"sys.monitoring" --- Execution event monitoring
***********************************************

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

Note:

  "sys.monitoring" is a namespace within the "sys" module, not an
  independent module, so there is no need to "import sys.monitoring",
  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 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.


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

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

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

sys.monitoring.free_tool_id(id: int) -> None

   Should be called once a tool no longer requires "id".

sys.monitoring.get_tool(id: int) -> str | None

   Returns the name of the tool if "id" is in use, otherwise it
   returns "None". "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

There is no obligation to set an ID, nor is there anything preventing
a tool from using an ID even it is already in use. However, tools are
encouraged to use a unique ID and respect other tools.


Events
======

The following events are supported:

BRANCH
   A conditional branch is taken (or not).

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

C_RAISE
   Exception raised from any callable, except Python functions (event
   occurs after the exit).

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

EXCEPTION_HANDLED
   An exception is handled.

INSTRUCTION
   A VM instruction is about to be executed.

JUMP
   An unconditional jump in the control flow graph is made.

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

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

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

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

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

PY_UNWIND
   Exit from a Python function during exception unwinding.

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

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

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

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".

Events are divided into three groups:


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

* STOP_ITERATION


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 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.

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".


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

In order to monitor an event, it must be turned on and a callback
registered. Events can be turned on or off by setting the events
either globally or for a particular code object.


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)

   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.

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)

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

Local events add to global events, but do not mask them. In other
words, all global events will trigger for a code object, regardless of
the local events.


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

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.


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

To register a callable for events call

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".

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

Callback functions can be registered and unregistered at any time.

Registering or unregistering a callback function will generate a
"sys.audit" event.


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

When an active event occurs, the registered callback function is
called. Different events will provide the callback function with
different arguments, as follows:

* "PY_START" and "PY_RESUME":

     func(code: CodeType, instruction_offset: int) -> DISABLE | Any

* "PY_RETURN" and "PY_YIELD":

     "func(code: CodeType, instruction_offset: int, retval: object) ->
     DISABLE | Any"

* "CALL", "C_RAISE" and "C_RETURN":

     "func(code: CodeType, instruction_offset: int, callable: object,
     arg0: object | MISSING) -> DISABLE | Any"

     If there are no arguments, "arg0" is set to "MISSING".

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

     "func(code: CodeType, instruction_offset: int, exception:
     BaseException) -> DISABLE | Any"

* "LINE":

     "func(code: CodeType, line_number: int) -> DISABLE | Any"

* "BRANCH" and "JUMP":

     "func(code: CodeType, instruction_offset: int,
     destination_offset: int) -> DISABLE | Any"

  Note that the "destination_offset" is where the code will next
  execute. For an untaken branch this will be the offset of the
  instruction following the branch.

* "INSTRUCTION":

     "func(code: CodeType, instruction_offset: int) -> DISABLE | Any"
