sys.monitoring
— Execution event monitoring
Added in version 3.12.
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
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.free_tool_id(tool_id: int, /) None
Should be called once a tool no longer requires tool_id.
Note
free_tool_id()
will not disable global or local events associated
with tool_id, nor will it unregister any callback functions. This
function is only intended to be used to notify the VM that the
particular tool_id is no longer in use.
- 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
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:
- sys.monitoring.events.BRANCH
A conditional branch is taken (or not).
- 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.
- 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: ...
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:
Ancillary events
Ancillary events can be monitored like other events, but are controlled by another event:
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.
The other events that can be monitored are:
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 corresponding callback must be 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.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.
- 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.
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
- 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.
- sys.monitoring.restart_events() None
Enable all the events that were disabled by
sys.monitoring.DISABLE
for all tools.
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()
returnsNone
.
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
- 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. Different events will provide the callback function with different arguments, as follows:
-
func(code: CodeType, instruction_offset: int) -> DISABLE | Any
-
func(code: CodeType, instruction_offset: int, retval: object) -> DISABLE | Any
-
func(code: CodeType, instruction_offset: int, callable: object, arg0: object | MISSING) -> DISABLE | Any
If there are no arguments, arg0 is set to
sys.monitoring.MISSING
. RAISE
,RERAISE
,EXCEPTION_HANDLED
,PY_UNWIND
,PY_THROW
andSTOP_ITERATION
:func(code: CodeType, instruction_offset: int, exception: BaseException) -> DISABLE | Any
LINE
:func(code: CodeType, line_number: int) -> DISABLE | Any
-
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.
-
func(code: CodeType, instruction_offset: int) -> DISABLE | Any