Castrated traceback in sys.exc_info()
Pakal
I've just realized recently that sys.exc_info() didn't return a full
traceback for exception concerned : it actually only contains the
frame below the point of exception catching.
That's very annoying to me, because I planned to log such tracebacks
with logging.critical(*****, exc_info=True), and these partial
tracebacks, like the one below, are clearly unsufficient to determine
where the problem comes from. A whole traceback, from program entry
point to exception raising point, would be much better.
2010-03-17 09:28:59,184 - pims - CRITICAL - This is just a test to
ensure critical emails are properly sent
Traceback (most recent call last):
<< HERE, lots of frames missing >>
File "test_common.py", line 34, in test_email_sending
os.open("qsdsdqsdsdqsd", "r")
TypeError: an integer is required
Is there any workaround for this ? I've thought about a custom logging
formatter, which would take both the exc_info traceback AND its own
full backtrace, and to connect them together on their relevant part,
but it's awkward and error prone... why can't we just have all the
precious traceback info under the hand there (an additional attribute
might have pointed the precise frame in which the exception was
caught).
Tanks for the attention,
Regards,
Pascal
Michael Ricordeau
to log tracebacks, you can probably try traceback module.
I use it like this :
import traceback
.... # your code
for line in traceback.format_exc().splitlines():
log.trace(line)
Le Wed, 17 Mar 2010 03:42:44 -0700 (PDT),
Pakal <chambon...@gmail.com> a écrit :
Vinay Sajip
> Hello
>
> I've just realized recently that sys.exc_info() didn't return a full
> traceback for exception concerned : it actually only contains the
> frame below the point of exception catching.
>
> That's very annoying to me, because I planned to log such tracebacks
> tracebacks, like the one below, are clearly unsufficient to determine
> where the problem comes from. A whole traceback, from program entry
> point to exception raising point, would be much better.
>
> 2010-03-17 09:28:59,184 - pims - CRITICAL - This is just a test to
> ensure critical emails are properly sent
> Traceback (most recent call last):
> << HERE, lots of frames missing >>
> File "test_common.py", line 34, in test_email_sending
> os.open("qsdsdqsdsdqsd", "r")
> TypeError: an integer is required
>
> Is there any workaround for this ? I've thought about a customlogging
> formatter, which would take both the exc_info traceback AND its own
> full backtrace, and to connect them together on their relevant part,
> but it's awkward and error prone... why can't we just have all the
> precious traceback info under the hand there (an additional attribute
> might have pointed the precise frame in which the exception was
> caught).
>
> Tanks for the attention,
> Regards,
> Pascal
Do you have a short script which demonstrates the problem? Some more
context is needed. For example, if you have multiple threads of
execution, the traceback will only go up to the top-level function in
the thread.
Regards,
Vinay Sajip
Gabriel Genellina
<chambon...@gmail.com> escribió:
> traceback functions indeed allow the manipulation of exception
> tracebacks,
> but the root problem is that anyway, since that traceback is incomplete,
> your "traceback.format_exc().splitlines()" will only provide frames for
> callee (downward) functions, not caller (upward) ones, starting from the
> exception catching frame.
Either I don't understand what you mean, or I can't reproduce it:
<code>
import logging
def a(): return b()
def b(): return c()
def c(): return d()
def d(): raise ValueError
def main():
logging.basicConfig(level=logging.DEBUG)
try: a()
except: logging.exception("An error")
main()
</code>
Output:
D:\temp>python test_logging.py
ERROR:root:An error
Traceback (most recent call last):
File "test_logging.py", line 10, in main
try: a()
File "test_logging.py", line 3, in a
def a(): return b()
File "test_logging.py", line 4, in b
def b(): return c()
File "test_logging.py", line 5, in c
def c(): return d()
File "test_logging.py", line 6, in d
def d(): raise ValueError
ValueError
--
Gabriel Genellina
Pascal Chambon
>
> En Wed, 17 Mar 2010 09:42:06 -0300, Pascal Chambon
> <chambon...@gmail.com> escribió:
>
>> traceback functions indeed allow the manipulation of exception
>> tracebacks,
>> but the root problem is that anyway, since that traceback is incomplete,
>> your "traceback.format_exc().splitlines()" will only provide frames for
>> callee (downward) functions, not caller (upward) ones, starting from the
>> exception catching frame.
>
> Either I don't understand what you mean, or I can't reproduce it:
>
>
Allright, here is more concretely the problem :
<code>
import logging
def a(): return b()
def b(): return c()
def c():
try:
return d()
except:
logging.exception("An error")
def d(): raise ValueError
def main():
logging.basicConfig(level=logging.DEBUG)
a()
main()
</code>
OUTPUT:
>>>
ERROR:root:An error
Traceback (most recent call last):
File "C:/Users/Pakal/Desktop/aaa.py", line 7, in c
return d()
File "C:/Users/Pakal/Desktop/aaa.py", line 11, in d
def d(): raise ValueError
ValueError
>>>
As you see, the traceback only starts from function c, which handles the
exception.
It doesn't show main(), a() and b(), which might however be (and are, in
my case) critical to diagnose the severity of the problem (since many
different paths would lead to calling c()).
So the question is : is that possible to enforce, by a way or another,
the retrieval of the FULL traceback at exception raising point, instead
of that incomplete one ?
Thank you for your help,
regards,
Pascal
Gabriel Genellina
<chambon...@wanadoo.fr> escribi�:
>
> Allright, here is more concretely the problem :
>
> Traceback (most recent call last):
> return d()
> File "C:/Users/Pakal/Desktop/aaa.py", line 11, in d
> def d(): raise ValueError
> ValueError
> >>>
>
> As you see, the traceback only starts from function c, which handles the
> exception.
> It doesn't show main(), a() and b(), which might however be (and are, in
> my case) critical to diagnose the severity of the problem (since many
> different paths would lead to calling c()).
>
> So the question is : is that possible to enforce, by a way or another,
> the retrieval of the FULL traceback at exception raising point, instead
> of that incomplete one ?
Thanks for bringing this topic! I learned a lot trying to understand what
happens.
The exception traceback (what sys.exc_info()[2] returns) is *not* a
complete stack trace. The sys module documentation is wrong [1] when it
says "...encapsulates the call stack at the point where the exception
originally occurred."
The Language Reference is more clear [2]: "Traceback objects represent a
stack trace of an exception. A traceback object is created when an
exception occurs. When the search for an exception handler unwinds the
execution stack, at each unwound level a traceback object is inserted in
front of the current traceback. When an exception handler is entered, the
stack trace is made available to the program."
That is, a traceback holds only the *forward* part of the stack: the
frames already exited when looking for an exception handler. Frames going
from the program starting point up to the current execution point are
*not* included.
Conceptually, it's like having two lists: stack and traceback. The
complete stack trace is always stack+traceback. At each step (when
unwinding the stack, looking for a frame able to handle the current
exception) an item is popped from the top of the stack (last item) and
inserted at the head of the traceback.
The traceback holds the "forward" path (from the current execution point,
to the frame where the exception was actually raised). It's a linked list,
its tb_next attribute holds a reference to the next item; None marks the
last one.
The "back" path (going from the current execution point to its caller and
all the way to the program entry point) is a linked list of frames; the
f_back attribute points to the previous one, or None.
In order to show a complete stack trace, one should combine both. The
traceback module contains several useful functions: extract_stack() +
extract_tb() are a starting point. The simplest way I could find to make
the logging module report a complete stack is to monkey patch
logging.Formatter.formatException so it uses format_exception() and
format_stack() combined (in fact it is simpler than the current
implementation using a StringIO object):
<code>
import logging
import traceback
def formatException(self, ei):
"""
Format and return the specified exception information as a string.
This implementation builds the complete stack trace, combining
traceback.format_exception and traceback.format_stack.
"""
lines = traceback.format_exception(*ei)
if ei[2]:
lines[1:1] = traceback.format_stack(ei[2].tb_frame.f_back)
return ''.join(lines)
# monkey patch the logging module
logging.Formatter.formatException = formatException
def a(): return b()
def b(): return c()
def c():
try:
return d()
except:
logging.exception("An error")
raise
def d(): raise ValueError
def main():
a()
main()
</code>
Output:
ERROR:root:An error
Traceback (most recent call last):
File "test_logging.py", line 32, in <module>
main()
File "test_logging.py", line 30, in main
a()
File "test_logging.py", line 19, in a
def a(): return b()
File "test_logging.py", line 20, in b
def b(): return c()
File "test_logging.py", line 23, in c
return d()
File "test_logging.py", line 27, in d
def d(): raise ValueError
ValueError
Traceback (most recent call last):
File "test_logging.py", line 32, in <module>
main()
File "test_logging.py", line 30, in main
a()
File "test_logging.py", line 19, in a
def a(): return b()
File "test_logging.py", line 20, in b
def b(): return c()
File "test_logging.py", line 23, in c
return d()
File "test_logging.py", line 27, in d
def d(): raise ValueError
ValueError
Note that both tracebacks are identical: the first comes from the patched
logging module, the second is the standard Python one.
[1] http://docs.python.org/library/sys.html#sys.exc_info
[2]
http://docs.python.org/reference/datamodel.html#the-standard-type-hierarchy
--
Gabriel Genellina
Vinay Sajip
wrote:
> En Mon, 22 Mar 2010 15:20:39 -0300, Pascal Chambon
> format_stack() combined (in fact it is simpler than the current
> implementation using a StringIO object):
>
> <code>
> importlogging
> import traceback
>
> def formatException(self, ei):
> """
> Format and return the specified exception information as a string.
>
> This implementation builds the complete stack trace, combining
> traceback.format_exception and traceback.format_stack.
> """
> lines = traceback.format_exception(*ei)
> if ei[2]:
> lines[1:1] = traceback.format_stack(ei[2].tb_frame.f_back)
> return ''.join(lines)
>
>
> [1]http://docs.python.org/library/sys.html#sys.exc_info
> [2] http://docs.python.org/reference/datamodel.html#the-standard-type-hie...
>
> --
> Gabriel Genellina
Good catch, Gabriel.
There should be no need to monkey-patch the logging module - it's
better if I include the change in the module itself. The only
remaining question is that of backward compatibility, but I can do
this for Python 2.7/3.2 only so that won't be an issue. It's probably
a good idea to log an issue on the bug tracker, though, so we have
some history for the change - do you want to do that, or shall I?
Regards,
Vinay Sajip
Pascal Chambon
>
> En Mon, 22 Mar 2010 15:20:39 -0300, Pascal Chambon
>>
>> Allright, here is more concretely the problem :
>>
>> ERROR:root:An error
> format_stack() combined (in fact it is simpler than the current
> implementation using a StringIO object):
"exception traceback" that I didn't know (actually, it seems no one
makes it in computer literature).
> Good catch, Gabriel.
>
> There should be no need to monkey-patch the logging module - it's
> better if I include the change in the module itself. The only
> remaining question is that of backward compatibility, but I can do
> this for Python 2.7/3.2 only so that won't be an issue. It's probably
> a good idea to log an issue on the bug tracker, though, so we have
> some history for the change - do you want to do that, or shall I?
>
> Regards,
>
> Vinay Sajip
>
Well having it fixed in logging would be great, but that kind of
information is good to have in other circumstances, so shouldn't we
rather advocate the availability of this "stack trace part" in exc_info
too ?
This way, people like me who consider frames as black magic wouldn't
need to meet complex stuffs as
"traceback.format_stack(ei[2].tb_frame.f_back" :p
Should I open an issue for this evolution of exceptiuon handling, or
should we content ourselves of this "hacking" of frame stck ?
Regards,
Pascal
Vinay Sajip
>
> Should I open an issue for this evolution of exceptiuon handling, or
> should we content ourselves of this "hacking" of frame stck ?
>
Possibly worth raising an issue (not logging-related), but perhaps
it's worth seeing if this has come up before creating the issue.
Regards,
Vinay Sajip
Gabriel Genellina
escribiￜ:
There is, from 2006: http://bugs.python.org/issue1553375
I'll add a patch+tests to this issue.
--
Gabriel Genellina
Andreas Löscher
> As you see, the traceback only starts from function c, which handles the
> exception.
> It doesn't show main(), a() and b(), which might however be (and are, in
> my case) critical to diagnose the severity of the problem (since many
> different paths would lead to calling c()).
This results in the way the tb is created.
If an exception occours, and there is no one who handles it in the
current function, a traceback object is created. Because the the calling
function could have a try-except clause, the function returns with NULL
and an set exception. Now if there is no one to handle the exception, a
traceback object is created and linked with the existing one.
In your case does the traceback start from the function c, because no
exception occoured in main() etc. You catched it. If you want to
determine the calling path, you can take a look at the encapsuled frame
in the tb.
Best