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Meaning of tracebacklimit differs between sys.tracebacklimit and traceback module #82378
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cfbolz commented
on Sep 17, 2019 cfbolzmannequinMannequinAuthorThe meaning of sys.tracebacklimit seems to be different than the meaning of the various limit parameters in the traceback module. One shows the top n stack frames, the other the bottom n.
Is this intentional, and if yes, is that difference documented somewhere? (it came up because PyPy just uses the traceback module and has no equivalent of PyTraceBack_Print).
See the attached script to understand the problem. The script formats the same exception twice, once with the traceback module, once by the interpreter. I would have expected them to look the same for all limits, but instead:
$ ./python /tmp/x.py 3 limit 3 from traceback module: Traceback (most recent call last): File "/tmp/x.py", line 19, in <module> main() File "/tmp/x.py", line 16, in main x3() File "/tmp/x.py", line 14, in x3 x2() ZeroDivisionError: division by zero from interpreter: Traceback (most recent call last): File "/tmp/x.py", line 14, in x3 x2() File "/tmp/x.py", line 12, in x2 x1() File "/tmp/x.py", line 10, in x1 1 / 0 ZeroDivisionError: division by zero
If you change your script to do
sys.tracebacklimit = abs(limit)
and run it with arg -3, then you get the output you expect:C:\Users\User\src\cpython>python.bat x.py -3 Running Release|Win32 interpreter... limit -3 from traceback module: Traceback (most recent call last): File "C:\Users\User\src\cpython\x.py", line 14, in x3 x2() File "C:\Users\User\src\cpython\x.py", line 12, in x2 x1() File "C:\Users\User\src\cpython\x.py", line 10, in x1 1 / 0 ZeroDivisionError: division by zero from interpreter: Traceback (most recent call last): File "C:\Users\User\src\cpython\x.py", line 14, in x3 x2() File "C:\Users\User\src\cpython\x.py", line 12, in x2 x1() File "C:\Users\User\src\cpython\x.py", line 10, in x1 1 / 0 ZeroDivisionError: division by zero
The documentation for traceback mentions the possibility of using negative limits and their meaning:
Print up to limit stack trace entries from traceback object tb (starting from the caller’s frame) if limit is positive. Otherwise, print the last abs(limit) entries.
https://docs.python.org/3/library/traceback.html#traceback.print_tb
cfbolz commented
on Nov 4, 2020 cfbolzmannequinMannequinAuthorIt's still inconsistent between the two ways to get a traceback, and the inconsistency is not documented.
This issue also becomes more relevant now, with pyrepl using
code, which uses thetracebackmodule. Here's a behaviour difference between cpy3.12 and cpy3.13 (current 3.13 branch):cfbolz@triacontahedron:~/projects/cpython$ cat x.py def x1(): 1 / 0 def x2(): x1() def x3(): x2() cfbolz@triacontahedron:~/projects/cpython$ python3.12 -i x.py >>> x3() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/cfbolz/projects/cpython/x.py", line 6, in x3 x2() File "/home/cfbolz/projects/cpython/x.py", line 4, in x2 x1() File "/home/cfbolz/projects/cpython/x.py", line 2, in x1 1 / 0 ~~^~~ ZeroDivisionError: division by zero >>> import sys >>> sys.tracebacklimit = 2 >>> x3() Traceback (most recent call last): File "/home/cfbolz/projects/cpython/x.py", line 4, in x2 x1() File "/home/cfbolz/projects/cpython/x.py", line 2, in x1 1 / 0 ~~^~~ ZeroDivisionError: division by zero >>> cfbolz@triacontahedron:~/projects/cpython$ ./python -i x.py >>> x3() Traceback (most recent call last): File "<python-input-0>", line 1, in <module> x3() ~~^^ File "/home/cfbolz/projects/cpython/x.py", line 6, in x3 x2() ~~^^ File "/home/cfbolz/projects/cpython/x.py", line 4, in x2 x1() ~~^^ File "/home/cfbolz/projects/cpython/x.py", line 2, in x1 1 / 0 ~~^~~ ZeroDivisionError: division by zero >>> import sys >>> sys.tracebacklimit = 2 >>> x3() Traceback (most recent call last): File "<python-input-3>", line 1, in <module> x3() ~~^^ File "/home/cfbolz/projects/cpython/x.py", line 6, in x3 x2() ~~^^ ZeroDivisionError: division by zero >>>note how on 3.12 the traceback is missing the frame for
x3whensys.tracebacklimitis set, and 3.13 is missing the frame forx1.I would still argue that
sys.tracebacklimithas basically been broken since essentially forever in thetracebackmodule. It might still be too late to fix this, of course, because people have come to rely on this undocumented behavior.I would still argue that sys.tracebacklimit has basically been broken since essentially forever in the traceback module. It might still be too late to fix this, of course, because people have come to rely on this undocumented behavior.
Yes, we'd need to add a new limit and deprecate this one.
Ok, but that means we still have a very clear behavior change in the repl from 3.12 to 3.13 if someone is setting
sys.tracebacklimitCC @pablogsal @ambv re repl.
10 remaining items
Ok, I think we are done with this one. Thanks @cfbolz for the PRs and @serhiy-storchaka for the reviews!
@pablogsal I kind of would like to still add a paragraph to the
tracebackmodule docs somewhere that mentions the differences between the meaning oflimitin that module and the meaning ofsys.tracebacklimit. I just don't quite know where in the page to put it. Maybe simply in the text afterprint_tbwhere the meaning oflimitis explained for the first time?I'll re-open the issue to make sure I don't forget (or should I open a new one for the doc change?).
or should I open a new one for the doc change?
No, we can reuse this one 👍
Maybe simply in the text after print_tb where the meaning of limit is explained for the first time?
I think that would be a good place. Alternatively we can add it at the start after the "The module defines the following functions:
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