don't close figures every cycle in inline backend

This allows gcf(), getfigs(), etc. to be useful again in the qtconsole and notebook.

draw_if_interactive() signals that the current figure is to be drawn.

Figures that are created and closed in the same cell will not be drawn.

See  #638

IPython/zmq/pylab/backend_inline.py

@@ -1,116 +1,139 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.lib.pylabtools import print_figure, select_figure_format
 from IPython.utils.traitlets import Dict, Instance, CaselessStrEnum
 #-----------------------------------------------------------------------------
 # Configurable for inline backend options
 #-----------------------------------------------------------------------------
 
 class InlineBackendConfig(SingletonConfigurable):
     """An object to store configuration of the inline backend."""
 
     # 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.  This is configurable via Global.pylab_inline_rc,
     # or rather it will be once the zmq kernel is hooked up to
     # the config system.
     rc = Dict({'figure.figsize': (6.0,4.0),
         # 12pt labels get cutoff on 6x4 logplots, so use 10pt.
         'font.size': 10,
         # 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)
 
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC')
 
 
 #-----------------------------------------------------------------------------
 # Functions
 #-----------------------------------------------------------------------------
 
-def show(close=True):
+def show(close=False):
     """Show all figures as SVG payloads sent to the IPython clients.
 
     Parameters
     ----------
     close : bool, optional
       If true, a ``plt.close('all')`` call is automatically issued after
-      sending all the SVG figures. If this is set, the figures will entirely
+      sending all the figures. If this is set, the figures will entirely
       removed from the internal list of figures.
     """
     for figure_manager in Gcf.get_all_fig_managers():
         send_figure(figure_manager.canvas.figure)
     if close:
         matplotlib.pyplot.close('all')
+    show._to_draw = []
+        
 
 
 # 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
     """
-    # We simply flag we were called and otherwise do nothing.  At the end of
-    # the code execution, a separate call to show_close() will act upon this.
+    # 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():
-    """Call show, close all open figures, sending all figure images.
+    """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.
     """
-    if show._draw_called:
-        show()
-        show._draw_called = False
+    if not show._draw_called:
+        return
+    # 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)
+    # 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 = InlineBackendConfig.instance().figure_format
     data = print_figure(fig, fmt)
     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}
     )