Doc/library/sys.monitoring.rst
!sys.monitoring --- Execution event monitoring.. module:: sys.monitoring :synopsis: Access and control event monitoring
.. versionadded:: 3.12
.. note::
:mod:`!sys.monitoring` is a namespace within the :mod:`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 :mod:!sys.monitoring namespace provides means to
receive callbacks when events of interest occur.
The monitoring API consists of three components:
Tool identifiers_Events_Callbacks <callbacks>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 '''''''''''''''''''''''''''
.. function:: 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 :exc:ValueError if tool_id is in use.
.. function:: clear_tool_id(tool_id: int, /) -> None
Unregister all events and callback functions associated with tool_id.
.. function:: free_tool_id(tool_id: int, /) -> None
Should be called once a tool no longer requires tool_id.
Will call :func:clear_tool_id before releasing tool_id.
.. function:: 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
The following events are supported:
.. monitoring-event:: 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.
.. monitoring-event:: BRANCH_RIGHT
A conditional branch goes right.
.. monitoring-event:: CALL
A call in Python code (event occurs before the call).
.. monitoring-event:: C_RAISE
An exception raised from any callable, except for Python functions (event occurs after the exit).
.. monitoring-event:: C_RETURN
Return from any callable, except for Python functions (event occurs after the return).
.. monitoring-event:: EXCEPTION_HANDLED
An exception is handled.
.. monitoring-event:: INSTRUCTION
A VM instruction is about to be executed.
.. monitoring-event:: JUMP
An unconditional jump in the control flow graph is made.
.. monitoring-event:: LINE
An instruction is about to be executed that has a different line number from the preceding instruction.
.. monitoring-event:: PY_RESUME
Resumption of a Python function (for generator and coroutine functions), except for throw() calls.
.. monitoring-event:: PY_RETURN
Return from a Python function (occurs immediately before the return, the callee's frame will be on the stack).
.. monitoring-event:: PY_START
Start of a Python function (occurs immediately after the call, the callee's frame will be on the stack)
.. monitoring-event:: PY_THROW
A Python function is resumed by a throw() call.
.. monitoring-event:: 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.
.. monitoring-event:: PY_YIELD
Yield from a Python function (occurs immediately before the yield, the callee's frame will be on the stack).
.. monitoring-event:: RAISE
An exception is raised, except those that cause a :monitoring-event:STOP_ITERATION event.
.. monitoring-event:: RERAISE
An exception is re-raised, for example at the end of a :keyword:finally block.
.. monitoring-event:: STOP_ITERATION
An artificial :exc:StopIteration is raised; see the STOP_ITERATION event_.
More events may be added in the future.
These events are attributes of the :mod:!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 :monitoring-event:PY_RETURN and :monitoring-event:PY_START
events, use the expression PY_RETURN | PY_START.
.. monitoring-event:: 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.
.. _monitoring-event-local:
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_STARTPY_RESUMEPY_RETURNPY_YIELDCALLLINEINSTRUCTIONJUMPBRANCH_LEFTBRANCH_RIGHTSTOP_ITERATIONDeprecated event ''''''''''''''''
BRANCHThe BRANCH event is deprecated in 3.14.
Using :monitoring-event:BRANCH_LEFT and :monitoring-event: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_RAISEC_RETURNThe :monitoring-event:C_RETURN and :monitoring-event:C_RAISE events
are controlled by the :monitoring-event:CALL event.
:monitoring-event:C_RETURN and :monitoring-event:C_RAISE events will only be
seen if the corresponding :monitoring-event:CALL event is being monitored.
.. _monitoring-event-global:
Other events ''''''''''''
Other events are not necessarily tied to a specific location in the
program and cannot be individually disabled via :data:DISABLE.
The other events that can be monitored are:
PY_THROWPY_UNWINDRAISEEXCEPTION_HANDLEDThe STOP_ITERATION event ''''''''''''''''''''''''
:pep:PEP 380 <380#use-of-stopiteration-to-return-values>
specifies that a :exc: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 :monitoring-event:STOP_ITERATION event is provided.
:monitoring-event:STOP_ITERATION can be locally disabled, unlike
:monitoring-event:RAISE.
Note that the :monitoring-event:STOP_ITERATION event and the
:monitoring-event:RAISE event for a :exc:StopIteration exception are
equivalent, and are treated as interchangeable when generating events.
Implementations will favor :monitoring-event:STOP_ITERATION for performance
reasons, but may generate a :monitoring-event:RAISE event with a
:exc:StopIteration.
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.
.. function:: get_events(tool_id: int, /) -> int
Returns the int representing all the active events.
.. function:: set_events(tool_id: int, event_set: int, /) -> None
Activates all events which are set in event_set.
Raises a :exc: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 :class:types.CodeType should be prepared
to accept a look-alike object from functions which are not defined
in Python (see :ref:c-api-monitoring).
.. function:: get_local_events(tool_id: int, code: CodeType, /) -> int
Returns all the :ref:local events <monitoring-event-local> for code
.. function:: set_local_events(tool_id: int, code: CodeType, event_set: int, /) -> None
Activates all the :ref:local events <monitoring-event-local> for code
which are set in event_set. Raises a :exc:ValueError if tool_id is not
in use.
Disabling events ''''''''''''''''
.. data:: DISABLE
A special value that can be returned from a callback function to disable events for the current code location.
:ref:Local events <monitoring-event-local> can be disabled for a specific code
location by returning :data: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 :data:DISABLE is returned by a callback for a
:ref:global event <monitoring-event-global>, :exc:ValueError will be raised
by the interpreter in a non-specific location (that is, no traceback will be
provided).
.. function:: restart_events() -> None
Enable all the events that were disabled by :data:sys.monitoring.DISABLE
for all tools.
.. _callbacks:
.. function:: 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 :func:register_callback returns None.
.. audit-event:: sys.monitoring.register_callback func sys.monitoring.register_callback
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 '''''''''''''''''''''''''''
.. data:: 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 :data:DISABLE will have no effect.
Different events will provide the callback function with different arguments, as follows:
:monitoring-event:PY_START and :monitoring-event:PY_RESUME::
func(code: CodeType, instruction_offset: int) -> object
:monitoring-event:PY_RETURN and :monitoring-event:PY_YIELD::
func(code: CodeType, instruction_offset: int, retval: object) -> object
:monitoring-event:CALL, :monitoring-event:C_RAISE and :monitoring-event:C_RETURN
(arg0 can be :data: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 :data: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).
:monitoring-event:RAISE, :monitoring-event:RERAISE, :monitoring-event:EXCEPTION_HANDLED,
:monitoring-event:PY_UNWIND, :monitoring-event:PY_THROW and :monitoring-event:STOP_ITERATION::
func(code: CodeType, instruction_offset: int, exception: BaseException) -> object
:monitoring-event:LINE::
func(code: CodeType, line_number: int) -> object
:monitoring-event:BRANCH_LEFT, :monitoring-event:BRANCH_RIGHT and :monitoring-event:JUMP::
func(code: CodeType, instruction_offset: int, destination_offset: int) -> object
Note that the destination_offset is where the code will next execute.
:monitoring-event:INSTRUCTION::
func(code: CodeType, instruction_offset: int) -> object