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 aValueError
ifid
is in use.
- sys.monitoring.get_tool(id: int) str | None ¶
Returns the name of the tool if
id
is in use, otherwise it returnsNone
.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.set_events(tool_id: int, event_set: int)¶
Activates all events which are set in
event_set
. Raises aValueError
iftool_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 inevent_set
. Raises aValueError
iftool_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 theevent
with the giventool_id
If another callback was registered for the given
tool_id
andevent
, it is unregistered and returned. Otherwiseregister_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¶
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
andPY_RESUME
:func(code: CodeType, instruction_offset: int) -> DISABLE | Any
PY_RETURN
andPY_YIELD
:func(code: CodeType, instruction_offset: int, retval: object) -> DISABLE | Any
CALL
,C_RAISE
andC_RETURN
:func(code: CodeType, instruction_offset: int, callable: object, arg0: object | MISSING) -> DISABLE | Any
If there are no arguments,
arg0
is set toMISSING
.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
BRANCH
andJUMP
: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