sudden exceptions in PDB when entering a callback function

Question

I am trying to debug my python application which uses the "mailbot" module together with triggered user callbacks. These get triggered when a specific email has been received. Yesterday, debugging the application with PDB worked perfectly fine and I kind of finished my work. But today it seems that the world has changed... I can't remember having changed any relevant code since yesterday, but when I invoke my application within the PDB setting, it causes many exceptions which I don't know how to interpret.

When omitting PDB and starting the application with python application.py, the application runs fine. So (and the error messages suggest it as well), the problems do not seem to have anything to do with my application, the usage of callbacks or the mailbot module.

Here is the story until the error messages. Sorry, it appears to be lengthy.

starting python in PDB mode...

C:\...> python -m pdb application.py
> c:\...\application.py(2)<module>()
-> from __future__ import print_function

setting breakpoint to one step before exception...

(Pdb) b c:\PROGS\Python34\Lib\site-packages\mailbot\mailbot.py:52
Breakpoint 1 at c:\progs\python34\lib\site-packages\mailbot\mailbot.py:52

starting execution until breakpoint...

(Pdb) c
> c:\progs\python34\lib\site-packages\mailbot\mailbot.py(52)process_message()
-> return callback.trigger()

show relevant listing...

(Pdb) l
 47
 48         def process_message(self, message, callback_class, rules):
 49             """Check if callback matches rules, and if so, trigger."""
 50             callback = callback_class(message, rules)
 51             if callback.check_rules():
 52 B->             return callback.trigger()
 53
 54         def process_messages(self):
 55             """Process messages: check which callbacks should be triggered."""
 56             from . import CALLBACKS_MAP
 57             self.reset_timeout_messages()

examining next statement...

(Pdb) callback.trigger
<bound method cbLookUpMindmapAndSendText.trigger of <callbacks.cbLookUpMindmapAndSendText object at 0x02EB95B0>>

stepping into the callback...

(Pdb) s
--Call--
Traceback (most recent call last):
  File "C:\PROGS\Python34\lib\pdb.py", line 1661, in main
    pdb._runscript(mainpyfile)
  File "C:\PROGS\Python34\lib\pdb.py", line 1542, in _runscript
    self.run(statement)
  File "C:\PROGS\Python34\lib\bdb.py", line 431, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "c:\...\application.py", line 25, in <module>
    mailbox.process_messages()
  File "C:\PROGS\Python34\lib\site-packages\mailbot\mailbot.py", line 64, in process_messages
    self.process_message(message, callback_class, rules)
  File "C:\PROGS\Python34\lib\site-packages\mailbot\mailbot.py", line 52, in process_message
    return callback.trigger()
Traceback (most recent call last):
  File "C:\PROGS\Python34\lib\pdb.py", line 1661, in main
    pdb._runscript(mainpyfile)
  File "C:\PROGS\Python34\lib\pdb.py", line 1542, in _runscript
    self.run(statement)
  File "C:\PROGS\Python34\lib\bdb.py", line 431, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "c:\...\application.py", line 25, in <module>
    mailbox.process_messages()
  File "C:\PROGS\Python34\lib\site-packages\mailbot\mailbot.py", line 64, in process_messages
    self.process_message(message, callback_class, rules)
  File "C:\PROGS\Python34\lib\site-packages\mailbot\mailbot.py", line 52, in process_message
    return callback.trigger()
  File "C:\...\callbacks.py", line 20, in trigger
  File "C:\PROGS\Python34\lib\bdb.py", line 50, in trace_dispatch
    return self.dispatch_call(frame, arg)
  File "C:\PROGS\Python34\lib\bdb.py", line 82, in dispatch_call
    self.user_call(frame, arg)
  File "C:\PROGS\Python34\lib\pdb.py", line 249, in user_call
    self.interaction(frame, None)
  File "C:\PROGS\Python34\lib\pdb.py", line 345, in interaction
    self.print_stack_entry(self.stack[self.curindex])
  File "C:\PROGS\Python34\lib\pdb.py", line 1447, in print_stack_entry
    self.format_stack_entry(frame_lineno, prompt_prefix))
  File "C:\PROGS\Python34\lib\bdb.py", line 411, in format_stack_entry
    line = linecache.getline(filename, lineno, frame.f_globals)
  File "C:\PROGS\Python34\lib\linecache.py", line 15, in getline
    lines = getlines(filename, module_globals)
  File "C:\PROGS\Python34\lib\linecache.py", line 41, in getlines
    return updatecache(filename, module_globals)
  File "C:\PROGS\Python34\lib\linecache.py", line 127, in updatecache
    lines = fp.readlines()
  File "C:\PROGS\Python34\lib\codecs.py", line 319, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xfc in position 351: invalid start byte

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\PROGS\Python34\lib\runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\PROGS\Python34\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\PROGS\Python34\lib\pdb.py", line 1685, in <module>
    pdb.main()
  File "C:\PROGS\Python34\lib\pdb.py", line 1673, in main
    traceback.print_exc()
  File "C:\PROGS\Python34\lib\traceback.py", line 252, in print_exc
    print_exception(*sys.exc_info(), limit=limit, file=file, chain=chain)
  File "C:\PROGS\Python34\lib\traceback.py", line 169, in print_exception
    for line in _format_exception_iter(etype, value, tb, limit, chain):
  File "C:\PROGS\Python34\lib\traceback.py", line 153, in _format_exception_iter
    yield from _format_list_iter(_extract_tb_iter(tb, limit=limit))
  File "C:\PROGS\Python34\lib\traceback.py", line 18, in _format_list_iter
    for filename, lineno, name, line in extracted_list:
  File "C:\PROGS\Python34\lib\traceback.py", line 65, in _extract_tb_or_stack_iter
    line = linecache.getline(filename, lineno, f.f_globals)
  File "C:\PROGS\Python34\lib\linecache.py", line 15, in getline
    lines = getlines(filename, module_globals)
  File "C:\PROGS\Python34\lib\linecache.py", line 41, in getlines
    return updatecache(filename, module_globals)
  File "C:\PROGS\Python34\lib\linecache.py", line 127, in updatecache
    lines = fp.readlines()
  File "C:\PROGS\Python34\lib\codecs.py", line 319, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xfc in position 351: invalid start byte

C:\...>

Any hints, where or how to work for a solution?

Thank you. nnako

Answer 1

finally, I found the solution by trying to run the application on a Linux machine. Running it on Windows (without PDB) resulted in NO ERRORS. Linux showed this output while trying to run the command python application.py:

Traceback (most recent call last):
  File "application.py", line 12, in <module>
    import callbacks
  File "/home/pi/examples/callbacks.py", line 16
SyntaxError: Non-ASCII character '\xfc' in file /home/pi/examples/callbacks.py on line 16, but no encoding declared; see http://www.python.org/peps/pep-0263.html for details

So I found out that the oddity was caused by a non printable character "\xfc" hidden somewhere in a comment line. Removing the line and re-typing its contents solved the problem. Even though I do not have a clue how this could possibly cause the exception messages described in the post above.

Hope, this would be some help for someone some day.

Thanks. nnako