debugging with ipython does not work well outside ipython

[Robert Cimrman]

Python 2.5.4, IPython 0.9.1, gentoo linux

The Tracer does not work outside ipython, short summary follows below, for details see the thread [1].

[1] http://mail.scipy.org/pipermail/ipython-user/2009-July/006451.html

When running the attachment file ipython_debug1.py:

  $ python ipython_debug1.py
Traceback (most recent call last):
   File "ipython_debug1.py", line 6, in <module>
     debug = Tracer()
   File "/usr/lib64/python2.5/site-packages/IPython/Debugger.py", line
135, in __init__
     self.debugger = Pdb(colors)
   File "/usr/lib64/python2.5/site-packages/IPython/Debugger.py", line
213, in __init__
     self.color_scheme_table = ExceptionColors.copy()
   File "/usr/lib64/python2.5/site-packages/IPython/ColorANSI.py", line
151, in copy
     return ColorSchemeTable(self.values(),self.active_scheme_name)
   File "/usr/lib64/python2.5/site-packages/IPython/ColorANSI.py", line
144, in __init__
     raise ValueError,'you must specify the default color scheme'
ValueError: you must specify the default color scheme

The second file ipython_debug2.py performs better - the debugging is actually possible, but any exception after the 'continue' command causes the ipython crash dump:

$ python ipython_debug2.py
hello
 > ipython_debug2.py(14)<module>()
      13
---> 14 print 'hello again'
      15

ipdb>

then type 'c' to continue, and you get the error. I would like to see
just the ZeroDivisionError, not the ipython crash dump that follows.

[Fernando Perez (fdo.perez)]

I'm afraid that for now, IPython's debugger is pretty much welded to living inside a 'real ipython'. It simply assumes in too many places that the ipython machinery is already up and running.

It would be definitely great to decouple them, but it's a fairly non-trivial job. I'm marking this as a wishlist item so we don't forget about it, but I don't think it will happen soon, unfortunately.

[Robert Cimrman (cimrman3)]

Thanks Fernando.

As a workaround, is there a way to capture/silence/redirect to /dev/null somehow the ipython crash report output after pressing Ctrl-C? The crash per se does not matter, it's just the length of the output, that makes the interesting things I saw while debugging to scroll out of sight (and often even the terminal buffer).

[Fernando Perez (fdo.perez)]

Hi Robert,

On Mon, Apr 26, 2010 at 12:43 AM, Robert Cimrman <email address hidden> wrote:
> Thanks Fernando.
>
> As a workaround, is there a way to capture/silence/redirect to /dev/null
> somehow the ipython crash report output after pressing Ctrl-C? The crash
> per se does not matter, it's just the length of the output, that makes
> the interesting things I saw while debugging to scroll out of sight (and
> often even the terminal buffer).

Certainly, I suspect the problem is that ipython sets sys.excepthook
to be its own (noisy) crash handler.

Just save sys.excepthook before you start ipython's machinery to
sys.old_excepthook, and restore it immediately once you exit the
ipython part and you shouldn't see the ipython internal handler
anymore.

Let me know if that works for you.

Cheers,

f

[Robert Cimrman (cimrman3)]

On 04/27/10 07:41, Fernando Perez wrote:
> Hi Robert,
>
> On Mon, Apr 26, 2010 at 12:43 AM, Robert Cimrman<email address hidden> wrote:
>> Thanks Fernando.
>>
>> As a workaround, is there a way to capture/silence/redirect to /dev/null
>> somehow the ipython crash report output after pressing Ctrl-C? The crash
>> per se does not matter, it's just the length of the output, that makes
>> the interesting things I saw while debugging to scroll out of sight (and
>> often even the terminal buffer).
>
> Certainly, I suspect the problem is that ipython sets sys.excepthook
> to be its own (noisy) crash handler.
>
> Just save sys.excepthook before you start ipython's machinery to
> sys.old_excepthook, and restore it immediately once you exit the
> ipython part and you shouldn't see the ipython internal handler
> anymore.
>
> Let me know if that works for you.

Perfect! I will use the following function:

def debug():
     try:
         old_excepthook = sys.excepthook
         import ipdb as pdb
         sys.excepthook = old_excepthook

     except:
         import pdb

     pdb.set_trace()

Do you happen to know how to avoid pressing 'n' to get to the caller's frame?

cheers,
r.

[Fernando Perez (fdo.perez)]

Hi Robert,

On Tue, Apr 27, 2010 at 4:30 AM, Robert Cimrman <email address hidden> wrote:
> Perfect! I will use the following function:

Great, glad it worked.

> Do you happen to know how to avoid pressing 'n' to get to the caller's
> frame?

No idea, sorry.

Cheers,

f

[Robert Cimrman (cimrman3)]

On 04/27/10 19:22, Fernando Perez wrote:
> Hi Robert,
>
> On Tue, Apr 27, 2010 at 4:30 AM, Robert Cimrman<email address hidden> wrote:
>> Perfect! I will use the following function:
>
> Great, glad it worked.
>
>> Do you happen to know how to avoid pressing 'n' to get to the caller's
>> frame?
>
> No idea, sorry.

I have a working solution now, see below. The only problem is, that sphinx'
automodule directive ignores the returned debug() function, but this problem
can be solved by adding it directly using autofunction:

.. automodule:: my_module
    :members:
    :undoc-members:

    .. autofunction:: my_module.debug

In my_module:

def get_debug():
     """
     Utility function providing ``debug()`` function.
     """
     old_excepthook = sys.excepthook

     try:
         import ipdb
     except ImportError:
         debug = None
     else:
         debug = ipdb.set_trace

     if debug is None:
         try:
             from IPython.Debugger import Pdb
             from IPython.Shell import IPShell
             from IPython import ipapi
         except ImportError:
             pass
         else:
             shell = IPShell(argv=[''])
             def debug():
                 ip = ipapi.get()
                 def_colors = ip.options.colors
                 Pdb(def_colors).set_trace(sys._getframe().f_back)

     if debug is None:
         import pdb
         debug = pdb.set_trace

     sys.excepthook = old_excepthook

     debug.__doc__ = """
     Start debugger on line where it is called, roughly equivalent to::

         import pdb; pdb.set_trace()

     First, this function tries to use `ipdb` (`IPython`-enabled `pdb`).

     If it is not installed, the function resorts to trying to start the
     debugger directly using the `IPython` API.

     When this fails too, the plain old `pdb` is used instead.
     """

     return debug

debug = get_debug()

cheers,
r.

[Fernando Perez (fdo.perez)]

OK, great. This bug stays tagged as wishlist, so we can effectively provide something like what you are doing, with a publicly supported api.