bpo-17799: Explain real behaviour of sys.settrace and sys.setprofile (GH-4056) (#5298)

(cherry picked from commit 131fd7f96c619bc7eaea956e45c6337175f4b27f)

1 files changed, 32 insertions, 17 deletions

diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index dd51ffd56c..68521df37e 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1005,13 +1005,38 @@ always available.
    Set the system's profile function, which allows you to implement a Python source
    code profiler in Python.  See chapter :ref:`profile` for more information on the
    Python profiler.  The system's profile function is called similarly to the
-   system's trace function (see :func:`settrace`), but it isn't called for each
-   executed line of code (only on call and return, but the return event is reported
-   even when an exception has been set).  The function is thread-specific, but
-   there is no way for the profiler to know about context switches between threads,
-   so it does not make sense to use this in the presence of multiple threads. Also,
+   system's trace function (see :func:`settrace`), but it is called with different events,
+   for example it isn't called for each executed line of code (only on call and return,
+   but the return event is reported even when an exception has been set). The function is
+   thread-specific, but there is no way for the profiler to know about context switches between
+   threads, so it does not make sense to use this in the presence of multiple threads. Also,
    its return value is not used, so it can simply return ``None``.
 
+   Profile functions should have three arguments: *frame*, *event*, and
+   *arg*. *frame* is the current stack frame.  *event* is a string: ``'call'``,
+   ``'return'``, ``'c_call'``, ``'c_return'``, or ``'c_exception'``. *arg* depends
+   on the event type.
+
+   The events have the following meaning:
+
+   ``'call'``
+      A function is called (or some other code block entered).  The
+      profile function is called; *arg* is ``None``.
+
+   ``'return'``
+      A function (or other code block) is about to return.  The profile
+      function is called; *arg* is the value that will be returned, or ``None``
+      if the event is caused by an exception being raised.
+
+   ``'c_call'``
+      A C function is about to be called.  This may be an extension function or
+      a built-in.  *arg* is the C function object.
+
+   ``'c_return'``
+      A C function has returned. *arg* is the C function object.
+
+   ``'c_exception'``
+      A C function has raised an exception.  *arg* is the C function object.
 
 .. function:: setrecursionlimit(limit)
 
@@ -1058,8 +1083,8 @@ always available.
 
    Trace functions should have three arguments: *frame*, *event*, and
    *arg*. *frame* is the current stack frame.  *event* is a string: ``'call'``,
-   ``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
-   ``'c_exception'``. *arg* depends on the event type.
+   ``'line'``, ``'return'`` or ``'exception'``.  *arg* depends on
+   the event type.
 
    The trace function is invoked (with *event* set to ``'call'``) whenever a new
    local scope is entered; it should return a reference to a local trace
@@ -1094,16 +1119,6 @@ always available.
       tuple ``(exception, value, traceback)``; the return value specifies the
       new local trace function.
 
-   ``'c_call'``
-      A C function is about to be called.  This may be an extension function or
-      a built-in.  *arg* is the C function object.
-
-   ``'c_return'``
-      A C function has returned. *arg* is the C function object.
-
-   ``'c_exception'``
-      A C function has raised an exception.  *arg* is the C function object.
-
    Note that as an exception is propagated down the chain of callers, an
    ``'exception'`` event is generated at each level.