Backport PR #8271: Allow to set same mode as existing if load fails.

Setting the mode many times in a row, would fail to actually set the
mode if it was failing to load the mode the first time.

The prevent the above behavior by falling back **explicitly** to 'null'
if mode does not exist yet, or can't be loaded.
...

IPython/html/static/notebook/js/notebook.js

@@ -1,2478 +1,2498 b''
 // Copyright (c) IPython Development Team.
 // Distributed under the terms of the Modified BSD License.
 
 /**
  * @module notebook
  */
 define(function (require) {
     "use strict";
     var IPython = require('base/js/namespace');
     var $ = require('jquery');
     var utils = require('base/js/utils');
     var dialog = require('base/js/dialog');
     var cellmod = require('notebook/js/cell');
     var textcell = require('notebook/js/textcell');
     var codecell = require('notebook/js/codecell');
     var moment = require('moment');
     var configmod = require('services/config');
     var session = require('services/sessions/session');
     var celltoolbar = require('notebook/js/celltoolbar');
     var marked = require('components/marked/lib/marked');
     var CodeMirror = require('codemirror/lib/codemirror');
     var runMode = require('codemirror/addon/runmode/runmode');
     var mathjaxutils = require('notebook/js/mathjaxutils');
     var keyboard = require('base/js/keyboard');
     var tooltip = require('notebook/js/tooltip');
     var default_celltoolbar = require('notebook/js/celltoolbarpresets/default');
     var rawcell_celltoolbar = require('notebook/js/celltoolbarpresets/rawcell');
     var slideshow_celltoolbar = require('notebook/js/celltoolbarpresets/slideshow');
     var scrollmanager = require('notebook/js/scrollmanager');
 
     /**
      * Contains and manages cells.
      * 
      * @class Notebook
      * @param {string}          selector
      * @param {object}          options - Dictionary of keyword arguments.  
      * @param {jQuery}          options.events - selector of Events
      * @param {KeyboardManager} options.keyboard_manager
      * @param {Contents}        options.contents
      * @param {SaveWidget}      options.save_widget
      * @param {object}          options.config
      * @param {string}          options.base_url
      * @param {string}          options.notebook_path
      * @param {string}          options.notebook_name
      */
     var Notebook = function (selector, options) {
         this.config = options.config;
         this.class_config = new configmod.ConfigWithDefaults(this.config, 
                                         Notebook.options_default, 'Notebook');
         this.base_url = options.base_url;
         this.notebook_path = options.notebook_path;
         this.notebook_name = options.notebook_name;
         this.events = options.events;
         this.keyboard_manager = options.keyboard_manager;
         this.contents = options.contents;
         this.save_widget = options.save_widget;
         this.tooltip = new tooltip.Tooltip(this.events);
         this.ws_url = options.ws_url;
         this._session_starting = false;
         this.last_modified = null;
 
         //  Create default scroll manager.
         this.scroll_manager = new scrollmanager.ScrollManager(this);
 
         // TODO: This code smells (and the other `= this` line a couple lines down)
         // We need a better way to deal with circular instance references.
         this.keyboard_manager.notebook = this;
         this.save_widget.notebook = this;
         
         mathjaxutils.init();
 
         if (marked) {
             marked.setOptions({
                 gfm : true,
                 tables: true,
                 // FIXME: probably want central config for CodeMirror theme when we have js config
                 langPrefix: "cm-s-ipython language-",
                 highlight: function(code, lang, callback) {
                     if (!lang) {
                         // no language, no highlight
                         if (callback) {
                             callback(null, code);
                             return;
                         } else {
                             return code;
                         }
                     }
                     utils.requireCodeMirrorMode(lang, function (spec) {
                         var el = document.createElement("div");
                         var mode = CodeMirror.getMode({}, spec);
                         if (!mode) {
                             console.log("No CodeMirror mode: " + lang);
                             callback(null, code);
                             return;
                         }
                         try {
                             CodeMirror.runMode(code, spec, el);
                             callback(null, el.innerHTML);
                         } catch (err) {
                             console.log("Failed to highlight " + lang + " code", err);
                             callback(err, code);
                         }
                     }, function (err) {
                         console.log("No CodeMirror mode: " + lang);
                         callback(err, code);
                     });
                 }
             });
         }
 
         this.element = $(selector);
         this.element.scroll();
         this.element.data("notebook", this);
         this.next_prompt_number = 1;
         this.session = null;
         this.kernel = null;
         this.clipboard = null;
         this.undelete_backup = null;
         this.undelete_index = null;
         this.undelete_below = false;
         this.paste_enabled = false;
         this.writable = false;
         // It is important to start out in command mode to match the intial mode
         // of the KeyboardManager.
         this.mode = 'command';
         this.set_dirty(false);
         this.metadata = {};
         this._checkpoint_after_save = false;
         this.last_checkpoint = null;
         this.checkpoints = [];
         this.autosave_interval = 0;
         this.autosave_timer = null;
         // autosave *at most* every two minutes
         this.minimum_autosave_interval = 120000;
         this.notebook_name_blacklist_re = /[\/\\:]/;
         this.nbformat = 4; // Increment this when changing the nbformat
         this.nbformat_minor = this.current_nbformat_minor = 0; // Increment this when changing the nbformat
         this.codemirror_mode = 'text';
         this.create_elements();
         this.bind_events();
         this.kernel_selector = null;
         this.dirty = null;
         this.trusted = null;
         this._fully_loaded = false;
 
         // Trigger cell toolbar registration.
         default_celltoolbar.register(this);
         rawcell_celltoolbar.register(this);
         slideshow_celltoolbar.register(this);
 
         // prevent assign to miss-typed properties.
         Object.seal(this);
     };
 
     Notebook.options_default = {
         // can be any cell type, or the special values of
         // 'above', 'below', or 'selected' to get the value from another cell.
         default_cell_type: 'code'
     };
 
     /**
      * Create an HTML and CSS representation of the notebook.
      */
     Notebook.prototype.create_elements = function () {
         var that = this;
         this.element.attr('tabindex','-1');
         this.container = $("<div/>").addClass("container").attr("id", "notebook-container");
         // We add this end_space div to the end of the notebook div to:
         // i) provide a margin between the last cell and the end of the notebook
         // ii) to prevent the div from scrolling up when the last cell is being
         // edited, but is too low on the page, which browsers will do automatically.
         var end_space = $('<div/>').addClass('end_space');
         end_space.dblclick(function (e) {
             var ncells = that.ncells();
             that.insert_cell_below('code',ncells-1);
         });
         this.element.append(this.container);
         this.container.after(end_space);
     };
 
     /**
      * Bind JavaScript events: key presses and custom IPython events.
      */
     Notebook.prototype.bind_events = function () {
         var that = this;
 
         this.events.on('set_next_input.Notebook', function (event, data) {
             if (data.replace) {
                 data.cell.set_text(data.text);
                 data.cell.clear_output();
             } else {
                 var index = that.find_cell_index(data.cell);
                 var new_cell = that.insert_cell_below('code',index);
                 new_cell.set_text(data.text);
             }
             that.dirty = true;
         });
 
         this.events.on('unrecognized_cell.Cell', function () {
             that.warn_nbformat_minor();
         });
 
         this.events.on('unrecognized_output.OutputArea', function () {
             that.warn_nbformat_minor();
         });
 
         this.events.on('set_dirty.Notebook', function (event, data) {
             that.dirty = data.value;
         });
 
         this.events.on('trust_changed.Notebook', function (event, trusted) {
             that.trusted = trusted;
         });
 
         this.events.on('select.Cell', function (event, data) {
             var index = that.find_cell_index(data.cell);
             that.select(index);
         });
 
         this.events.on('edit_mode.Cell', function (event, data) {
             that.handle_edit_mode(data.cell);
         });
 
         this.events.on('command_mode.Cell', function (event, data) {
             that.handle_command_mode(data.cell);
         });
         
         this.events.on('spec_changed.Kernel', function(event, data) {
             that.metadata.kernelspec = {
                 name: data.name,
                 display_name: data.spec.display_name,
                 language: data.spec.language,
             };
             // start session if the current session isn't already correct
             if (!(that.session && that.session.kernel && that.session.kernel.name === data.name)) {
                 that.start_session(data.name);
             }
         });
 
         this.events.on('kernel_ready.Kernel', function(event, data) {
             var kinfo = data.kernel.info_reply;
             if (!kinfo.language_info) {
                 delete that.metadata.language_info;
                 return;
             }
             var langinfo = kinfo.language_info;
             that.metadata.language_info = langinfo;
             // Mode 'null' should be plain, unhighlighted text.
             var cm_mode = langinfo.codemirror_mode || langinfo.name || 'null';
             that.set_codemirror_mode(cm_mode);
         });
 
         var collapse_time = function (time) {
             var app_height = $('#ipython-main-app').height(); // content height
             var splitter_height = $('div#pager_splitter').outerHeight(true);
             var new_height = app_height - splitter_height;
             that.element.animate({height : new_height + 'px'}, time);
         };
 
         this.element.bind('collapse_pager', function (event, extrap) {
             var time = (extrap !== undefined) ? ((extrap.duration !== undefined ) ? extrap.duration : 'fast') : 'fast';
             collapse_time(time);
         });
 
         var expand_time = function (time) {
             var app_height = $('#ipython-main-app').height(); // content height
             var splitter_height = $('div#pager_splitter').outerHeight(true);
             var pager_height = $('div#pager').outerHeight(true);
             var new_height = app_height - pager_height - splitter_height;
             that.element.animate({height : new_height + 'px'}, time);
         };
 
         this.element.bind('expand_pager', function (event, extrap) {
             var time = (extrap !== undefined) ? ((extrap.duration !== undefined ) ? extrap.duration : 'fast') : 'fast';
             expand_time(time);
         });
         
         // Firefox 22 broke $(window).on("beforeunload")
         // I'm not sure why or how.
         window.onbeforeunload = function (e) {
             // TODO: Make killing the kernel configurable.
             var kill_kernel = false;
             if (kill_kernel) {
                 that.session.delete();
             }
             // if we are autosaving, trigger an autosave on nav-away.
             // still warn, because if we don't the autosave may fail.
             if (that.dirty) {
                 if ( that.autosave_interval ) {
                     // schedule autosave in a timeout
                     // this gives you a chance to forcefully discard changes
                     // by reloading the page if you *really* want to.
                     // the timer doesn't start until you *dismiss* the dialog.
                     setTimeout(function () {
                         if (that.dirty) {
                             that.save_notebook();
                         }
                     }, 1000);
                     return "Autosave in progress, latest changes may be lost.";
                 } else {
                     return "Unsaved changes will be lost.";
                 }
             }
             // IE treats null as a string.  Instead just return which will avoid the dialog.
             return;
         };
     };
     
     /**
      * Trigger a warning dialog about missing functionality from newer minor versions
      */
     Notebook.prototype.warn_nbformat_minor = function (event) {
         var v = 'v' + this.nbformat + '.';
         var orig_vs = v + this.nbformat_minor;
         var this_vs = v + this.current_nbformat_minor;
         var msg = "This notebook is version " + orig_vs + ", but we only fully support up to " +
         this_vs + ".  You can still work with this notebook, but cell and output types " +
         "introduced in later notebook versions will not be available.";
 
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
             title : "Newer Notebook",
             body : msg,
             buttons : {
                 OK : {
                     "class" : "btn-danger"
                 }
             }
         });
     };
 
     /**
      * Set the dirty flag, and trigger the set_dirty.Notebook event
      */
     Notebook.prototype.set_dirty = function (value) {
         if (value === undefined) {
             value = true;
         }
         if (this.dirty === value) {
             return;
         }
         this.events.trigger('set_dirty.Notebook', {value: value});
     };
 
     /**
      * Scroll the top of the page to a given cell.
      * 
      * @param {integer}  index - An index of the cell to view
      * @param {integer}  time - Animation time in milliseconds
      * @return {integer} Pixel offset from the top of the container
      */
     Notebook.prototype.scroll_to_cell = function (index, time) {
         var cells = this.get_cells();
         time = time || 0;
         index = Math.min(cells.length-1,index);
         index = Math.max(0             ,index);
         var scroll_value = cells[index].element.position().top-cells[0].element.position().top ;
         this.scroll_manager.element.animate({scrollTop:scroll_value}, time);
         return scroll_value;
     };
 
     /**
      * Scroll to the bottom of the page.
      */
     Notebook.prototype.scroll_to_bottom = function () {
         this.scroll_manager.element.animate({scrollTop:this.element.get(0).scrollHeight}, 0);
     };
 
     /**
      * Scroll to the top of the page.
      */
     Notebook.prototype.scroll_to_top = function () {
         this.scroll_manager.element.animate({scrollTop:0}, 0);
     };
 
     // Edit Notebook metadata
 
     /**
      * Display a dialog that allows the user to edit the Notebook's metadata.
      */
     Notebook.prototype.edit_metadata = function () {
         var that = this;
         dialog.edit_metadata({
             md: this.metadata, 
             callback: function (md) {
                 that.metadata = md;
             },
             name: 'Notebook',
             notebook: this,
             keyboard_manager: this.keyboard_manager});
     };
 
     // Cell indexing, retrieval, etc.
 
     /**
      * Get all cell elements in the notebook.
      * 
      * @return {jQuery} A selector of all cell elements
      */
     Notebook.prototype.get_cell_elements = function () {
         return this.container.find(".cell").not('.cell .cell');
     };
 
     /**
      * Get a particular cell element.
      * 
      * @param {integer} index An index of a cell to select
      * @return {jQuery} A selector of the given cell.
      */
     Notebook.prototype.get_cell_element = function (index) {
         var result = null;
         var e = this.get_cell_elements().eq(index);
         if (e.length !== 0) {
             result = e;
         }
         return result;
     };
 
     /**
      * Try to get a particular cell by msg_id.
      * 
      * @param {string} msg_id A message UUID
      * @return {Cell} Cell or null if no cell was found.
      */
     Notebook.prototype.get_msg_cell = function (msg_id) {
         return codecell.CodeCell.msg_cells[msg_id] || null;
     };
 
     /**
      * Count the cells in this notebook.
      * 
      * @return {integer} The number of cells in this notebook
      */
     Notebook.prototype.ncells = function () {
         return this.get_cell_elements().length;
     };
 
     /**
      * Get all Cell objects in this notebook.
      * 
      * @return {Array} This notebook's Cell objects
      */
     Notebook.prototype.get_cells = function () {
         // TODO: we are often calling cells as cells()[i], which we should optimize
         // to cells(i) or a new method.
         return this.get_cell_elements().toArray().map(function (e) {
             return $(e).data("cell");
         });
     };
 
     /**
      * Get a Cell objects from this notebook.
      * 
      * @param {integer} index - An index of a cell to retrieve
      * @return {Cell} Cell or null if no cell was found.
      */
     Notebook.prototype.get_cell = function (index) {
         var result = null;
         var ce = this.get_cell_element(index);
         if (ce !== null) {
             result = ce.data('cell');
         }
         return result;
     };
 
     /**
      * Get the cell below a given cell.
      * 
      * @param {Cell} cell
      * @return {Cell} the next cell or null if no cell was found.
      */
     Notebook.prototype.get_next_cell = function (cell) {
         var result = null;
         var index = this.find_cell_index(cell);
         if (this.is_valid_cell_index(index+1)) {
             result = this.get_cell(index+1);
         }
         return result;
     };
 
     /**
      * Get the cell above a given cell.
      * 
      * @param {Cell} cell
      * @return {Cell} The previous cell or null if no cell was found.
      */
     Notebook.prototype.get_prev_cell = function (cell) {
         var result = null;
         var index = this.find_cell_index(cell);
         if (index !== null && index > 0) {
             result = this.get_cell(index-1);
         }
         return result;
     };
     
     /**
      * Get the numeric index of a given cell.
      * 
      * @param {Cell} cell
      * @return {integer} The cell's numeric index or null if no cell was found.
      */
     Notebook.prototype.find_cell_index = function (cell) {
         var result = null;
         this.get_cell_elements().filter(function (index) {
             if ($(this).data("cell") === cell) {
                 result = index;
             }
         });
         return result;
     };
 
     /**
      * Return given index if defined, or the selected index if not.
      * 
      * @param {integer} [index] - A cell's index
      * @return {integer} cell index
      */
     Notebook.prototype.index_or_selected = function (index) {
         var i;
         if (index === undefined || index === null) {
             i = this.get_selected_index();
             if (i === null) {
                 i = 0;
             }
         } else {
             i = index;
         }
         return i;
     };
 
     /**
      * Get the currently selected cell.
      * 
      * @return {Cell} The selected cell
      */
     Notebook.prototype.get_selected_cell = function () {
         var index = this.get_selected_index();
         return this.get_cell(index);
     };
 
     /**
      * Check whether a cell index is valid.
      * 
      * @param {integer} index - A cell index
      * @return True if the index is valid, false otherwise
      */
     Notebook.prototype.is_valid_cell_index = function (index) {
         if (index !== null && index >= 0 && index < this.ncells()) {
             return true;
         } else {
             return false;
         }
     };
 
     /**
      * Get the index of the currently selected cell.
      *
      * @return {integer} The selected cell's numeric index
      */
     Notebook.prototype.get_selected_index = function () {
         var result = null;
         this.get_cell_elements().filter(function (index) {
             if ($(this).data("cell").selected === true) {
                 result = index;
             }
         });
         return result;
     };
 
 
     // Cell selection.
 
     /**
      * Programmatically select a cell.
      * 
      * @param {integer} index - A cell's index
      * @return {Notebook} This notebook
      */
     Notebook.prototype.select = function (index) {
         if (this.is_valid_cell_index(index)) {
             var sindex = this.get_selected_index();
             if (sindex !== null && index !== sindex) {
                 // If we are about to select a different cell, make sure we are
                 // first in command mode.
                 if (this.mode !== 'command') {
                     this.command_mode();
                 }
                 this.get_cell(sindex).unselect();
             }
             var cell = this.get_cell(index);
             cell.select();
             if (cell.cell_type === 'heading') {
                 this.events.trigger('selected_cell_type_changed.Notebook',
                     {'cell_type':cell.cell_type,level:cell.level}
                 );
             } else {
                 this.events.trigger('selected_cell_type_changed.Notebook',
                     {'cell_type':cell.cell_type}
                 );
             }
         }
         return this;
     };
 
     /**
      * Programmatically select the next cell.
      *
      * @return {Notebook} This notebook
      */
     Notebook.prototype.select_next = function () {
         var index = this.get_selected_index();
         this.select(index+1);
         return this;
     };
 
     /**
      * Programmatically select the previous cell.
      *
      * @return {Notebook} This notebook
      */
     Notebook.prototype.select_prev = function () {
         var index = this.get_selected_index();
         this.select(index-1);
         return this;
     };
 
 
     // Edit/Command mode
 
     /**
      * Gets the index of the cell that is in edit mode.
      *
      * @return {integer} index
      */
     Notebook.prototype.get_edit_index = function () {
         var result = null;
         this.get_cell_elements().filter(function (index) {
             if ($(this).data("cell").mode === 'edit') {
                 result = index;
             }
         });
         return result;
     };
 
     /**
      * Handle when a a cell blurs and the notebook should enter command mode.
      *
      * @param {Cell} [cell] - Cell to enter command mode on.
      */
     Notebook.prototype.handle_command_mode = function (cell) {
         if (this.mode !== 'command') {
             cell.command_mode();
             this.mode = 'command';
             this.events.trigger('command_mode.Notebook');
             this.keyboard_manager.command_mode();
         }
     };
 
     /**
      * Make the notebook enter command mode.
      */
     Notebook.prototype.command_mode = function () {
         var cell = this.get_cell(this.get_edit_index());
         if (cell && this.mode !== 'command') {
             // We don't call cell.command_mode, but rather blur the CM editor
             // which will trigger the call to handle_command_mode.
             cell.code_mirror.getInputField().blur();
         }
     };
 
     /**
      * Handle when a cell fires it's edit_mode event.
      *
      * @param {Cell} [cell] Cell to enter edit mode on.
      */
     Notebook.prototype.handle_edit_mode = function (cell) {
         if (cell && this.mode !== 'edit') {
             cell.edit_mode();
             this.mode = 'edit';
             this.events.trigger('edit_mode.Notebook');
             this.keyboard_manager.edit_mode();
         }
     };
 
     /**
      * Make a cell enter edit mode.
      */
     Notebook.prototype.edit_mode = function () {
         var cell = this.get_selected_cell();
         if (cell && this.mode !== 'edit') {
             cell.unrender();
             cell.focus_editor();
         }
     };
     
     /**
      * Ensure either cell, or codemirror is focused. Is none 
      * is focused, focus the cell.
      */
     Notebook.prototype.ensure_focused = function(){
         var cell = this.get_selected_cell();
         if (cell === null) {return;}  // No cell is selected
         cell.ensure_focused();
     }
 
     /**
      * Focus the currently selected cell.
      */
     Notebook.prototype.focus_cell = function () {
         var cell = this.get_selected_cell();
         if (cell === null) {return;}  // No cell is selected
         cell.focus_cell();
     };
 
     // Cell movement
 
     /**
      * Move given (or selected) cell up and select it.
      * 
      * @param {integer} [index] - cell index
      * @return {Notebook} This notebook
      */
     Notebook.prototype.move_cell_up = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i) && i > 0) {
             var pivot = this.get_cell_element(i-1);
             var tomove = this.get_cell_element(i);
             if (pivot !== null && tomove !== null) {
                 tomove.detach();
                 pivot.before(tomove);
                 this.select(i-1);
                 var cell = this.get_selected_cell();
                 cell.focus_cell();
             }
             this.set_dirty(true);
         }
         return this;
     };
 
 
     /**
      * Move given (or selected) cell down and select it.
      * 
      * @param {integer} [index] - cell index
      * @return {Notebook} This notebook
      */
     Notebook.prototype.move_cell_down = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i) && this.is_valid_cell_index(i+1)) {
             var pivot = this.get_cell_element(i+1);
             var tomove = this.get_cell_element(i);
             if (pivot !== null && tomove !== null) {
                 tomove.detach();
                 pivot.after(tomove);
                 this.select(i+1);
                 var cell = this.get_selected_cell();
                 cell.focus_cell();
             }
         }
         this.set_dirty();
         return this;
     };
 
 
     // Insertion, deletion.
 
     /**
      * Delete a cell from the notebook without any precautions
      * Needed to reload checkpoints and other things like that.
      * 
      * @param {integer} [index] - cell's numeric index
      * @return {Notebook} This notebook
      */
     Notebook.prototype._unsafe_delete_cell = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
 
         $('#undelete_cell').addClass('disabled');
         if (this.is_valid_cell_index(i)) {
             var old_ncells = this.ncells();
             var ce = this.get_cell_element(i);
             ce.remove();
             this.set_dirty(true);
         }
         return this;
     };
 
     /**
      * Delete a cell from the notebook.
      * 
      * @param {integer} [index] - cell's numeric index
      * @return {Notebook} This notebook
      */
     Notebook.prototype.delete_cell = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (!cell.is_deletable()) {
             return this;
         }
 
         this.undelete_backup = cell.toJSON();
         $('#undelete_cell').removeClass('disabled');
         if (this.is_valid_cell_index(i)) {
             var old_ncells = this.ncells();
             var ce = this.get_cell_element(i);
             ce.remove();
             if (i === 0) {
                 // Always make sure we have at least one cell.
                 if (old_ncells === 1) {
                     this.insert_cell_below('code');
                 }
                 this.select(0);
                 this.undelete_index = 0;
                 this.undelete_below = false;
             } else if (i === old_ncells-1 && i !== 0) {
                 this.select(i-1);
                 this.undelete_index = i - 1;
                 this.undelete_below = true;
             } else {
                 this.select(i);
                 this.undelete_index = i;
                 this.undelete_below = false;
             }
             this.events.trigger('delete.Cell', {'cell': cell, 'index': i});
             this.set_dirty(true);
         }
         return this;
     };
 
     /**
      * Restore the most recently deleted cell.
      */
     Notebook.prototype.undelete_cell = function() {
         if (this.undelete_backup !== null && this.undelete_index !== null) {
             var current_index = this.get_selected_index();
             if (this.undelete_index < current_index) {
                 current_index = current_index + 1;
             }
             if (this.undelete_index >= this.ncells()) {
                 this.select(this.ncells() - 1);
             }
             else {
                 this.select(this.undelete_index);
             }
             var cell_data = this.undelete_backup;
             var new_cell = null;
             if (this.undelete_below) {
                 new_cell = this.insert_cell_below(cell_data.cell_type);
             } else {
                 new_cell = this.insert_cell_above(cell_data.cell_type);
             }
             new_cell.fromJSON(cell_data);
             if (this.undelete_below) {
                 this.select(current_index+1);
             } else {
                 this.select(current_index);
             }
             this.undelete_backup = null;
             this.undelete_index = null;
         }
         $('#undelete_cell').addClass('disabled');
     };
 
     /**
      * Insert a cell so that after insertion the cell is at given index.
      *
      * If cell type is not provided, it will default to the type of the
      * currently active cell.
      *
      * Similar to insert_above, but index parameter is mandatory.
      *
      * Index will be brought back into the accessible range [0,n].
      *
      * @param {string} [type] - in ['code','markdown', 'raw'], defaults to 'code'
      * @param {integer} [index] - a valid index where to insert cell
      * @return {Cell|null} created cell or null
      */
     Notebook.prototype.insert_cell_at_index = function(type, index){
 
         var ncells = this.ncells();
         index = Math.min(index, ncells);
         index = Math.max(index, 0);
         var cell = null;
         type = type || this.class_config.get_sync('default_cell_type');
         if (type === 'above') {
             if (index > 0) {
                 type = this.get_cell(index-1).cell_type;
             } else {
                 type = 'code';
             }
         } else if (type === 'below') {
             if (index < ncells) {
                 type = this.get_cell(index).cell_type;
             } else {
                 type = 'code';
             }
         } else if (type === 'selected') {
             type = this.get_selected_cell().cell_type;
         }
 
         if (ncells === 0 || this.is_valid_cell_index(index) || index === ncells) {
             var cell_options = {
                 events: this.events, 
                 config: this.config, 
                 keyboard_manager: this.keyboard_manager, 
                 notebook: this,
                 tooltip: this.tooltip
             };
             switch(type) {
             case 'code':
                 cell = new codecell.CodeCell(this.kernel, cell_options);
                 cell.set_input_prompt();
                 break;
             case 'markdown':
                 cell = new textcell.MarkdownCell(cell_options);
                 break;
             case 'raw':
                 cell = new textcell.RawCell(cell_options);
                 break;
             default:
                 console.log("Unrecognized cell type: ", type, cellmod);
                 cell = new cellmod.UnrecognizedCell(cell_options);
             }
 
             if(this._insert_element_at_index(cell.element,index)) {
                 cell.render();
                 this.events.trigger('create.Cell', {'cell': cell, 'index': index});
                 cell.refresh();
                 // We used to select the cell after we refresh it, but there
                 // are now cases were this method is called where select is
                 // not appropriate. The selection logic should be handled by the
                 // caller of the the top level insert_cell methods.
                 this.set_dirty(true);
             }
         }
         return cell;
 
     };
 
     /**
      * Insert an element at given cell index.
      *
      * @param {HTMLElement} element - a cell element
      * @param {integer}     [index] - a valid index where to inser cell
      * @returns {boolean}   success
      */
     Notebook.prototype._insert_element_at_index = function(element, index){
         if (element === undefined){
             return false;
         }
 
         var ncells = this.ncells();
 
         if (ncells === 0) {
             // special case append if empty
             this.container.append(element);
         } else if ( ncells === index ) {
             // special case append it the end, but not empty
             this.get_cell_element(index-1).after(element);
         } else if (this.is_valid_cell_index(index)) {
             // otherwise always somewhere to append to
             this.get_cell_element(index).before(element);
         } else {
             return false;
         }
 
         if (this.undelete_index !== null && index <= this.undelete_index) {
             this.undelete_index = this.undelete_index + 1;
             this.set_dirty(true);
         }
         return true;
     };
 
     /**
      * Insert a cell of given type above given index, or at top
      * of notebook if index smaller than 0.
      *
      * @param {string}     [type] - cell type
      * @param {integer}    [index] - defaults to the currently selected cell
      * @return {Cell|null} handle to created cell or null
      */
     Notebook.prototype.insert_cell_above = function (type, index) {
         index = this.index_or_selected(index);
         return this.insert_cell_at_index(type, index);
     };
 
     /**
      * Insert a cell of given type below given index, or at bottom
      * of notebook if index greater than number of cells
      *
      * @param {string}     [type] - cell type
      * @param {integer}    [index] - defaults to the currently selected cell
      * @return {Cell|null} handle to created cell or null
      */
     Notebook.prototype.insert_cell_below = function (type, index) {
         index = this.index_or_selected(index);
         return this.insert_cell_at_index(type, index+1);
     };
 
 
     /**
      * Insert cell at end of notebook
      *
      * @param {string} type - cell type
      * @return {Cell|null} handle to created cell or null
      */
     Notebook.prototype.insert_cell_at_bottom = function (type){
         var len = this.ncells();
         return this.insert_cell_below(type,len-1);
     };
 
     /**
      * Turn a cell into a code cell.
      * 
      * @param {integer} [index] - cell index
      */
     Notebook.prototype.to_code = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_cell = this.get_cell(i);
             if (!(source_cell instanceof codecell.CodeCell)) {
                 var target_cell = this.insert_cell_below('code',i);
                 var text = source_cell.get_text();
                 if (text === source_cell.placeholder) {
                     text = '';
                 }
                 //metadata
                 target_cell.metadata = source_cell.metadata;
 
                 target_cell.set_text(text);
                 // make this value the starting point, so that we can only undo
                 // to this state, instead of a blank cell
                 target_cell.code_mirror.clearHistory();
                 source_cell.element.remove();
                 this.select(i);
                 var cursor = source_cell.code_mirror.getCursor();
                 target_cell.code_mirror.setCursor(cursor);
                 this.set_dirty(true);
             }
         }
     };
 
     /**
      * Turn a cell into a Markdown cell.
      * 
      * @param {integer} [index] - cell index
      */
     Notebook.prototype.to_markdown = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_cell = this.get_cell(i);
 
             if (!(source_cell instanceof textcell.MarkdownCell)) {
                 var target_cell = this.insert_cell_below('markdown',i);
                 var text = source_cell.get_text();
 
                 if (text === source_cell.placeholder) {
                     text = '';
                 }
                 // metadata
                 target_cell.metadata = source_cell.metadata;
                 // We must show the editor before setting its contents
                 target_cell.unrender();
                 target_cell.set_text(text);
                 // make this value the starting point, so that we can only undo
                 // to this state, instead of a blank cell
                 target_cell.code_mirror.clearHistory();
                 source_cell.element.remove();
                 this.select(i);
                 if ((source_cell instanceof textcell.TextCell) && source_cell.rendered) {
                     target_cell.render();
                 }
                 var cursor = source_cell.code_mirror.getCursor();
                 target_cell.code_mirror.setCursor(cursor);
                 this.set_dirty(true);
             }
         }
     };
 
     /**
      * Turn a cell into a raw text cell.
      * 
      * @param {integer} [index] - cell index
      */
     Notebook.prototype.to_raw = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var target_cell = null;
             var source_cell = this.get_cell(i);
 
             if (!(source_cell instanceof textcell.RawCell)) {
                 target_cell = this.insert_cell_below('raw',i);
                 var text = source_cell.get_text();
                 if (text === source_cell.placeholder) {
                     text = '';
                 }
                 //metadata
                 target_cell.metadata = source_cell.metadata;
                 // We must show the editor before setting its contents
                 target_cell.unrender();
                 target_cell.set_text(text);
                 // make this value the starting point, so that we can only undo
                 // to this state, instead of a blank cell
                 target_cell.code_mirror.clearHistory();
                 source_cell.element.remove();
                 this.select(i);
                 var cursor = source_cell.code_mirror.getCursor();
                 target_cell.code_mirror.setCursor(cursor);
                 this.set_dirty(true);
             }
         }
     };
     
     /**
      * Warn about heading cell support removal.
      */
     Notebook.prototype._warn_heading = function () {
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
             title : "Use markdown headings",
             body : $("<p/>").text(
                 'IPython no longer uses special heading cells. ' + 
                 'Instead, write your headings in Markdown cells using # characters:'
             ).append($('<pre/>').text(
                 '## This is a level 2 heading'
             )),
             buttons : {
                 "OK" : {}
             }
         });
     };
     
     /**
      * Turn a cell into a heading containing markdown cell.
      * 
      * @param {integer} [index] - cell index
      * @param {integer} [level] - heading level (e.g., 1 for h1)
      */
     Notebook.prototype.to_heading = function (index, level) {
         this.to_markdown(index);
         level = level || 1;
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var cell = this.get_cell(i);
             cell.set_heading_level(level);
             this.set_dirty(true);
         }
     };
 
 
     // Cut/Copy/Paste
 
     /**
      * Enable the UI elements for pasting cells.
      */
     Notebook.prototype.enable_paste = function () {
         var that = this;
         if (!this.paste_enabled) {
             $('#paste_cell_replace').removeClass('disabled')
                 .on('click', function () {that.paste_cell_replace();});
             $('#paste_cell_above').removeClass('disabled')
                 .on('click', function () {that.paste_cell_above();});
             $('#paste_cell_below').removeClass('disabled')
                 .on('click', function () {that.paste_cell_below();});
             this.paste_enabled = true;
         }
     };
 
     /**
      * Disable the UI elements for pasting cells.
      */
     Notebook.prototype.disable_paste = function () {
         if (this.paste_enabled) {
             $('#paste_cell_replace').addClass('disabled').off('click');
             $('#paste_cell_above').addClass('disabled').off('click');
             $('#paste_cell_below').addClass('disabled').off('click');
             this.paste_enabled = false;
         }
     };
 
     /**
      * Cut a cell.
      */
     Notebook.prototype.cut_cell = function () {
         this.copy_cell();
         this.delete_cell();
     };
 
     /**
      * Copy a cell.
      */
     Notebook.prototype.copy_cell = function () {
         var cell = this.get_selected_cell();
         this.clipboard = cell.toJSON();
         // remove undeletable status from the copied cell
         if (this.clipboard.metadata.deletable !== undefined) {
             delete this.clipboard.metadata.deletable;
         }
         this.enable_paste();
     };
 
     /**
      * Replace the selected cell with the cell in the clipboard.
      */
     Notebook.prototype.paste_cell_replace = function () {
         if (this.clipboard !== null && this.paste_enabled) {
             var cell_data = this.clipboard;
             var new_cell = this.insert_cell_above(cell_data.cell_type);
             new_cell.fromJSON(cell_data);
             var old_cell = this.get_next_cell(new_cell);
             this.delete_cell(this.find_cell_index(old_cell));
             this.select(this.find_cell_index(new_cell));
         }
     };
 
     /**
      * Paste a cell from the clipboard above the selected cell.
      */
     Notebook.prototype.paste_cell_above = function () {
         if (this.clipboard !== null && this.paste_enabled) {
             var cell_data = this.clipboard;
             var new_cell = this.insert_cell_above(cell_data.cell_type);
             new_cell.fromJSON(cell_data);
             new_cell.focus_cell();
         }
     };
 
     /**
      * Paste a cell from the clipboard below the selected cell.
      */
     Notebook.prototype.paste_cell_below = function () {
         if (this.clipboard !== null && this.paste_enabled) {
             var cell_data = this.clipboard;
             var new_cell = this.insert_cell_below(cell_data.cell_type);
             new_cell.fromJSON(cell_data);
             new_cell.focus_cell();
         }
     };
 
     // Split/merge
 
     /**
      * Split the selected cell into two cells.
      */
     Notebook.prototype.split_cell = function () {
         var cell = this.get_selected_cell();
         if (cell.is_splittable()) {
             var texta = cell.get_pre_cursor();
             var textb = cell.get_post_cursor();
             cell.set_text(textb);
             var new_cell = this.insert_cell_above(cell.cell_type);
             // Unrender the new cell so we can call set_text.
             new_cell.unrender();
             new_cell.set_text(texta);
         }
     };
 
     /**
      * Merge the selected cell into the cell above it.
      */
     Notebook.prototype.merge_cell_above = function () {
         var index = this.get_selected_index();
         var cell = this.get_cell(index);
         var render = cell.rendered;
         if (!cell.is_mergeable()) {
             return;
         }
         if (index > 0) {
             var upper_cell = this.get_cell(index-1);
             if (!upper_cell.is_mergeable()) {
                 return;
             }
             var upper_text = upper_cell.get_text();
             var text = cell.get_text();
             if (cell instanceof codecell.CodeCell) {
                 cell.set_text(upper_text+'\n'+text);
             } else {
                 cell.unrender(); // Must unrender before we set_text.
                 cell.set_text(upper_text+'\n\n'+text);
                 if (render) {
                     // The rendered state of the final cell should match
                     // that of the original selected cell;
                     cell.render();
                 }
             }
             this.delete_cell(index-1);
             this.select(this.find_cell_index(cell));
         }
     };
 
     /**
      * Merge the selected cell into the cell below it.
      */
     Notebook.prototype.merge_cell_below = function () {
         var index = this.get_selected_index();
         var cell = this.get_cell(index);
         var render = cell.rendered;
         if (!cell.is_mergeable()) {
             return;
         }
         if (index < this.ncells()-1) {
             var lower_cell = this.get_cell(index+1);
             if (!lower_cell.is_mergeable()) {
                 return;
             }
             var lower_text = lower_cell.get_text();
             var text = cell.get_text();
             if (cell instanceof codecell.CodeCell) {
                 cell.set_text(text+'\n'+lower_text);
             } else {
                 cell.unrender(); // Must unrender before we set_text.
                 cell.set_text(text+'\n\n'+lower_text);
                 if (render) {
                     // The rendered state of the final cell should match
                     // that of the original selected cell;
                     cell.render();
                 }
             }
             this.delete_cell(index+1);
             this.select(this.find_cell_index(cell));
         }
     };
 
 
     // Cell collapsing and output clearing
 
     /**
      * Hide a cell's output.
      * 
      * @param {integer} index - cell index
      */
     Notebook.prototype.collapse_output = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (cell !== null && (cell instanceof codecell.CodeCell)) {
             cell.collapse_output();
             this.set_dirty(true);
         }
     };
 
     /**
      * Hide each code cell's output area.
      */
     Notebook.prototype.collapse_all_output = function () {
         this.get_cells().map(function (cell, i) {
             if (cell instanceof codecell.CodeCell) {
                 cell.collapse_output();
             }
         });
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /**
      * Show a cell's output.
      * 
      * @param {integer} index - cell index
      */
     Notebook.prototype.expand_output = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (cell !== null && (cell instanceof codecell.CodeCell)) {
             cell.expand_output();
             this.set_dirty(true);
         }
     };
 
     /**
      * Expand each code cell's output area, and remove scrollbars.
      */
     Notebook.prototype.expand_all_output = function () {
         this.get_cells().map(function (cell, i) {
             if (cell instanceof codecell.CodeCell) {
                 cell.expand_output();
             }
         });
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /**
      * Clear the selected CodeCell's output area.
      * 
      * @param {integer} index - cell index
      */
     Notebook.prototype.clear_output = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (cell !== null && (cell instanceof codecell.CodeCell)) {
             cell.clear_output();
             this.set_dirty(true);
         }
     };
 
     /**
      * Clear each code cell's output area.
      */
     Notebook.prototype.clear_all_output = function () {
         this.get_cells().map(function (cell, i) {
             if (cell instanceof codecell.CodeCell) {
                 cell.clear_output();
             }
         });
         this.set_dirty(true);
     };
 
     /**
      * Scroll the selected CodeCell's output area.
      * 
      * @param {integer} index - cell index
      */
     Notebook.prototype.scroll_output = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (cell !== null && (cell instanceof codecell.CodeCell)) {
             cell.scroll_output();
             this.set_dirty(true);
         }
     };
 
     /**
      * Expand each code cell's output area and add a scrollbar for long output.
      */
     Notebook.prototype.scroll_all_output = function () {
         this.get_cells().map(function (cell, i) {
             if (cell instanceof codecell.CodeCell) {
                 cell.scroll_output();
             }
         });
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /** 
      * Toggle whether a cell's output is collapsed or expanded.
      * 
      * @param {integer} index - cell index
      */
     Notebook.prototype.toggle_output = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (cell !== null && (cell instanceof codecell.CodeCell)) {
             cell.toggle_output();
             this.set_dirty(true);
         }
     };
 
     /**
      * Toggle the output of all cells.
      */
     Notebook.prototype.toggle_all_output = function () {
         this.get_cells().map(function (cell, i) {
             if (cell instanceof codecell.CodeCell) {
                 cell.toggle_output();
             }
         });
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /**
      * Toggle a scrollbar for long cell outputs.
      * 
      * @param {integer} index - cell index
      */
     Notebook.prototype.toggle_output_scroll = function (index) {
         var i = this.index_or_selected(index);
         var cell = this.get_cell(i);
         if (cell !== null && (cell instanceof codecell.CodeCell)) {
             cell.toggle_output_scroll();
             this.set_dirty(true);
         }
     };
 
     /**
      * Toggle the scrolling of long output on all cells.
      */
     Notebook.prototype.toggle_all_output_scroll = function () {
         this.get_cells().map(function (cell, i) {
             if (cell instanceof codecell.CodeCell) {
                 cell.toggle_output_scroll();
             }
         });
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     // Other cell functions: line numbers, ...
 
     /**
      * Toggle line numbers in the selected cell's input area.
      */
     Notebook.prototype.cell_toggle_line_numbers = function() {
         this.get_selected_cell().toggle_line_numbers();
     };
+
+
+    //dispatch codemirror mode to all cells.
+    Notebook.prototype._dispatch_mode = function(spec, newmode){
+        this.codemirror_mode = newmode;
+        codecell.CodeCell.options_default.cm_config.mode = newmode;
+        this.get_cells().map(function(cell, i) {
+            if (cell.cell_type === 'code'){
+                cell.code_mirror.setOption('mode', spec);
+                // This is currently redundant, because cm_config ends up as
+                // codemirror's own .options object, but I don't want to
+                // rely on that.
+                cell.cm_config.mode = spec;
+            }
+        });
+
+    };
+
+    // roughly try to check mode equality
+    var _mode_equal = function(mode1, mode2){
+        return ((mode1||{}).name||mode1)===((mode2||{}).name||mode2);
+    };
     
     /**
      * Set the codemirror mode for all code cells, including the default for
      * new code cells.
+     * Set the mode to 'null' (no highlighting) if it can't be found.
      */
     Notebook.prototype.set_codemirror_mode = function(newmode){
-        if (newmode === this.codemirror_mode) {
+        // if mode is the same don't reset,
+        // to avoid n-time re-highlighting.
+        if (_mode_equal(newmode, this.codemirror_mode)) {
             return;
         }
-        this.codemirror_mode = newmode;
-        codecell.CodeCell.options_default.cm_config.mode = newmode;
         
         var that = this;
         utils.requireCodeMirrorMode(newmode, function (spec) {
-            that.get_cells().map(function(cell, i) {
-                if (cell.cell_type === 'code'){
-                    cell.code_mirror.setOption('mode', spec);
-                    // This is currently redundant, because cm_config ends up as
-                    // codemirror's own .options object, but I don't want to
-                    // rely on that.
-                    cell._options.cm_config.mode = spec;
-                }
-            });
+            that._dispatch_mode(spec, newmode);
+        }, function(){
+            // on error don't dispatch the new mode as re-setting it later will not work.
+            // don't either set to null mode if it has been changed in the meantime
+            if( _mode_equal(newmode, this.codemirror_mode) ){
+                that._dispatch_mode('null','null');
+            }
         });
     };
 
     // Session related things
 
     /**
      * Start a new session and set it on each code cell.
      */
     Notebook.prototype.start_session = function (kernel_name) {
         if (this._session_starting) {
             throw new session.SessionAlreadyStarting();
         }
         this._session_starting = true;
 
         var options = {
             base_url: this.base_url,
             ws_url: this.ws_url,
             notebook_path: this.notebook_path,
             notebook_name: this.notebook_name,
             kernel_name: kernel_name,
             notebook: this
         };
 
         var success = $.proxy(this._session_started, this);
         var failure = $.proxy(this._session_start_failed, this);
 
         if (this.session !== null) {
             this.session.restart(options, success, failure);
         } else {
             this.session = new session.Session(options);
             this.session.start(success, failure);
         }
     };
 
 
     /**
      * Once a session is started, link the code cells to the kernel and pass the 
      * comm manager to the widget manager.
      */
     Notebook.prototype._session_started = function (){
         this._session_starting = false;
         this.kernel = this.session.kernel;
         var ncells = this.ncells();
         for (var i=0; i<ncells; i++) {
             var cell = this.get_cell(i);
             if (cell instanceof codecell.CodeCell) {
                 cell.set_kernel(this.session.kernel);
             }
         }
     };
 
     /**
      * Called when the session fails to start.
      */
     Notebook.prototype._session_start_failed = function(jqxhr, status, error){
         this._session_starting = false;
         utils.log_ajax_error(jqxhr, status, error);
     };
     
     /**
      * Prompt the user to restart the IPython kernel.
      */
     Notebook.prototype.restart_kernel = function () {
         var that = this;
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
             title : "Restart kernel or continue running?",
             body : $("<p/>").text(
                 'Do you want to restart the current kernel?  You will lose all variables defined in it.'
             ),
             buttons : {
                 "Continue running" : {},
                 "Restart" : {
                     "class" : "btn-danger",
                     "click" : function() {
                         that.kernel.restart();
                     }
                 }
             }
         });
     };
     
     /**
      * Execute or render cell outputs and go into command mode.
      */
     Notebook.prototype.execute_cell = function () {
         // mode = shift, ctrl, alt
         var cell = this.get_selected_cell();
         
         cell.execute();
         this.command_mode();
         this.set_dirty(true);
     };
 
     /**
      * Execute or render cell outputs and insert a new cell below.
      */
     Notebook.prototype.execute_cell_and_insert_below = function () {
         var cell = this.get_selected_cell();
         var cell_index = this.find_cell_index(cell);
         
         cell.execute();
 
         // If we are at the end always insert a new cell and return
         if (cell_index === (this.ncells()-1)) {
             this.command_mode();
             this.insert_cell_below();
             this.select(cell_index+1);
             this.edit_mode();
             this.scroll_to_bottom();
             this.set_dirty(true);
             return;
         }
 
         this.command_mode();
         this.insert_cell_below();
         this.select(cell_index+1);
         this.edit_mode();
         this.set_dirty(true);
     };
 
     /**
      * Execute or render cell outputs and select the next cell.
      */
     Notebook.prototype.execute_cell_and_select_below = function () {
 
         var cell = this.get_selected_cell();
         var cell_index = this.find_cell_index(cell);
         
         cell.execute();
 
         // If we are at the end always insert a new cell and return
         if (cell_index === (this.ncells()-1)) {
             this.command_mode();
             this.insert_cell_below();
             this.select(cell_index+1);
             this.edit_mode();
             this.scroll_to_bottom();
             this.set_dirty(true);
             return;
         }
 
         this.command_mode();
         this.select(cell_index+1);
         this.focus_cell();
         this.set_dirty(true);
     };
 
     /**
      * Execute all cells below the selected cell.
      */
     Notebook.prototype.execute_cells_below = function () {
         this.execute_cell_range(this.get_selected_index(), this.ncells());
         this.scroll_to_bottom();
     };
 
     /**
      * Execute all cells above the selected cell.
      */
     Notebook.prototype.execute_cells_above = function () {
         this.execute_cell_range(0, this.get_selected_index());
     };
 
     /**
      * Execute all cells.
      */
     Notebook.prototype.execute_all_cells = function () {
         this.execute_cell_range(0, this.ncells());
         this.scroll_to_bottom();
     };
 
     /**
      * Execute a contiguous range of cells.
      * 
      * @param {integer} start - index of the first cell to execute (inclusive)
      * @param {integer} end - index of the last cell to execute (exclusive)
      */
     Notebook.prototype.execute_cell_range = function (start, end) {
         this.command_mode();
         for (var i=start; i<end; i++) {
             this.select(i);
             this.execute_cell();
         }
     };
 
     // Persistance and loading
 
     /**
      * Getter method for this notebook's name.
      * 
      * @return {string} This notebook's name (excluding file extension)
      */
     Notebook.prototype.get_notebook_name = function () {
         var nbname = this.notebook_name.substring(0,this.notebook_name.length-6);
         return nbname;
     };
 
     /**
      * Setter method for this notebook's name.
      *
      * @param {string} name
      */
     Notebook.prototype.set_notebook_name = function (name) {
         var parent = utils.url_path_split(this.notebook_path)[0];
         this.notebook_name = name;
         this.notebook_path = utils.url_path_join(parent, name);
     };
 
     /**
      * Check that a notebook's name is valid.
      * 
      * @param {string} nbname - A name for this notebook
      * @return {boolean} True if the name is valid, false if invalid
      */
     Notebook.prototype.test_notebook_name = function (nbname) {
         nbname = nbname || '';
         if (nbname.length>0 && !this.notebook_name_blacklist_re.test(nbname)) {
             return true;
         } else {
             return false;
         }
     };
 
     /**
      * Load a notebook from JSON (.ipynb).
      * 
      * @param {object} data - JSON representation of a notebook
      */
     Notebook.prototype.fromJSON = function (data) {
 
         var content = data.content;
         var ncells = this.ncells();
         var i;
         for (i=0; i<ncells; i++) {
             // Always delete cell 0 as they get renumbered as they are deleted.
             this._unsafe_delete_cell(0);
         }
         // Save the metadata and name.
         this.metadata = content.metadata;
         this.notebook_name = data.name;
         this.notebook_path = data.path;
         var trusted = true;
         
         // Set the codemirror mode from language_info metadata
         if (this.metadata.language_info !== undefined) {
             var langinfo = this.metadata.language_info;
             // Mode 'null' should be plain, unhighlighted text.
             var cm_mode = langinfo.codemirror_mode || langinfo.name || 'null';
             this.set_codemirror_mode(cm_mode);
         }
         
         var new_cells = content.cells;
         ncells = new_cells.length;
         var cell_data = null;
         var new_cell = null;
         for (i=0; i<ncells; i++) {
             cell_data = new_cells[i];
             new_cell = this.insert_cell_at_index(cell_data.cell_type, i);
             new_cell.fromJSON(cell_data);
             if (new_cell.cell_type === 'code' && !new_cell.output_area.trusted) {
                 trusted = false;
             }
         }
         if (trusted !== this.trusted) {
             this.trusted = trusted;
             this.events.trigger("trust_changed.Notebook", trusted);
         }
     };
 
     /**
      * Dump this notebook into a JSON-friendly object.
      * 
      * @return {object} A JSON-friendly representation of this notebook.
      */
     Notebook.prototype.toJSON = function () {
         // remove the conversion indicator, which only belongs in-memory
         delete this.metadata.orig_nbformat;
         delete this.metadata.orig_nbformat_minor;
 
         var cells = this.get_cells();
         var ncells = cells.length;
         var cell_array = new Array(ncells);
         var trusted = true;
         for (var i=0; i<ncells; i++) {
             var cell = cells[i];
             if (cell.cell_type === 'code' && !cell.output_area.trusted) {
                 trusted = false;
             }
             cell_array[i] = cell.toJSON();
         }
         var data = {
             cells: cell_array,
             metadata: this.metadata,
             nbformat: this.nbformat,
             nbformat_minor: this.nbformat_minor
         };
         if (trusted !== this.trusted) {
             this.trusted = trusted;
             this.events.trigger("trust_changed.Notebook", trusted);
         }
         return data;
     };
 
     /**
      * Start an autosave timer which periodically saves the notebook.
      * 
      * @param {integer} interval - the autosave interval in milliseconds
      */
     Notebook.prototype.set_autosave_interval = function (interval) {
         var that = this;
         // clear previous interval, so we don't get simultaneous timers
         if (this.autosave_timer) {
             clearInterval(this.autosave_timer);
         }
         if (!this.writable) {
             // disable autosave if not writable
             interval = 0;
         }
         
         this.autosave_interval = this.minimum_autosave_interval = interval;
         if (interval) {
             this.autosave_timer = setInterval(function() {
                 if (that.dirty) {
                     that.save_notebook();
                 }
             }, interval);
             this.events.trigger("autosave_enabled.Notebook", interval);
         } else {
             this.autosave_timer = null;
             this.events.trigger("autosave_disabled.Notebook");
         }
     };
     
     /**
      * Save this notebook on the server. This becomes a notebook instance's
      * .save_notebook method *after* the entire notebook has been loaded.
      */
     Notebook.prototype.save_notebook = function (check_last_modified) {
         if (check_last_modified === undefined) {
             check_last_modified = true;
         }
         if (!this._fully_loaded) {
             this.events.trigger('notebook_save_failed.Notebook',
                 new Error("Load failed, save is disabled")
             );
             return;
         } else if (!this.writable) {
             this.events.trigger('notebook_save_failed.Notebook',
                 new Error("Notebook is read-only")
             );
             return;
         }
 
         // Trigger an event before save, which allows listeners to modify
         // the notebook as needed.
         this.events.trigger('before_save.Notebook');
 
         // Create a JSON model to be sent to the server.
         var model = {
             type : "notebook",
             content : this.toJSON()
         };
         // time the ajax call for autosave tuning purposes.
         var start =  new Date().getTime();
 
         var that = this;
         var _save = function () {
             return that.contents.save(that.notebook_path, model).then(
                 $.proxy(that.save_notebook_success, that, start),
                 function (error) {
                     that.events.trigger('notebook_save_failed.Notebook', error);
                 }
             );
         };
 
         if (check_last_modified) {
             return this.contents.get(this.notebook_path, {content: false}).then(
                 function (data) {
                     var last_modified = new Date(data.last_modified);
                     if (last_modified > that.last_modified) {
                         dialog.modal({
                             notebook: that,
                             keyboard_manager: that.keyboard_manager,
                             title: "Notebook changed",
                             body: "Notebook has changed since we opened it. Overwrite the changed file?",
                             buttons: {
                                 Cancel: {},
                                 Overwrite: {
                                     class: 'btn-danger',
                                     click: function () {
                                         _save();
                                     }
                                 },
                             }
                         });
                     } else {
                         return _save();
                     }
                 }, function (error) {
                     // maybe it has been deleted or renamed? Go ahead and save.
                     return _save();
                 }
             );
         } else {
             return _save();
         }
     };
     
     /**
      * Success callback for saving a notebook.
      * 
      * @param {integer} start - Time when the save request start
      * @param {object}  data - JSON representation of a notebook
      */
     Notebook.prototype.save_notebook_success = function (start, data) {
         this.set_dirty(false);
         this.last_modified = new Date(data.last_modified);
         if (data.message) {
             // save succeeded, but validation failed.
             var body = $("<div>");
             var title = "Notebook validation failed";
 
             body.append($("<p>").text(
                 "The save operation succeeded," +
                 " but the notebook does not appear to be valid." +
                 " The validation error was:"
             )).append($("<div>").addClass("validation-error").append(
                 $("<pre>").text(data.message)
             ));
             dialog.modal({
                 notebook: this,
                 keyboard_manager: this.keyboard_manager,
                 title: title,
                 body: body,
                 buttons : {
                     OK : {
                         "class" : "btn-primary"
                     }
                 }
             });
         }
         this.events.trigger('notebook_saved.Notebook');
         this._update_autosave_interval(start);
         if (this._checkpoint_after_save) {
             this.create_checkpoint();
             this._checkpoint_after_save = false;
         }
     };
     
     /**
      * Update the autosave interval based on the duration of the last save.
      * 
      * @param {integer} timestamp - when the save request started
      */
     Notebook.prototype._update_autosave_interval = function (start) {
         var duration = (new Date().getTime() - start);
         if (this.autosave_interval) {
             // new save interval: higher of 10x save duration or parameter (default 30 seconds)
             var interval = Math.max(10 * duration, this.minimum_autosave_interval);
             // round to 10 seconds, otherwise we will be setting a new interval too often
             interval = 10000 * Math.round(interval / 10000);
             // set new interval, if it's changed
             if (interval !== this.autosave_interval) {
                 this.set_autosave_interval(interval);
             }
         }
     };
 
     /**
      * Explicitly trust the output of this notebook.
      */
     Notebook.prototype.trust_notebook = function () {
         var body = $("<div>").append($("<p>")
             .text("A trusted IPython notebook may execute hidden malicious code ")
             .append($("<strong>")
                 .append(
                     $("<em>").text("when you open it")
                 )
             ).append(".").append(
                 " Selecting trust will immediately reload this notebook in a trusted state."
             ).append(
                 " For more information, see the "
             ).append($("<a>").attr("href", "http://ipython.org/ipython-doc/2/notebook/security.html")
                 .text("IPython security documentation")
             ).append(".")
         );
 
         var nb = this;
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
             title: "Trust this notebook?",
             body: body,
 
             buttons: {
                 Cancel : {},
                 Trust : {
                     class : "btn-danger",
                     click : function () {
                         var cells = nb.get_cells();
                         for (var i = 0; i < cells.length; i++) {
                             var cell = cells[i];
                             if (cell.cell_type === 'code') {
                                 cell.output_area.trusted = true;
                             }
                         }
                         nb.events.on('notebook_saved.Notebook', function () {
                             window.location.reload();
                         });
                         nb.save_notebook();
                     }
                 }
             }
         });
     };
 
     /**
      * Make a copy of the current notebook.
      */
     Notebook.prototype.copy_notebook = function () {
         var that = this;
         var base_url = this.base_url;
         var w = window.open('', IPython._target);
         var parent = utils.url_path_split(this.notebook_path)[0];
         this.contents.copy(this.notebook_path, parent).then(
             function (data) {
                 w.location = utils.url_join_encode(
                     base_url, 'notebooks', data.path
                 );
             },
             function(error) {
                 w.close();
                 that.events.trigger('notebook_copy_failed', error);
             }
         );
     };
     
     /**
      * Ensure a filename has the right extension
      * Returns the filename with the appropriate extension, appending if necessary.
      */
     Notebook.prototype.ensure_extension = function (name) {
         if (!name.match(/\.ipynb$/)) {
             name = name + ".ipynb";
         }
         return name;
     };
 
     /**
      * Rename the notebook.
      * @param  {string} new_name
      * @return {Promise} promise that resolves when the notebook is renamed.
      */
     Notebook.prototype.rename = function (new_name) {
         new_name = this.ensure_extension(new_name);
 
         var that = this;
         var parent = utils.url_path_split(this.notebook_path)[0];
         var new_path = utils.url_path_join(parent, new_name);
         return this.contents.rename(this.notebook_path, new_path).then(
             function (json) {
                 that.notebook_name = json.name;
                 that.notebook_path = json.path;
                 that.last_modified = new Date(json.last_modified);
                 that.session.rename_notebook(json.path);
                 that.events.trigger('notebook_renamed.Notebook', json);
             }
         );
     };
 
     /**
      * Delete this notebook
      */
     Notebook.prototype.delete = function () {
         this.contents.delete(this.notebook_path);
     };
 
     /**
      * Request a notebook's data from the server.
      * 
      * @param {string} notebook_path - A notebook to load
      */
     Notebook.prototype.load_notebook = function (notebook_path) {
-        var that = this;
         this.notebook_path = notebook_path;
         this.notebook_name = utils.url_path_split(this.notebook_path)[1];
         this.events.trigger('notebook_loading.Notebook');
         this.contents.get(notebook_path, {type: 'notebook'}).then(
             $.proxy(this.load_notebook_success, this),
             $.proxy(this.load_notebook_error, this)
         );
     };
 
     /**
      * Success callback for loading a notebook from the server.
      * 
      * Load notebook data from the JSON response.
      * 
      * @param {object} data JSON representation of a notebook
      */
     Notebook.prototype.load_notebook_success = function (data) {
         var failed, msg;
         try {
             this.fromJSON(data);
         } catch (e) {
             failed = e;
             console.log("Notebook failed to load from JSON:", e);
         }
         if (failed || data.message) {
             // *either* fromJSON failed or validation failed
             var body = $("<div>");
             var title;
             if (failed) {
                 title = "Notebook failed to load";
                 body.append($("<p>").text(
                     "The error was: "
                 )).append($("<div>").addClass("js-error").text(
                     failed.toString()
                 )).append($("<p>").text(
                     "See the error console for details."
                 ));
             } else {
                 title = "Notebook validation failed";
             }
 
             if (data.message) {
                 if (failed) {
                     msg = "The notebook also failed validation:";
                 } else {
                     msg = "An invalid notebook may not function properly." +
                     " The validation error was:";
                 }
                 body.append($("<p>").text(
                     msg
                 )).append($("<div>").addClass("validation-error").append(
                     $("<pre>").text(data.message)
                 ));
             }
 
             dialog.modal({
                 notebook: this,
                 keyboard_manager: this.keyboard_manager,
                 title: title,
                 body: body,
                 buttons : {
                     OK : {
                         "class" : "btn-primary"
                     }
                 }
             });
         }
         if (this.ncells() === 0) {
             this.insert_cell_below('code');
             this.edit_mode(0);
         } else {
             this.select(0);
             this.handle_command_mode(this.get_cell(0));
         }
         this.set_dirty(false);
         this.scroll_to_top();
         this.writable = data.writable || false;
         this.last_modified = new Date(data.last_modified);
         var nbmodel = data.content;
         var orig_nbformat = nbmodel.metadata.orig_nbformat;
         var orig_nbformat_minor = nbmodel.metadata.orig_nbformat_minor;
         if (orig_nbformat !== undefined && nbmodel.nbformat !== orig_nbformat) {
             var src;
             if (nbmodel.nbformat > orig_nbformat) {
                 src = " an older notebook format ";
             } else {
                 src = " a newer notebook format ";
             }
             
             msg = "This notebook has been converted from" + src +
             "(v"+orig_nbformat+") to the current notebook " +
             "format (v"+nbmodel.nbformat+"). The next time you save this notebook, the " +
             "current notebook format will be used.";
             
             if (nbmodel.nbformat > orig_nbformat) {
                 msg += " Older versions of IPython may not be able to read the new format.";
             } else {
                 msg += " Some features of the original notebook may not be available.";
             }
             msg += " To preserve the original version, close the " +
                 "notebook without saving it.";
             dialog.modal({
                 notebook: this,
                 keyboard_manager: this.keyboard_manager,
                 title : "Notebook converted",
                 body : msg,
                 buttons : {
                     OK : {
                         class : "btn-primary"
                     }
                 }
             });
         } else if (this.nbformat_minor < nbmodel.nbformat_minor) {
             this.nbformat_minor = nbmodel.nbformat_minor;
         }
 
         if (this.session === null) {
             var kernel_name = utils.get_url_param('kernel_name');
             if (kernel_name) {
                 this.kernel_selector.set_kernel(kernel_name);
             } else if (this.metadata.kernelspec) {
                 this.kernel_selector.set_kernel(this.metadata.kernelspec);
             } else if (this.metadata.language) {
                 // compat with IJulia, IHaskell, and other early kernels
                 // adopters that where setting a language metadata.
                 this.kernel_selector.set_kernel({
                     name: "(No name)",
                     language: this.metadata.language
                   });
                 // this should be stored in kspec now, delete it.
                 // remove once we do not support notebook v3 anymore.
                 delete this.metadata.language;
             } else {
                 // setting kernel via set_kernel above triggers start_session,
                 // otherwise start a new session with the server's default kernel
                 // spec_changed events will fire after kernel is loaded
                 this.start_session();
             }
         }
         // load our checkpoint list
         this.list_checkpoints();
         
         // load toolbar state
         if (this.metadata.celltoolbar) {
             celltoolbar.CellToolbar.global_show();
             celltoolbar.CellToolbar.activate_preset(this.metadata.celltoolbar);
         } else {
             celltoolbar.CellToolbar.global_hide();
         }
         
         if (!this.writable) {
             this.set_autosave_interval(0);
             this.events.trigger('notebook_read_only.Notebook');
         }
         
         // now that we're fully loaded, it is safe to restore save functionality
         this._fully_loaded = true;
         this.events.trigger('notebook_loaded.Notebook');
     };
 
     Notebook.prototype.set_kernelselector = function(k_selector){
         this.kernel_selector = k_selector;
     };
 
     /**
      * Failure callback for loading a notebook from the server.
      * 
      * @param {Error} error
      */
     Notebook.prototype.load_notebook_error = function (error) {
         this.events.trigger('notebook_load_failed.Notebook', error);
         var msg;
         if (error.name === utils.XHR_ERROR && error.xhr.status === 500) {
             utils.log_ajax_error(error.xhr, error.xhr_status, error.xhr_error);
             msg = "An unknown error occurred while loading this notebook. " +
             "This version can load notebook formats " +
             "v" + this.nbformat + " or earlier. See the server log for details.";
         } else {
             msg = error.message;
             console.warn('Error stack trace while loading notebook was:');
             console.warn(error.stack);
         }
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
             title: "Error loading notebook",
             body : msg,
             buttons : {
                 "OK": {}
             }
         });
     };
 
     /*********************  checkpoint-related  ********************/
     
     /**
      * Save the notebook then immediately create a checkpoint.
      */
     Notebook.prototype.save_checkpoint = function () {
         this._checkpoint_after_save = true;
         this.save_notebook();
     };
     
     /**
      * Add a checkpoint for this notebook.
      */
     Notebook.prototype.add_checkpoint = function (checkpoint) {
         var found = false;
         for (var i = 0; i < this.checkpoints.length; i++) {
             var existing = this.checkpoints[i];
             if (existing.id === checkpoint.id) {
                 found = true;
                 this.checkpoints[i] = checkpoint;
                 break;
             }
         }
         if (!found) {
             this.checkpoints.push(checkpoint);
         }
         this.last_checkpoint = this.checkpoints[this.checkpoints.length - 1];
     };
     
     /**
      * List checkpoints for this notebook.
      */
     Notebook.prototype.list_checkpoints = function () {
         var that = this;
         this.contents.list_checkpoints(this.notebook_path).then(
             $.proxy(this.list_checkpoints_success, this),
             function(error) {
                 that.events.trigger('list_checkpoints_failed.Notebook', error);
             }
         );
     };
 
     /**
      * Success callback for listing checkpoints.
      * 
      * @param {object} data - JSON representation of a checkpoint
      */
     Notebook.prototype.list_checkpoints_success = function (data) {
         this.checkpoints = data;
         if (data.length) {
             this.last_checkpoint = data[data.length - 1];
         } else {
             this.last_checkpoint = null;
         }
         this.events.trigger('checkpoints_listed.Notebook', [data]);
     };
 
     /**
      * Create a checkpoint of this notebook on the server from the most recent save.
      */
     Notebook.prototype.create_checkpoint = function () {
         var that = this;
         this.contents.create_checkpoint(this.notebook_path).then(
             $.proxy(this.create_checkpoint_success, this),
             function (error) {
                 that.events.trigger('checkpoint_failed.Notebook', error);
             }
         );
     };
 
     /**
      * Success callback for creating a checkpoint.
      * 
      * @param {object} data - JSON representation of a checkpoint
      */
     Notebook.prototype.create_checkpoint_success = function (data) {
         this.add_checkpoint(data);
         this.events.trigger('checkpoint_created.Notebook', data);
     };
 
     /**
      * Display the restore checkpoint dialog
      * @param  {string} checkpoint ID
      */
     Notebook.prototype.restore_checkpoint_dialog = function (checkpoint) {
         var that = this;
         checkpoint = checkpoint || this.last_checkpoint;
         if ( ! checkpoint ) {
             console.log("restore dialog, but no checkpoint to restore to!");
             return;
         }
         var body = $('<div/>').append(
             $('<p/>').addClass("p-space").text(
                 "Are you sure you want to revert the notebook to " +
                 "the latest checkpoint?"
             ).append(
                 $("<strong/>").text(
                     " This cannot be undone."
                 )
             )
         ).append(
             $('<p/>').addClass("p-space").text("The checkpoint was last updated at:")
         ).append(
             $('<p/>').addClass("p-space").text(
                 moment(checkpoint.last_modified).format('LLLL') +
                 ' ('+moment(checkpoint.last_modified).fromNow()+')'// Long form:  Tuesday, January 27, 2015 12:15 PM
             ).css("text-align", "center")
         );
         
         dialog.modal({
             notebook: this,
             keyboard_manager: this.keyboard_manager,
             title : "Revert notebook to checkpoint",
             body : body,
             buttons : {
                 Revert : {
                     class : "btn-danger",
                     click : function () {
                         that.restore_checkpoint(checkpoint.id);
                     }
                 },
                 Cancel : {}
                 }
         });
     };
     
     /**
      * Restore the notebook to a checkpoint state.
      * 
      * @param {string} checkpoint ID
      */
     Notebook.prototype.restore_checkpoint = function (checkpoint) {
         this.events.trigger('notebook_restoring.Notebook', checkpoint);
         var that = this;
         this.contents.restore_checkpoint(this.notebook_path, checkpoint).then(
             $.proxy(this.restore_checkpoint_success, this),
             function (error) {
                 that.events.trigger('checkpoint_restore_failed.Notebook', error);
             }
         );
     };
     
     /**
      * Success callback for restoring a notebook to a checkpoint.
      */
     Notebook.prototype.restore_checkpoint_success = function () {
         this.events.trigger('checkpoint_restored.Notebook');
         this.load_notebook(this.notebook_path);
     };
 
     /**
      * Delete a notebook checkpoint.
      * 
      * @param {string} checkpoint ID
      */
     Notebook.prototype.delete_checkpoint = function (checkpoint) {
         this.events.trigger('notebook_restoring.Notebook', checkpoint);
         var that = this;
         this.contents.delete_checkpoint(this.notebook_path, checkpoint).then(
             $.proxy(this.delete_checkpoint_success, this),
             function (error) {
                 that.events.trigger('checkpoint_delete_failed.Notebook', error);
             }
         );
     };
     
     /**
      * Success callback for deleting a notebook checkpoint.
      */
     Notebook.prototype.delete_checkpoint_success = function () {
         this.events.trigger('checkpoint_deleted.Notebook');
         this.load_notebook(this.notebook_path);
     };
 
 
     // For backwards compatability.
     IPython.Notebook = Notebook;
 
     return {'Notebook': Notebook};
 });