pdb.set_trace() is overwriting the actual traceback when exiting with an error. See minimal recreation here: https://gist.github.com/4079971

The top level frame line number is not updated because it has a local
trace function while the global trace function is None. This is
related to bpo-7238.

The following patch fixes the issue. The patch removes the local trace
at the top level frame and makes sure it is not reinstalled when
returning from the current trace function.

diff --git a/Lib/bdb.py b/Lib/bdb.py
--- a/Lib/bdb.py
+++ b/Lib/bdb.py
@@ -64,6 +64,10 @@
         if self.stop_here(frame) or self.break_here(frame):
             self.user_line(frame)
             if self.quitting: raise BdbQuit
+            # Do not re-install the local trace when we are finished debugging,
+            # see issues 16482 and 7238.
+            if not sys.gettrace():
+                return None
         return self.trace_dispatch
 
     def dispatch_call(self, frame, arg):
@@ -231,8 +235,10 @@
             # no breakpoints; run without debugger overhead
             sys.settrace(None)
             frame = sys._getframe().f_back
-            while frame and frame is not self.botframe:
+            while frame:
                 del frame.f_trace
+                if frame is self.botframe:
+                    break
                 frame = frame.f_back
 
     def set_quit(self):

The following code is a minimum implementation of pdb with the patch
applied and the associated code to test it.

class Bdb:

    def trace_dispatch(self, frame, event, arg):
        self.set_continue()
        if sys.gettrace():
            return self.trace_dispatch

    def set_trace(self, frame):
        self.botframe = frame
        frame.f_trace = self.trace_dispatch
        sys.settrace(self.trace_dispatch)

    def set_continue(self):
        sys.settrace(None)
        del self.botframe.f_trace

frame = sys._getframe()
d = Bdb()
d.set_trace(frame)

y = "line of code not triggering an error"
x = 1
assert x != 1

See also how it is fixed at
http://code.google.com/p/pdb-clone/source/detail?r=123d1b6db6649f4b54712db321865fda55395488&name=default

See also bpo-17697. Xavier, would you be willing to submit a patch with a test?

The last patch proposed at bpo-17277 and named traced_frame.patch
fixes also bpo-17697, and issues 7238, 16482. IMHO one should remove
the assumption in PyFrame_GetLineNumber() that f_lineno is correct
when f_trace is not NULL, as there does not seem to be a way to remove
the local trace function of a generator frame. The traced_frame patch
does that.

Forgot to say that traced_frame.patch includes a test.

I've just hit this. Is there anything I can do to help get this fixed?`

Just for reference: #6233 is about fixing this.

I too just hit this issue with pdb and checked out the PR locally. It works fine though it has merge conflicts with latest master. I am adding 3.8, 3.7 and 3.6 removing the older versions that don't support bug fixes. Since the PR has unknown repository I suppose @xdegaye has deleted their fork and I don't know if the PR can be revived without opening a new one.

I have merged the latest master in my own fork resolving conflicts in case a new PR has to be generated : master...tirkarthi:[pr-6233](https://github.com/python/cpython/pull/6233)-latest. Adding Serhiy to the issue. The PR also solves many linked issues in the PR and I thought to add the update here since it seemed appropriate.

Thanks much @xdegaye for the PR

You can download the branch for a pull request even if the repo is deleted using this https://stackoverflow.com/a/28622034/161801. That will let you keep the original commits intact.

iritkatriel@Irits-MBP cpython % cat pdb_traceback.py

import pdb

x = 0

while True:
	pdb.set_trace()
	y = "line of code not triggering an error"
	x += 1
	assert x != 3
iritkatriel@Irits-MBP cpython % cat pdb_traceback.py

import pdb

x = 0

while True:
	pdb.set_trace()
	y = "line of code not triggering an error"
	x += 1
	assert x != 3
iritkatriel@Irits-MBP cpython % ./python.exe pdb_traceback.py
> /Users/iritkatriel/src/cpython/pdb_traceback.py(8)<module>()
-> y = "line of code not triggering an error"
(Pdb) c
> /Users/iritkatriel/src/cpython/pdb_traceback.py(8)<module>()
-> y = "line of code not triggering an error"
(Pdb) c
> /Users/iritkatriel/src/cpython/pdb_traceback.py(8)<module>()
-> y = "line of code not triggering an error"
(Pdb) c
Traceback (most recent call last):
  File "/Users/iritkatriel/src/cpython/pdb_traceback.py", line 10, in <module>
    assert x != 3
    ^^^^^^^^^^^^^
AssertionError