New submission from Sriram Rajagopalan:
Consider this simple python program -
1 #!/usr/bin/python
2
3 import pdb
4 import sys
5 import traceback
6
7 def trace_exception( type, value, tb ):
8 traceback.print_tb( tb )
9 pdb.post_mortem( tb )
10
11 sys.excepthook = trace_exception
12
13 def func():
14 print ( "Am here in func" )
15 pdb.set_trace()
16 print ( "Am here after pdb" )
17 print ( "Am going to assert now" )
18 assert False
19 print ( "Am here after exception" )
20
21 def main():
22 func()
23
24 if __name__ == "__main__":
25 main()
On running this program -
% ./python /tmp/test.py
Am here in func
> /tmp/test.py(16)func()
-> print ( "Am here after pdb" )
(Pdb) c
Am here after pdb
Am going to assert now
File "/tmp/test.py", line 25, in
main()
File "/tmp/test.py", line 22, in main
func()
File "/tmp/test.py", line 16, in func
print ( "Am here after pdb" )
> /tmp/test.py(16)func()
-> print ( "Am here after pdb" ) >>>> This should have been at the line
corresponding to "Am going to assert now"
(Pdb)
This seems to be an bug ( due to a performance consideration ) with the way
python bdb's set_continue() has been implemented -
https://hg.python.org/cpython/file/2.7/Lib/bdb.py#l227
def set_continue(self):
# Don't stop except at breakpoints or when finished
self._set_stopinfo(self.botframe, None, -1)
if not self.breaks:
# no breakpoints; run without debugger overhead
sys.settrace(None)
frame = sys._getframe().f_back
while frame and frame is not self.botframe:
del frame.f_trace
frame = frame.f_back
Basically what happens after "c" in a "(Pdb)" prompt is that bdb optimizes for
the case where there are no more break points found by cleaning up the trace
callback from all the frames.
However, all of this happens in the context of tracing itself and hence the
trace_dispatch function in
https://hg.python.org/cpython/file/2.7/Lib/bdb.py#l45 still returns back the
trace_dispatch as the new system trace function. For more details on
sys.settrace(), check https://docs.python.org/2/library/sys.html#sys.settrace
Check, the function trace_trampoline at
https://hg.python.org/cpython/file/2.7/Python/sysmodule.c#l353
which sets f->f_trace back to result at
https://hg.python.org/cpython/file/2.7/Python/sysmodule.c#l377
This seems to be an bug ( due to a performance consideration ) with the way
python bdb's set_continue() has been implemented -
https://hg.python.org/cpython/file/2.7/Lib/bdb.py#l227
def set_continue(self):
# Don't stop except at breakpoints or when finished
self._set_stopinfo(self.botframe, None, -1)
if not self.breaks:
# no breakpoints; run without debugger overhead
sys.settrace(None)
frame = sys._getframe().f_back
while frame and frame is not self.botframe:
del frame.f_trace
frame = frame.f_back
Basically what happens after "c" in a "(Pdb)" prompt is that bdb optimizes for
the case where there are no more break points found by cleaning up the trace
callback from all the frames.
However, all of this happens in the context of tracing itself and hence the
trace_dispatch function in
https://hg.python.org/cpython/file/2.7/Lib/bdb.py#l45 still returns back the
trace_dispatch as the new system trace function. For more details on
sys.settrace(), check https://docs.python.org/2/library/sys.html#sys.settrace
Check, the function trace_trampoline at
https://hg.python.org/cpython/file/2.7/Python/sysmodule.c#l353
which sets f->f_trace back to result at
https://hg.python.org/cpython/file/2.7/Python/sysmodule.c#l377
Now, check the function PyFrame_GetLineNumber() which is used by the traceback
to get the frame line number
https://hg.python.org/cpython/file/2.7/Objects/frameobject.c#l63
int
PyFrame_GetLineNumber(PyFrameObject *f)
{
if (f->f_trace)
return f->f_lineno;
else
return PyCode_Addr2Line(f->f_code, f->f_lasti);
}
Basically this function returns back the stored f->f_lineno in case the
f->f_trace is enabled.
The fix is fortunately simple -
Just set self.trace_dispatch to None if pdb set_continue decides to run without
debugger overhead.
--
components: Library (Lib)
files: bdbfix.patch
keywords: patch
messages: 263553
nosy: Sriram Rajagopalan
priority: normal
severity: normal
status: open
title: pdb continue followed by an exception in the same frame shows incorrect
frame linenumber
type: behavior
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6
Added file: http://bugs.python.org/
