Using _view_count in ipywidget to lazily update an 'expensive' plot

ExpensivePlot.ipynb

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    "import traitlets\n",
    "import ipywidgets\n",
    "import matplotlib.pylab as plt\n",
    "import numpy as np\n",
    "import IPython.display"
   ]
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       "<function IPython.core.display.clear_output>"
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    "IPython.display.clear_output"
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    "class ExpensivePlot(traitlets.HasTraits):\n",
    "    gamma = traitlets.CFloat(default_value=2, help='Exponent for our expensive computation')\n",
    "    dirty = traitlets.CBool(default_value=True, help='Keep track if out plot is dirty (needs an update) or not')\n",
    "    def __init__(self, **kwargs):\n",
    "        super(ExpensivePlot, self).__init__(**kwargs)\n",
    "        # _view_count is None by default to avoid keeping track of the number of views\n",
    "        self.output_plot = ipywidgets.Output(_view_count=0)\n",
    "        self.output_log = ipywidgets.Output()\n",
    "        # if the number of views changes, or the plot is 'dirty'\n",
    "        # we need to check if we need to update the plot\n",
    "        self.output_plot.observe(self._check_plot_update, '_view_count')\n",
    "        self.observe(self._check_plot_update, 'dirty')\n",
    "\n",
    "        # track if a parameter has changed, which will mark the plot as dirty\n",
    "        self.observe(self._parameter_changed, 'gamma')\n",
    "        \n",
    "    def _parameter_changed(self, change):\n",
    "        with self.output_log:\n",
    "            print('A parameter has changed')\n",
    "            print(change)\n",
    "            self.dirty = True\n",
    "        \n",
    "    def _check_plot_update(self, change):\n",
    "        if self.dirty and self.output_plot._view_count > 0:\n",
    "            with self.output_log:\n",
    "                print('it seems our plot is dirty, and is visible, we need to update')\n",
    "            self.update_expensive_plot()\n",
    "        else:\n",
    "            with self.output_log:\n",
    "                if self.dirty:\n",
    "                    print('it seems our plot is dirty, but nothing is visible, so we postpone updating')\n",
    "                else:\n",
    "                    print('nothing to update')\n",
    "\n",
    "    def _ipython_display_(self):\n",
    "        # forward to the output for the plot\n",
    "        return self.output_plot._ipython_display_()\n",
    "\n",
    "    def update_expensive_plot(self):\n",
    "        with self.output_log:\n",
    "            print('Updating an expensive plot')\n",
    "        # clear display, but want till the new output is available to avoid flickering\n",
    "        with self.output_plot:\n",
    "            IPython.display.clear_output(wait=True)\n",
    "            x = np.arange(10)\n",
    "            y = x**self.gamma\n",
    "            plt.plot(x, y)\n",
    "            plt.show()\n",
    "        self.dirty = False\n",
    "        "
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      "application/vnd.jupyter.widget-view+json": {
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       "version_major": 2,
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       "<p>Failed to display Jupyter Widget of type <code>Output</code>.</p>\n",
       "<p>\n",
       "  If you're reading this message in Jupyter Notebook or JupyterLab, it may mean\n",
       "  that the widgets JavaScript is still loading. If this message persists, it\n",
       "  likely means that the widgets JavaScript library is either not installed or\n",
       "  not enabled. See the <a href=https://ipywidgets.readthedocs.io/en/latest/user_install.html>Jupyter\n",
       "  Widgets Documentation</a> for setup instructions.\n",
       "</p>\n",
       "<p>\n",
       "  If you're reading this message in another notebook frontend (e.g. a static\n",
       "  rendering on GitHub or <a href=\"https://nbviewer.jupyter.org/\">NBViewer</a>),\n",
       "  it may mean that your frontend doesn't currently support widgets.\n",
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       "Output()"
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   "source": [
    "plot = ExpensivePlot(gamma=1)\n",
    "# print out the log here\n",
    "plot.output_log"
   ]
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    "# change a parameter, plot  will not be updated\n",
    "plot.gamma = 0.2"
   ]
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       "<p>Failed to display Jupyter Widget of type <code>Output</code>.</p>\n",
       "<p>\n",
       "  If you're reading this message in Jupyter Notebook or JupyterLab, it may mean\n",
       "  that the widgets JavaScript is still loading. If this message persists, it\n",
       "  likely means that the widgets JavaScript library is either not installed or\n",
       "  not enabled. See the <a href=https://ipywidgets.readthedocs.io/en/latest/user_install.html>Jupyter\n",
       "  Widgets Documentation</a> for setup instructions.\n",
       "</p>\n",
       "<p>\n",
       "  If you're reading this message in another notebook frontend (e.g. a static\n",
       "  rendering on GitHub or <a href=\"https://nbviewer.jupyter.org/\">NBViewer</a>),\n",
       "  it may mean that your frontend doesn't currently support widgets.\n",
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       "Output()"
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   "source": [
    "# display the widget, now it's view count will change to 1, and the plot should update itself\n",
    "plot"
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   "source": [
    "# We change the parameter, and the plot should update\n",
    "plot.gamma = 2"
   ]
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    "# you could remove the widget above (for instance by commenting out the 'plot' statement and running the cell again)\n",
    "# and then change the parameter again\n",
    "# this will *not* update the plot\n",
    "plot.gamma = 1"
   ]
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   "source": [
    "# if we display the widget again, it will update itself, since it is marked dirty\n",
    "plot"
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