protect flush_figures post-exec function from user error

Invalid matplotlib data may raise inside print_figure.  flush_figures()
is a post-exec function, so user errors should not raise.  Instead,
call get_ipython().showtraceback() if called from IPython, raising as before
otherwise.

IPython/zmq/pylab/backend_inline.py

@@ -1,180 +1,199 b''
 """Produce SVG versions of active plots for display by the rich Qt frontend.
 """
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 from __future__ import print_function
 
 # Standard library imports
 import sys
 
 # Third-party imports
 import matplotlib
 from matplotlib.backends.backend_agg import new_figure_manager
 from matplotlib._pylab_helpers import Gcf
 
 # Local imports.
 from IPython.config.configurable import SingletonConfigurable
 from IPython.core.displaypub import publish_display_data
 from IPython.core.pylabtools import print_figure, select_figure_format
 from IPython.utils.traitlets import Dict, Instance, CaselessStrEnum, CBool
 from IPython.utils.warn import warn
 
 #-----------------------------------------------------------------------------
 # Configurable for inline backend options
 #-----------------------------------------------------------------------------
 # inherit from InlineBackendConfig for deprecation purposes
 class InlineBackendConfig(SingletonConfigurable):
     pass
 
 class InlineBackend(InlineBackendConfig):
     """An object to store configuration of the inline backend."""
 
     def _config_changed(self, name, old, new):
         # warn on change of renamed config section
         if new.InlineBackendConfig != old.InlineBackendConfig:
             warn("InlineBackendConfig has been renamed to InlineBackend")
         super(InlineBackend, self)._config_changed(name, old, new)
 
     # The typical default figure size is too large for inline use,
     # so we shrink the figure size to 6x4, and tweak fonts to
     # make that fit.
     rc = Dict({'figure.figsize': (6.0,4.0),
         # 12pt labels get cutoff on 6x4 logplots, so use 10pt.
         'font.size': 10,
         # 72 dpi matches SVG/qtconsole
         # this only affects PNG export, as SVG has no dpi setting
         'savefig.dpi': 72,
         # 10pt still needs a little more room on the xlabel:
         'figure.subplot.bottom' : .125
         }, config=True,
         help="""Subset of matplotlib rcParams that should be different for the
         inline backend."""
     )
 
     figure_format = CaselessStrEnum(['svg', 'png'], default_value='png', config=True,
         help="The image format for figures with the inline backend.")
 
     def _figure_format_changed(self, name, old, new):
         if self.shell is None:
             return
         else:
             select_figure_format(self.shell, new)
     
     close_figures = CBool(True, config=True,
         help="""Close all figures at the end of each cell.
         
         When True, ensures that each cell starts with no active figures, but it
         also means that one must keep track of references in order to edit or
         redraw figures in subsequent cells. This mode is ideal for the notebook,
         where residual plots from other cells might be surprising.
         
         When False, one must call figure() to create new figures. This means
         that gcf() and getfigs() can reference figures created in other cells,
         and the active figure can continue to be edited with pylab/pyplot
         methods that reference the current active figure. This mode facilitates
         iterative editing of figures, and behaves most consistently with
         other matplotlib backends, but figure barriers between cells must
         be explicit.
         """)
 
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC')
 
 
 #-----------------------------------------------------------------------------
 # Functions
 #-----------------------------------------------------------------------------
 
 def show(close=None):
     """Show all figures as SVG/PNG payloads sent to the IPython clients.
 
     Parameters
     ----------
     close : bool, optional
       If true, a ``plt.close('all')`` call is automatically issued after
       sending all the figures. If this is set, the figures will entirely
       removed from the internal list of figures.
     """
     if close is None:
         close = InlineBackend.instance().close_figures
     try:
         for figure_manager in Gcf.get_all_fig_managers():
             send_figure(figure_manager.canvas.figure)
     finally:
         show._to_draw = []
         if close:
             matplotlib.pyplot.close('all')
 
 
 
 # This flag will be reset by draw_if_interactive when called
 show._draw_called = False
 # list of figures to draw when flush_figures is called
 show._to_draw = []
 
 
 def draw_if_interactive():
     """
     Is called after every pylab drawing command
     """
     # signal that the current active figure should be sent at the end of execution.
     # Also sets the _draw_called flag, signaling that there will be something to send.
     # At the end of the code execution, a separate call to flush_figures()
     # will act upon these values
     
     fig = Gcf.get_active().canvas.figure
     
     # ensure current figure will be drawn, and each subsequent call
     # of draw_if_interactive() moves the active figure to ensure it is
     # drawn last
     try:
         show._to_draw.remove(fig)
     except ValueError:
         # ensure it only appears in the draw list once
         pass
     show._to_draw.append(fig)
     show._draw_called = True
 
 def flush_figures():
     """Send all figures that changed
 
     This is meant to be called automatically and will call show() if, during
     prior code execution, there had been any calls to draw_if_interactive.
+    
+    This function is meant to be used as a post_execute callback in IPython,
+    so user-caused errors are handled with showtraceback() instead of being
+    allowed to raise.  If this function is not called from within IPython,
+    then these exceptions will raise.
     """
     if not show._draw_called:
         return
     
     if InlineBackend.instance().close_figures:
         # ignore the tracking, just draw and close all figures
-        return show(True)
-    
+        try:
+            return show(True)
+        except Exception as e:
+            # safely show traceback if in IPython, else raise
+            try:
+                get_ipython().showtraceback()
+                return
+            except NameError:
+                raise e
     try:
         # exclude any figures that were closed:
         active = set([fm.canvas.figure for fm in Gcf.get_all_fig_managers()])
         for fig in [ fig for fig in show._to_draw if fig in active ]:
-            send_figure(fig)
+            try:
+                send_figure(fig)
+            except Exception as e:
+                # safely show traceback if in IPython, else raise
+                try:
+                    get_ipython().showtraceback()
+                    break
+                except NameError:
+                    raise e
     finally:
         # clear flags for next round
         show._to_draw = []
         show._draw_called = False
 
 
 def send_figure(fig):
     """Draw the current figure and send it as a PNG payload.
     """
-    # For an empty figure, don't even bother calling figure_to_svg, to avoid
-    # big blank spaces in the qt console
-    if not fig.axes:
-        return
     fmt = InlineBackend.instance().figure_format
     data = print_figure(fig, fmt)
+    # print_figure will return None if there's nothing to draw:
+    if data is None:
+        return
     mimetypes = { 'png' : 'image/png', 'svg' : 'image/svg+xml' }
     mime = mimetypes[fmt]
     # flush text streams before sending figures, helps a little with output
     # synchronization in the console (though it's a bandaid, not a real sln)
     sys.stdout.flush(); sys.stderr.flush()
     publish_display_data(
         'IPython.zmq.pylab.backend_inline.send_figure',
         {mime : data}
     )