Profiling and tracing
*********************

The Python interpreter provides some low-level support for attaching
profiling and execution tracing facilities.  These are used for
profiling, debugging, and coverage analysis tools.

This C interface allows the profiling or tracing code to avoid the
overhead of calling through Python-level callable objects, making a
direct C function call instead.  The essential attributes of the
facility have not changed; the interface allows trace functions to be
installed per-thread, and the basic events reported to the trace
function are the same as had been reported to the Python-level trace
functions in previous versions.

typedef int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg)

   The type of the trace function registered using
   "PyEval_SetProfile()" and "PyEval_SetTrace()". The first parameter
   is the object passed to the registration function as *obj*, *frame*
   is the frame object to which the event pertains, *what* is one of
   the constants "PyTrace_CALL", "PyTrace_EXCEPTION", "PyTrace_LINE",
   "PyTrace_RETURN", "PyTrace_C_CALL", "PyTrace_C_EXCEPTION",
   "PyTrace_C_RETURN", or "PyTrace_OPCODE", and *arg* depends on the
   value of *what*:

   +---------------------------------+------------------------------------------+
   | Value of *what*                 | Meaning of *arg*                         |
   |=================================|==========================================|
   | "PyTrace_CALL"                  | Always "Py_None".                        |
   +---------------------------------+------------------------------------------+
   | "PyTrace_EXCEPTION"             | Exception information as returned by     |
   |                                 | "sys.exc_info()".                        |
   +---------------------------------+------------------------------------------+
   | "PyTrace_LINE"                  | Always "Py_None".                        |
   +---------------------------------+------------------------------------------+
   | "PyTrace_RETURN"                | Value being returned to the caller, or   |
   |                                 | "NULL" if caused by an exception.        |
   +---------------------------------+------------------------------------------+
   | "PyTrace_C_CALL"                | Function object being called.            |
   +---------------------------------+------------------------------------------+
   | "PyTrace_C_EXCEPTION"           | Function object being called.            |
   +---------------------------------+------------------------------------------+
   | "PyTrace_C_RETURN"              | Function object being called.            |
   +---------------------------------+------------------------------------------+
   | "PyTrace_OPCODE"                | Always "Py_None".                        |
   +---------------------------------+------------------------------------------+

int PyTrace_CALL

   The value of the *what* parameter to a "Py_tracefunc" function when
   a new call to a function or method is being reported, or a new
   entry into a generator. Note that the creation of the iterator for
   a generator function is not reported as there is no control
   transfer to the Python bytecode in the corresponding frame.

int PyTrace_EXCEPTION

   The value of the *what* parameter to a "Py_tracefunc" function when
   an exception has been raised.  The callback function is called with
   this value for *what* when after any bytecode is processed after
   which the exception becomes set within the frame being executed.
   The effect of this is that as exception propagation causes the
   Python stack to unwind, the callback is called upon return to each
   frame as the exception propagates.  Only trace functions receive
   these events; they are not needed by the profiler.

int PyTrace_LINE

   The value passed as the *what* parameter to a "Py_tracefunc"
   function (but not a profiling function) when a line-number event is
   being reported. It may be disabled for a frame by setting
   "f_trace_lines" to *0* on that frame.

int PyTrace_RETURN

   The value for the *what* parameter to "Py_tracefunc" functions when
   a call is about to return.

int PyTrace_C_CALL

   The value for the *what* parameter to "Py_tracefunc" functions when
   a C function is about to be called.

int PyTrace_C_EXCEPTION

   The value for the *what* parameter to "Py_tracefunc" functions when
   a C function has raised an exception.

int PyTrace_C_RETURN

   The value for the *what* parameter to "Py_tracefunc" functions when
   a C function has returned.

int PyTrace_OPCODE

   The value for the *what* parameter to "Py_tracefunc" functions (but
   not profiling functions) when a new opcode is about to be executed.
   This event is not emitted by default: it must be explicitly
   requested by setting "f_trace_opcodes" to *1* on the frame.

void PyEval_SetProfile(Py_tracefunc func, PyObject *obj)

   Set the profiler function to *func*.  The *obj* parameter is passed
   to the function as its first parameter, and may be any Python
   object, or "NULL".  If the profile function needs to maintain
   state, using a different value for *obj* for each thread provides a
   convenient and thread-safe place to store it.  The profile function
   is called for all monitored events except "PyTrace_LINE"
   "PyTrace_OPCODE" and "PyTrace_EXCEPTION".

   See also the "sys.setprofile()" function.

   The caller must have an *attached thread state*.

void PyEval_SetProfileAllThreads(Py_tracefunc func, PyObject *obj)

   Like "PyEval_SetProfile()" but sets the profile function in all
   running threads belonging to the current interpreter instead of the
   setting it only on the current thread.

   The caller must have an *attached thread state*.

   As "PyEval_SetProfile()", this function ignores any exceptions
   raised while setting the profile functions in all threads.

Added in version 3.12.

void PyEval_SetTrace(Py_tracefunc func, PyObject *obj)

   Set the tracing function to *func*.  This is similar to
   "PyEval_SetProfile()", except the tracing function does receive
   line-number events and per-opcode events, but does not receive any
   event related to C function objects being called.  Any trace
   function registered using "PyEval_SetTrace()" will not receive
   "PyTrace_C_CALL", "PyTrace_C_EXCEPTION" or "PyTrace_C_RETURN" as a
   value for the *what* parameter.

   See also the "sys.settrace()" function.

   The caller must have an *attached thread state*.

void PyEval_SetTraceAllThreads(Py_tracefunc func, PyObject *obj)

   Like "PyEval_SetTrace()" but sets the tracing function in all
   running threads belonging to the current interpreter instead of the
   setting it only on the current thread.

   The caller must have an *attached thread state*.

   As "PyEval_SetTrace()", this function ignores any exceptions raised
   while setting the trace functions in all threads.

Added in version 3.12.


Reference tracing
*****************

Added in version 3.13.

typedef int (*PyRefTracer)(PyObject*, int event, void *data)

   The type of the trace function registered using
   "PyRefTracer_SetTracer()". The first parameter is a Python object
   that has been just created (when **event** is set to
   "PyRefTracer_CREATE") or about to be destroyed (when **event** is
   set to "PyRefTracer_DESTROY"). The **data** argument is the opaque
   pointer that was provided when "PyRefTracer_SetTracer()" was
   called.

   If a new tracing function is registered replacing the current one,
   a call to the trace function will be made with the object set to
   **NULL** and **event** set to "PyRefTracer_TRACKER_REMOVED". This
   will happen just before the new function is registered.

Added in version 3.13.

int PyRefTracer_CREATE

   The value for the *event* parameter to "PyRefTracer" functions when
   a Python object has been created.

int PyRefTracer_DESTROY

   The value for the *event* parameter to "PyRefTracer" functions when
   a Python object has been destroyed.

int PyRefTracer_TRACKER_REMOVED

   The value for the *event* parameter to "PyRefTracer" functions when
   the current tracer is about to be replaced by a new one.

   Added in version 3.14.

int PyRefTracer_SetTracer(PyRefTracer tracer, void *data)

   Register a reference tracer function. The function will be called
   when a new Python object has been created or when an object is
   going to be destroyed. If **data** is provided it must be an opaque
   pointer that will be provided when the tracer function is called.
   Return "0" on success. Set an exception and return "-1" on error.

   Note that tracer functions **must not** create Python objects
   inside or otherwise the call will be re-entrant. The tracer also
   **must not** clear any existing exception or set an exception.  A
   *thread state* will be active every time the tracer function is
   called.

   There must be an *attached thread state* when calling this
   function.

   If another tracer function was already registered, the old function
   will be called with **event** set to "PyRefTracer_TRACKER_REMOVED"
   just before the new function is registered.

Added in version 3.13.

PyRefTracer PyRefTracer_GetTracer(void **data)

   Get the registered reference tracer function and the value of the
   opaque data pointer that was registered when
   "PyRefTracer_SetTracer()" was called. If no tracer was registered
   this function will return NULL and will set the **data** pointer to
   NULL.

   There must be an *attached thread state* when calling this
   function.

Added in version 3.13.
