Re: [matplotlib-devel] Error handling in backends.

[John H. <jdh...@ac...>]

>>>>> "Ted" == Ted Drain <ted...@jp...> writes:

    Ted> OK - I think I've read most of those messages and had a few
    Ted> comments/ideas.  Soapbox mode on: Please don't change the
    Ted> system exception hook.  Matplotlib (MPL) is not an
    Ted> application, it's a library.  Libraries should never change
    Ted> global behavior - only applications.  By changing global
    Ted> behavior you severely limit how and where your library can be
    Ted> used by others.  Soapbox mode off.

Point taken.  One might view the pylab interface as an application,
but I won't quibble.

    Ted> I wonder if it helps to think about errors from a different
    Ted> stand point.  For example, I can think of two high level
    Ted> views that might help.  Users interact with MPL either
    Ted> through the Python command line (via typing or scripts) or
    Ted> through GUI front ends.

    Ted> 1) A python function is called which has an error (or
    Ted> something it calls has an error, etc, etc).  In this case, if
    Ted> the function throws an exception, python will propagate the
    Ted> exception normally.  If you're typing something on the prompt
    Ted> or running a script, the exception get's printed with a trace
    Ted> back.  Very useful and very normal for Python.

There is a wrinkle here.  On win32 (or another platform that binds
python scripts to an executable), you might double click on a script

  from pylab import *
  semilogy([0,1,2,3])
  show()

In pre 0.72 mpl, this would throw an exception.  On windows, if you
double clicked this script, a stdout window would launch and die
before you could read the traceback -- very annoying.  That's the
primary reason I tried to catch exceptions in the pylab interface and
forward them on to the error_msg handler function.  But the current
impl. is admittedly broken.  Do you have a suggestion for how to
handle this case?

We could do something like this: all GUIs define a function
show_message.  This pops up a message dialog with a single OK button.
pylab dies the following


def plot(*args, **kwargs):
    # allow callers to override the hold state by passing hold=True|False
    b = ishold()
    h = popd(kwargs, 'hold', None)
    if h is not None:
        hold(h)
    try:
        ret =  gca().plot(*args, **kwargs)
    except:
        msg = ...get the traceback as string ...
        show_message(msg)
        hold(b)
        ...rethrow the same exception....   
    else:
        try: draw_if_interactive()
        except:
            msg = ...get the traceback as string...
            show_message(msg)
            hold(b)
            ...rethrow the same exception....   
        
        hold(b)
        return ret

Someone please remind me how to rethrow the last exception --
Fernando?

Or we could just ignore it and let win32 users who are double clicking
scripts die the painful death they deserve.  It would certainly make
for cleaner pylab code.

    Ted> - Error trying to save the plot to the image file 'plot.png'.
    Ted> - Error in the automatic plot layout algorithm.  - Error in
    Ted> setting the size of the plot region.  The inputs size of
    Ted> [-10, 200] is invalid.

This gives you more diagnostic information in that you get helpful
messages at each level, but the standard traceback gives you all the
levels, no?

JDH