bpo-17799: Explain real behaviour of sys.settrace and sys.setprofile by pablogsal · Pull Request #4056 · python/cpython · GitHub

The docs for sys.settrace mention that:

event is a string: 'call', 'line', 'return', 'exception', 'c_call', 'c_return', or 'c_exception'

But in the code for ceval.c the only point where call_trace is invoked with PyTrace_C_CALL or PyTrace_C_RETURN is under the C_TRACE macro. In this macro this line prevents any function set up using sys.settrace to call call_trace with the mentioned arguments:

if (tstate->use_tracing && tstate->c_profilefunc)

Notice that from the code of PyEval_SetTrace and PyEval_SetProfile, only the later sets tstate->c_profilefunc and therefore only functions installed using sys.setprofile will recieve a c_call for the event:

void
PyEval_SetProfile(Py_tracefunc func, PyObject *arg)
{
     ...
    tstate->c_profilefunc = func;
    tstate->c_profileobj = arg;
    tstate->use_tracing = (func != NULL) || (tstate->c_tracefunc != NULL);
}

void
PyEval_SetTrace(Py_tracefunc func, PyObject *arg)
{
     ...
    tstate->c_tracefunc = func;
    tstate->c_traceobj = arg;
    tstate->use_tracing = ((func != NULL) || (tstate->c_profilefunc != NULL));
}

This PR updates the documentation clarifying the real behaviour of both functions.

https://bugs.python.org/issue17799

The C API part https://docs.python.org/3.7/c-api/init.html?highlight=pyeval_settrace#profiling-and-tracing also needs editing.

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@zhangyangyu: please review the changes made to this pull request.

/s/recieve/receive

Thanks! Corrected in 18b88ec

Why line event is also here?

Sorry, I misread that part.

It is does receive, so I think it is wrong here. Maybe just declare it does not receive PyTrace_C_CALL/RETURN/EXCEPTION here, so don't have to write the previous added block.

line will be received here?

Although the previous doc says profile function is called similiarly to trace function, but from the implementation, there is no concept like global trace function or local trace function. And at least we call it profile function here not trace function.

@zhangyangyu Thank you very much for the comments. I have addressed your feedback in a9f381f and 3887c6c.

Also here and the following return, exception part about the global/local and name.

opcode events won't be triggered for profile functions, also note the above list needs also be modified.

I prefer to combine this section with the specified PyEval_SetTrace part.

@zhangyangyu Upon inspection and testing I think that profile functions cannot receive "exception" events as well. Can you confirm this?

Yes. But profile functions receive even PyTrace_C_EXCEPTION, why not Python exception events?

From the unitest test_sys_setprofile, exception events should never be received by profile functions, so seems it's by design.

…not being received.

PyEval_SetProfile should also be updated that not only line number events but also Python exception events.

…ceived.

I think the following paragraph need some modification, something like "... but it is called with different events, for example ..."

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).

And your NEWS entry needs updated too. It's not only about C events now. BTW, we need a new issue to complete the C part about PyTrace_OPCODE.

Thanks @pablogsal for the PR, and @zhangyangyu for merging it 🌮🎉.. I'm working now to backport this PR to: 2.7, 3.6.
🐍🍒⛏🤖

@pablogsal I merged it! Nice work!

Sorry, @pablogsal and @zhangyangyu, I could not cleanly backport this to 3.6 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 131fd7f96c619bc7eaea956e45c6337175f4b27f 3.6

Sorry, @pablogsal and @zhangyangyu, I could not cleanly backport this to 2.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 131fd7f96c619bc7eaea956e45c6337175f4b27f 2.7

…ofile (pythonGH-4056). (cherry picked from commit 131fd7f)

GH-5298 is a backport of this pull request to the 3.6 branch.

…ofile (pythonGH-4056). (cherry picked from commit 131fd7f)

GH-5299 is a backport of this pull request to the 2.7 branch.

…H-4056) (#5298) (cherry picked from commit 131fd7f)

…H-4056). (#5299) (cherry picked from commit 131fd7f)

the-knights-who-say-ni added the CLA signed label Oct 20, 2017

bedevere-bot added the awaiting review label Oct 20, 2017

Explain real behaviour of sys.settrace and sys.setprofile

2199f40

pablogsal force-pushed the 17799 branch from 75ee0ee to 2199f40 Compare October 20, 2017 10:59

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Add documentation in the c-api section

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Add News entry

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csabella reviewed Jan 22, 2018

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Correct typo

18b88ec

zhangyangyu reviewed Jan 23, 2018

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pablogsal added 2 commits January 23, 2018 16:05

Update init.rst

a9f381f

Update sys.rst

3887c6c

zhangyangyu reviewed Jan 23, 2018

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Remove local/global references in setprofile. Remove events that are …

076a702

…not being received.

pablogsal added 2 commits January 23, 2018 16:57

Corrected typo in PyEval_SetTrace section

d0dce97

Remove exception event from profile paragraph

9301d56

pablogsal added 2 commits January 23, 2018 17:27

Update PyEval_SetProfile to indicate that exception events are not re…

b4531a9

…ceived.

Remove line about exception events in the setprofile section.

72ca9cb

pablogsal added 2 commits January 24, 2018 10:19

Improve description of profile function

3061991

Update NEWS entry

426368c

zhangyangyu added needs backport to 3.6 docs Documentation in the Doc dir labels Jan 24, 2018

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zhangyangyu merged commit 131fd7f into python:master Jan 24, 2018

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zhangyangyu pushed a commit to zhangyangyu/cpython that referenced this pull request Jan 24, 2018

[3.6] bpo-17799: Explain real behaviour of sys.settrace and sys.setpr…

3793d06

…ofile (pythonGH-4056). (cherry picked from commit 131fd7f)

bedevere-bot removed the needs backport to 3.6 label Jan 24, 2018

zhangyangyu pushed a commit to zhangyangyu/cpython that referenced this pull request Jan 24, 2018

[2.7] bpo-17799: Explain real behaviour of sys.settrace and sys.setpr…

7eaa652

…ofile (pythonGH-4056). (cherry picked from commit 131fd7f)

bedevere-bot removed the needs backport to 2.7 label Jan 24, 2018

zhangyangyu added a commit that referenced this pull request Jan 24, 2018

bpo-17799 : Explain real behaviour of sys.settrace and sys.setprofile (G…

fd844ef

…H-4056) (#5298) (cherry picked from commit 131fd7f)

zhangyangyu added a commit that referenced this pull request Jan 24, 2018

bpo-17799 : Explain real behaviour of sys.settrace and sys.setprofile (G…

e64a47b

…H-4056). (#5299) (cherry picked from commit 131fd7f)

pablogsal deleted the 17799 branch May 19, 2021 18:56

                  Set the tracing function to *func*.  This is similar to
                  :c:func:`PyEval_SetProfile`, except the tracing function does receive line-number
-                 events.
+                 events or any event related to C function objects being called.

                  | :const:`PyTrace_C_RETURN`    | Function object being called.        |
                  +------------------------------+--------------------------------------+
+                 Any trace function registered using :c:func:`PyEval_SetTrace` will not recieve

+                 Any trace function registered using :c:func:`PyEval_SetTrace` will not receive
+                 :const:`PyTrace_C_CALL`, :const:`PyTrace_C_EXCEPTION`, :const:`PyTrace_C_RETURN`
+                 or :const:`PyTrace_LINE` as a value for the *what* parameter.

+                 Profile 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

+                 ``'call'``
+                    A function is called (or some other code block entered).  The
+                    global trace function is called; *arg* is ``None``; the return value

+                    profile function is called; *arg* is ``None``.
+                 ``'return'``
+                    A function (or other code block) is about to return.  The local trace

+                 ``'c_exception'``
+                    A C function has raised an exception.  *arg* is the C function object.
+                 ``'opcode'``

bpo-17799: Explain real behaviour of sys.settrace and sys.setprofile by pablogsal · Pull Request #4056 · python/cpython · GitHub

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bpo-17799: Explain real behaviour of sys.settrace and sys.setprofile#4056
zhangyangyu merged 14 commits into
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