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.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:
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:
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
.
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.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.
- 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()
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.
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.
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.
Callback functions returning an object other than DISABLE
will have no effect.
Different events will provide the callback function with different arguments, as follows:
-
func(code: CodeType, instruction_offset: int) -> object
-
func(code: CodeType, instruction_offset: int, retval: object) -> object
CALL
,C_RAISE
andC_RETURN
(arg0 can beMISSING
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
andSTOP_ITERATION
:func(code: CodeType, instruction_offset: int, exception: BaseException) -> object
LINE
:func(code: CodeType, line_number: int) -> object
BRANCH_LEFT
,BRANCH_RIGHT
andJUMP
:func(code: CodeType, instruction_offset: int, destination_offset: int) -> object
Note that the destination_offset is where the code will next execute.
-
func(code: CodeType, instruction_offset: int) -> object