"sys.monitoring" — Execution event monitoring
*********************************************

Added in version 3.12.

======================================================================

Note:

  "sys.monitoring" is a namespace within the "sys" module, not an
  independent module, and "import sys.monitoring" would fail with a
  "ModuleNotFoundError". Instead, simply "import sys" and then use
  "sys.monitoring".

This namespace provides access to the functions and constants
necessary to activate and control event monitoring.

As programs execute, events occur that might be of interest to tools
that monitor execution. The "sys.monitoring" namespace provides means
to receive callbacks when events of interest occur.

The monitoring API consists of three components:

* Tool identifiers

* Events

* Callbacks


Tool identifiers
================

A tool identifier is an integer and the associated name. Tool
identifiers are used to discourage tools from interfering with each
other and to allow multiple tools to operate at the same time.
Currently tools are completely independent and cannot be used to
monitor each other. This restriction may be lifted in the future.

Before registering or activating events, a tool should choose an
identifier. Identifiers are integers in the range 0 to 5 inclusive.


Registering and using tools
---------------------------

sys.monitoring.use_tool_id(tool_id: int, name: str, /) -> None

   Must be called before *tool_id* can be used. *tool_id* must be in
   the range 0 to 5 inclusive. Raises a "ValueError" if *tool_id* is
   in use.

sys.monitoring.clear_tool_id(tool_id: int, /) -> None

   Unregister all events and callback functions associated with
   *tool_id*.

sys.monitoring.free_tool_id(tool_id: int, /) -> None

   Should be called once a tool no longer requires *tool_id*. Will
   call "clear_tool_id()" before releasing *tool_id*.

sys.monitoring.get_tool(tool_id: int, /) -> str | None

   Returns the name of the tool if *tool_id* is in use, otherwise it
   returns "None". *tool_id* must be in the range 0 to 5 inclusive.

All IDs are treated the same by the VM with regard to events, but the
following IDs are pre-defined to make co-operation of tools easier:

   sys.monitoring.DEBUGGER_ID = 0
   sys.monitoring.COVERAGE_ID = 1
   sys.monitoring.PROFILER_ID = 2
   sys.monitoring.OPTIMIZER_ID = 5


Events
======

The following events are supported:

sys.monitoring.events.BRANCH_LEFT

   A conditional branch goes left.

   It is up to the tool to determine how to present “left” and “right”
   branches. There is no guarantee which branch is “left” and which is
   “right”, except that it will be consistent for the duration of the
   program.

sys.monitoring.events.BRANCH_RIGHT

   A conditional branch goes right.

sys.monitoring.events.CALL

   A call in Python code (event occurs before the call).

sys.monitoring.events.C_RAISE

   An exception raised from any callable, except for Python functions
   (event occurs after the exit).

sys.monitoring.events.C_RETURN

   Return from any callable, except for Python functions (event occurs
   after the return).

sys.monitoring.events.EXCEPTION_HANDLED

   An exception is handled.

sys.monitoring.events.INSTRUCTION

   A VM instruction is about to be executed.

sys.monitoring.events.JUMP

   An unconditional jump in the control flow graph is made.

sys.monitoring.events.LINE

   An instruction is about to be executed that has a different line
   number from the preceding instruction.

sys.monitoring.events.PY_RESUME

   Resumption of a Python function (for generator and coroutine
   functions), except for "throw()" calls.

sys.monitoring.events.PY_RETURN

   Return from a Python function (occurs immediately before the
   return, the callee’s frame will be on the stack).

sys.monitoring.events.PY_START

   Start of a Python function (occurs immediately after the call, the
   callee’s frame will be on the stack)

sys.monitoring.events.PY_THROW

   A Python function is resumed by a "throw()" call.

sys.monitoring.events.PY_UNWIND

   Exit from a Python function during exception unwinding. This
   includes exceptions raised directly within the function and that
   are allowed to continue to propagate.

sys.monitoring.events.PY_YIELD

   Yield from a Python function (occurs immediately before the yield,
   the callee’s frame will be on the stack).

sys.monitoring.events.RAISE

   An exception is raised, except those that cause a "STOP_ITERATION"
   event.

sys.monitoring.events.RERAISE

   An exception is re-raised, for example at the end of a "finally"
   block.

sys.monitoring.events.STOP_ITERATION

   An artificial "StopIteration" is raised; see the STOP_ITERATION
   event.

More events may be added in the future.

These events are attributes of the "sys.monitoring.events" namespace.
Each event is represented as a power-of-2 integer constant. To define
a set of events, simply bitwise OR the individual events together. For
example, to specify both "PY_RETURN" and "PY_START" events, use the
expression "PY_RETURN | PY_START".

sys.monitoring.events.NO_EVENTS

   An alias for "0" so users can do explicit comparisons like:

      if get_events(DEBUGGER_ID) == NO_EVENTS:
          ...

   Setting this event deactivates all events.


Local events
------------

Local events are associated with normal execution of the program and
happen at clearly defined locations. All local events can be disabled.
The local events are:

* "PY_START"

* "PY_RESUME"

* "PY_RETURN"

* "PY_YIELD"

* "CALL"

* "LINE"

* "INSTRUCTION"

* "JUMP"

* "BRANCH_LEFT"

* "BRANCH_RIGHT"

* "STOP_ITERATION"


Deprecated event
----------------

* "BRANCH"

The "BRANCH" event is deprecated in 3.14. Using "BRANCH_LEFT" and
"BRANCH_RIGHT" events will give much better performance as they can be
disabled independently.


Ancillary events
----------------

Ancillary events can be monitored like other events, but are
controlled by another event:

* "C_RAISE"

* "C_RETURN"

The "C_RETURN" and "C_RAISE" events are controlled by the "CALL"
event. "C_RETURN" and "C_RAISE" events will only be seen if the
corresponding "CALL" event is being monitored.


Other events
------------

Other events are not necessarily tied to a specific location in the
program and cannot be individually disabled via "DISABLE".

The other events that can be monitored are:

* "PY_THROW"

* "PY_UNWIND"

* "RAISE"

* "EXCEPTION_HANDLED"


The STOP_ITERATION event
------------------------

**PEP 380** specifies that a "StopIteration" exception is raised when
returning a value from a generator or coroutine. However, this is a
very inefficient way to return a value, so some Python
implementations, notably CPython 3.12+, do not raise an exception
unless it would be visible to other code.

To allow tools to monitor for real exceptions without slowing down
generators and coroutines, the "STOP_ITERATION" event is provided.
"STOP_ITERATION" can be locally disabled, unlike "RAISE".

Note that the "STOP_ITERATION" event and the "RAISE" event for a
"StopIteration" exception are equivalent, and are treated as
interchangeable when generating events. Implementations will favor
"STOP_ITERATION" for performance reasons, but may generate a "RAISE"
event with a "StopIteration".


Turning events on and off
=========================

In order to monitor an event, it must be turned on and a corresponding
callback must be registered. Events can be turned on or off by setting
the events either globally and/or for a particular code object. An
event will trigger only once, even if it is turned on both globally
and locally.


Setting events globally
-----------------------

Events can be controlled globally by modifying the set of events being
monitored.

sys.monitoring.get_events(tool_id: int, /) -> int

   Returns the "int" representing all the active events.

sys.monitoring.set_events(tool_id: int, event_set: int, /) -> None

   Activates all events which are set in *event_set*. Raises a
   "ValueError" if *tool_id* is not in use.

No events are active by default.


Per code object events
----------------------

Events can also be controlled on a per code object basis. The
functions defined below which accept a "types.CodeType" should be
prepared to accept a look-alike object from functions which are not
defined in Python (see Monitoring C API).

sys.monitoring.get_local_events(tool_id: int, code: CodeType, /) -> int

   Returns all the local events for *code*

sys.monitoring.set_local_events(tool_id: int, code: CodeType, event_set: int, /) -> None

   Activates all the local events for *code* which are set in
   *event_set*. Raises a "ValueError" if *tool_id* is not in use.


Disabling events
----------------

sys.monitoring.DISABLE

   A special value that can be returned from a callback function to
   disable events for the current code location.

Local events can be disabled for a specific code location by returning
"sys.monitoring.DISABLE" from a callback function. This does not
change which events are set, or any other code locations for the same
event.

Disabling events for specific locations is very important for high
performance monitoring. For example, a program can be run under a
debugger with no overhead if the debugger disables all monitoring
except for a few breakpoints.

If "DISABLE" is returned by a callback for a global event,
"ValueError" will be raised by the interpreter in a non-specific
location (that is, no traceback will be provided).

sys.monitoring.restart_events() -> None

   Enable all the events that were disabled by
   "sys.monitoring.DISABLE" for all tools.


Registering callback functions
==============================

sys.monitoring.register_callback(tool_id: int, event: int, func: Callable | None, /) -> Callable | None

   Registers the callable *func* for the *event* with the given
   *tool_id*

   If another callback was registered for the given *tool_id* and
   *event*, it is unregistered and returned. Otherwise
   "register_callback()" returns "None".

   Raises an auditing event "sys.monitoring.register_callback" with
   argument "func".

Functions can be unregistered by calling
"sys.monitoring.register_callback(tool_id, event, None)".

Callback functions can be registered and unregistered at any time.

Callbacks are called only once regardless if the event is turned on
both globally and locally. As such, if an event could be turned on for
both global and local events by your code then the callback needs to
be written to handle either trigger.


Callback function arguments
---------------------------

sys.monitoring.MISSING

   A special value that is passed to a callback function to indicate
   that there are no arguments to the call.

When an active event occurs, the registered callback function is
called. Callback functions returning an object other than "DISABLE"
will have no effect. Different events will provide the callback
function with different arguments, as follows:

* "PY_START" and "PY_RESUME":

     func(code: CodeType, instruction_offset: int) -> object

* "PY_RETURN" and "PY_YIELD":

     func(code: CodeType, instruction_offset: int, retval: object) -> object

* "CALL", "C_RAISE" and "C_RETURN" (*arg0* can be "MISSING"
  specifically):

     func(code: CodeType, instruction_offset: int, callable: object, arg0: object) -> object

  *code* represents the code object where the call is being made,
  while *callable* is the object that is about to be called (and thus
  triggered the event). If there are no arguments, *arg0* is set to
  "sys.monitoring.MISSING".

  For instance methods, *callable* will be the function object as
  found on the class with *arg0* set to the instance (i.e. the "self"
  argument to the method).

* "RAISE", "RERAISE", "EXCEPTION_HANDLED", "PY_UNWIND", "PY_THROW" and
  "STOP_ITERATION":

     func(code: CodeType, instruction_offset: int, exception: BaseException) -> object

* "LINE":

     func(code: CodeType, line_number: int) -> object

* "BRANCH_LEFT", "BRANCH_RIGHT" and "JUMP":

     func(code: CodeType, instruction_offset: int, destination_offset: int) -> object

  Note that the *destination_offset* is where the code will next
  execute.

* "INSTRUCTION":

     func(code: CodeType, instruction_offset: int) -> object
