Re: [matplotlib-devel] matplotlib.use()

[Benjamin R. <ben...@ou...>]

On Fri, Jul 20, 2012 at 3:20 PM, Eric Firing <ef...@ha...> wrote:

> On 2012/07/20 9:34 AM, Ryan May wrote:
> > Hi,
> >
> > I'm here at the SciPy sprints trying to fix switching inline/gui for
> > the ipython notebook. I've noticed something weird about
> > matplotlib.use() :
> >
> >      if 'matplotlib.backends' in sys.modules:
> >          if warn: warnings.warn(_use_error_msg)
> >          return
> >      if arg.startswith('module://'):
> >          name = arg
> >      else:
> >          # Lowercase only non-module backend names (modules are
> case-sensitive)
> >          arg = arg.lower()
> >          name = validate_backend(arg)
> >      rcParams['backend'] = name
> >
> > Notice the return if it finds the module loaded. This return basically
> > makes it impossible to make use() have any effect one backends has
> > been loaded. However, matplotlib.pyplot.switch_backend(). Eric added
> > this in 2008 as a bug fix, but no more detail than that. Does anyone
> > have a problem putting the return with the warning? Therefore if you
> > pass warn=False, you can actually make the setting take effect, but
> > the average user won't accidentally shoot their foot off. Something
> > like:
> >
> >      if 'matplotlib.backends' in sys.modules:
> >          if warn:
> >              warnings.warn(_use_error_msg)
> >              return
>
> This would be defeating the basic idea: use() is *only* designed to take
> effect *once*.  If you need to switch backends, it takes more than what
> use() does, hence the need for switch_backend().  (But see below.)
>
> The warn=False invocation of use, with the return statement were it is
> now, is for the case where you may have a script or module that
> specifies a backend (in case it is the first or only import of
> matplotlib), but that may itself be imported or called by something else
> that has already imported matplotlib and set the backend.  In this case
> we want the warning to be suppressed, which is all the warn=False flag
> is supposed to do.  I use this myself.
>
> So it looks like the problem is that switch_backends is broken because
> what it needs matplotlib.use to do is go ahead and make the switch. This
> could be done by adding a force=True kwarg (default would be False) to
> matplotlib.use, or by deleting the reference to the backends module from
> sys.modules in switch_backends before calling matplotlib.use.  A
> variation would be to use the force kwarg and to include the
> switch_backends logic, or at least the reload line, in matplotlib.use.
> That probably makes sense: add the force kwarg, and when the module is
> replaced, reload it immediately within matplotlib.use.  Then
> pyplot.switch_backends only needs to do the pyplot-specific operations.
>   I think that is the cleanest solution.
>
> Eric
>
>
I like the "force" idea very much.  It means exactly what it says.

Ben Root