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