Re: [matplotlib-devel] switching to PS/SVG backends

[Steve C. <ste...@ya...>]

On Wed, 2004-11-17 at 10:54 -0700, Fernando Perez wrote:
> Jochen Voss schrieb:
> > Hello Steve,
> > 
> > On Wed, Nov 17, 2004 at 11:00:32PM +0800, Steve Chaplin wrote:
> > 
> >>I don't think failure to save a file should be a fatal error.
> >>Perhaps PS/SVG print_figure() could raise an exception for the GUI
> >>backend to catch so it can popup an error message and continue.
> > 
> > Even now you can probably (not tried) catch the SystemExit
> > exception and prevent the program from aborting.
> > Do you think raising SystemExit like
> > 
> >     raise SystemExit("error while writing file: permission denied")
> > 
> > would be good enough.  The GUI frontend could catch SystemExit,
> > check whether the associated value is a string, and then display
> > this string in an error message box.
> > 
> > What do you think?
> 
> Bad design.  Doing exception analysis based on string matching for the message 
> is very brittle.  A single change in capitalization of the message can break 
> things down the road.
> 
> It's _far_ better to either:
> 
> 1. have the ps/svg backends do whatever cleanup they want, and then reraise 
> the original exception unchanged
> 
> 2. or simply make a matplotlib.SaveError exception which can be explicitly 
> caught based on class matching, which is the preferred python way of doing this.
> 
> Best,
> 
> f
At the moment the PS backend does
  try:
      fh = file(outfile, 'w')
  except IOError:
      error_msg_ps('Could not open %s for writing' % outfile)

which translates into
  try:
      fh = file(outfile, 'w')
  except IOError:
      verbose.report_error('Error: Could not open %s for writing' %
outfile)
      sys.exit()

The backend does not do any cleanup after the exception, so I think it
could be changed to just
      fh = file(outfile, 'w')
allowing the exception terminate the program with the error message
appearing at the end of the traceback.

Then the GUI backends can then do
    from backend_svg import FigureCanvasSVG as FigureCanvas
    try:
        fc = self.switch_backends(FigureCanvas)
        fc.print_figure(filename, dpi, facecolor, edgecolor,
orientation)
    except IOError, exc:
        error_msg("%s: %s" % (exc.filename, exc.strerror), parent=self)

I tried this calling the SVG backend directly and by switching to the
SVG backend from the GTK backend and it works OK.

Steve