BdbQuit raised when debugging Python with pdb

Recently when adding the pdb debugger to my Python 2.7.10 code, I get this message:

Traceback (most recent call last):
  File "/Users/isaachess/Programming/vivint/Platform/MessageProcessing/vivint_cloud/queues/connectors/amqplib_connector.py", line 191, in acking_callback
    callback(message.body)
  File "/Users/isaachess/Programming/vivint/Platform/MessageProcessing/vivint_cloud/queues/consumable_message_queue.py", line 32, in deserialized_callback
    self._callback_method(msg)
  File "/Users/isaachess/Programming/vivint/Platform/BusinessLogic/businesslogic/util/statsd_util.py", line 95, in _time_func
    retVal = f(*args, **kwargs)
  File "/Users/isaachess/Programming/vivint/Platform/MessageProcessing/vivint_cloud/net/router.py", line 226, in handle
    try:
  File "/Users/isaachess/Programming/vivint/Platform/MessageProcessing/vivint_cloud/net/router.py", line 226, in handle
    try:
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/bdb.py", line 49, in trace_dispatch
    return self.dispatch_line(frame)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/bdb.py", line 68, in dispatch_line
    if self.quitting: raise BdbQuit
BdbQuit

This is after inserting the line:

import pdb; pdb.set_trace()

in the code.

I cannot figure out why this is happening. I've read up on Bdb and Bdbquit, but cannot figure out why this is happening in my code. Can anyone provide me with some hints of why this happens in general? I really want to get the debugger working again.

I ran into this when I left import pdb and a pdb.set_trace() in my production code. When the pdb.set_trace() line was executed, python was waiting for my input to tell it to continue or step into, etc... Because the python code was being called by a web server I wasn't there to press c to continue. After so long (not sure how long) it finally raised the BdbQuit exception.

I didn't have anything setup to catch that exception so it raised a 500 in my web server.

It took me a while to understand that my debug code running in an a daemon/background was causing the issue. I felt silly.

If you continue from the (pdb) prompt and allow your code to finish normally, I wouldn't expect output like the traceback you indicated, but if you quit pdb, with the quit command or ^D (EOF), a traceback like that occurs because there is nothing to catch the BdbQuit exception raised when the debugger quits. In bdb.py self.quitting gets set to True by the set_quit method (and by finally clauses in the various run methods). Dispatch methods called by trace_dispatch raise BdbQuit when self.quitting is True, and the typical except: clause for BdbQuit is a simple pass statement; pdb inherits all of that from gdb.

In short, exception handling is used to disable the system trace function used by the debugger, when the debugger interaction finishes early.

One way to avoid that traceback altogether is to use pdb differently. Rather than calling pdb.set_trace() from your code (and not handling BdbQuit at all), you can invoke your code within pdb (rather than vice versa), at which point the BdbQuit exception will be handled as intended by pdb. That will also allow you to choose breakpoint locations without modifying your code (using pdb's break command). Or you can mix the two approaches; run your code under pdb, pdb.set_trace() calls and all, and those calls will be breakpoints that you can remove only by modifying your code.

You can invoke your code within pdb by using the pdb command with your script invocation as its command line arguments, or with python -m pdb.