer execution to ``_PyEval_EvalFrameDefault()`` should continue to work.
 
-Thus, if ``_PyInterpreterState_SetEvalFrameFunc()`` has been called and has
-changed the frame evaluation function, then calling
-``sys.monitoring.set_events()``, ``sys.monitoring.set_local_events()``, or
-``sys.monitoring.insert_marker`` will raise an exception.
+:func:`sys.settrace` and :func:`sys.setprofile` will act as if they were tools
+6 and 7 respectively, so can be used along side this PEP.
 
-Likewise, if ``sys.monitoring.set_events()``,
-``sys.monitoring.set_local_events()``, or ``sys.monitoring.insert_marker``
-has been called, then calling ``_PyInterpreterState_SetEvalFrameFunc()``
-will raise an exception.
-
-``sys.settrace`` and ``sys.setprofile`` will act as if they were tools 6 and 7
-respectively, so can be used along side this PEP.
-
-This makes ``sys.settrace`` and ``sys.setprofile`` incompatible with :pep:`523`.
-Arguably, they already were as the author does not know of any PEP 523 plugin
-that supports ``sys.settrace`` or ``sys.setprofile`` correctly.
-This PEP merely formalizes that.
+This means that :func:`sys.settrace` and :func:`sys.setprofile` may not work
+correctly with all :pep:`523` plugins. Although, simple :pep:`523`
+plugins, as described above, should be fine.
 
 Performance
 -----------
 
 If no events are active, this PEP should have a small positive impact on
 performance. Experiments show between 1 and 2% speedup from not supporting
-``sys.settrace()`` directly.
+:func:`sys.settrace` directly.
 
-The performance of ``sys.settrace()`` will be worse.
-The performance of ``sys.setprofile()`` should be better.
-However, tools relying on ``sys.settrace()`` and ``sys.setprofile()`` 
-can be made a lot faster by using the API provided by this PEP.
+The performance of :func:`sys.settrace` will be worse.
+The performance of :func:`sys.setprofile` should be better.
+However, tools relying on :func:`sys.settrace` and
+:func:`sys.setprofile` can be made a lot faster by using the
+API provided by this PEP.
 
 If a small set of events are active, e.g. for a debugger, then the overhead
-of callbacks will be orders of magnitudes less than for ``sys.settrace`` and 
-much cheaper than using :pep:`523`.
+of callbacks will be orders of magnitudes less than for :func:`sys.settrace`
+and much cheaper than using :pep:`523`.
 
 Coverage tools can be implemented at very low cost,
 by returning ``DISABLE`` in all callbacks.