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When a breakpoint is set in one of the frames of the frame stack, Pdb
may not stop at that breakpoint when the frame does not have a trace
function. This problem is closely related to issue 13183 and issue
14728. The following scenario demonstrates this problem.
==== main.py ================================
import bar
def foo():
 bar.bar()
 x = 1
foo()
==== bar.py ==================================
def bar():
 pass
=================================================
$ python3 -m pdb main.py 
> /path_to/main.py(1)<module>()
-> import bar
(Pdb) import sys; print(sys.version)
3.2.2 (default, Dec 27 2011, 17:35:55) 
[GCC 4.3.2]
(Pdb) break bar.bar
Breakpoint 1 at /path_to/bar.py:1
(Pdb) continue
> /path_to/bar.py(2)bar()
-> pass
(Pdb) break main.py:5
Breakpoint 2 at /path_to/main.py:5
(Pdb) continue
The program finished and will be restarted
> /path_to/main.py(1)<module>()
-> import bar
(Pdb) quit
=================================================
The attached patch fixes this problem. A test case is included in the
patch. The patch is made against the proposed fix of issue 14728 (i.e.
assumes this patch is applied), the reason being that self._curframe
must be correctly set. Actually this issue and issue 14728 should
probably be merged.
Note that the trace function does not need anymore to be set in all
the frames of the frame stack in set_trace(), so setting the trace
function has been removed from the while loop.

Uploaded pdb_default_2.patch that corrects the initial patch: the
trace function must be set in all frames whose co_filename is the
breakpoint file name and not just the first frame of this set.