Split cell keyboard shortcut wired up. Merge markdown adds 2nd \n.

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

@@ -1,274 +1,279 b''
 //----------------------------------------------------------------------------
 //  Copyright (C) 2011  The IPython Development Team
 //
 //  Distributed under the terms of the BSD License.  The full license is in
 //  the file COPYING, distributed as part of this software.
 //----------------------------------------------------------------------------
 
 //============================================================================
 // Keyboard management
 //============================================================================
 
 var IPython = (function (IPython) {
     "use strict";
 
     var key   = IPython.utils.keycodes;
 
     var KeyboardManager = function () {
         this.mode = 'command';
         this.enabled = true;
         this.bind_events();
     };
 
     KeyboardManager.prototype.bind_events = function () {
         var that = this;
         $(document).keydown(function (event) {
             return that.handle_keydown(event);
         });
     };
 
     KeyboardManager.prototype.handle_keydown = function (event) {
         var notebook = IPython.notebook;
         
         console.log('keyboard_manager', this.mode, event.keyCode);
         
         if (event.which === key.ESC) {
             // Intercept escape at highest level to avoid closing
             // websocket connection with firefox
             event.preventDefault();
         }
         
         if (!this.enabled) {
             return true;
         }
         
         // Event handlers for both command and edit mode
         if ((event.ctrlKey || event.metaKey) && event.keyCode==83) {
             // Save (CTRL+S) or (Command+S on Mac)
             notebook.save_checkpoint();
             event.preventDefault();
             return false;
         } else if (event.which === key.ESC) {
             // Intercept escape at highest level to avoid closing
             // websocket connection with firefox
             event.preventDefault();
             // Don't return yet to allow edit/command modes to handle
         } else if (event.which === key.SHIFT) {
             // ignore shift keydown
             return true;
         } else if (event.which === key.ENTER && event.shiftKey) {
             notebook.execute_selected_cell('shift');
             return false;
         } else if (event.which === key.ENTER && event.altKey) {
             // Execute code cell, and insert new in place
             notebook.execute_selected_cell('alt');
             return false;
         } else if (event.which === key.ENTER && event.ctrlKey) {
             notebook.execute_selected_cell('ctrl');
             return false;
         }
         
         if (this.mode === 'edit') {
             return this.handle_edit_mode(event);
         } else if (this.mode === 'command' && !(event.ctrlKey || event.altKey || event.metaKey)) {
             return this.handle_command_mode(event);
         }
     }
 
     KeyboardManager.prototype.handle_edit_mode = function (event) {
         var notebook = IPython.notebook;
         
         if (event.which === key.ESC) {
             // ESC
             notebook.command_mode();
             return false;
         } else if (event.which === 77 && event.ctrlKey) {
             // Ctrl-m
             notebook.command_mode();
             return false;
         } else if (event.which === key.UPARROW && !event.shiftKey) {
             var cell = notebook.get_selected_cell();
             if (cell && cell.at_top()) {
                 event.preventDefault();
                 notebook.command_mode()
                 notebook.select_prev();
                 notebook.edit_mode();
                 return false;
             };
         } else if (event.which === key.DOWNARROW && !event.shiftKey) {
             var cell = notebook.get_selected_cell();
             if (cell && cell.at_bottom()) {
                 event.preventDefault();
                 notebook.command_mode()
                 notebook.select_next();
                 notebook.edit_mode();
                 return false;
             };
         };
         return true;
     }
 
     KeyboardManager.prototype.handle_command_mode = function (event) {
         var notebook = IPython.notebook;
         
         if (event.which === key.ENTER && !(event.ctrlKey || event.altKey || event.shiftKey)) {
             // Enter edit mode = ENTER alone
             notebook.edit_mode();
             return false;
         } else if (event.which === key.UPARROW && !event.shiftKey) {
             var index = notebook.get_selected_index();
             if (index !== 0 && index !== null) {
                 notebook.select_prev();
                 var cell = notebook.get_selected_cell();
                 cell.focus_cell();
             };
             return false;
         } else if (event.which === key.DOWNARROW && !event.shiftKey) {
             var index = notebook.get_selected_index();
             if (index !== (notebook.ncells()-1) && index !== null) {
                 notebook.select_next();
                 var cell = notebook.get_selected_cell();
                 cell.focus_cell();
             };
             return false;
         } else if (event.which === 88) {
             // Cut selected cell = x
             notebook.cut_cell();
             return false;
         } else if (event.which === 67) {
             // Copy selected cell = c
             notebook.copy_cell();
             return false;
         } else if (event.which === 86) {
             // Paste below selected cell = v
             notebook.paste_cell_below();
             return false;
         } else if (event.which === 68) {
             // Delete selected cell = d
             notebook.delete_cell();
             return false;
         } else if (event.which === 65) {
             // Insert code cell above selected = a
             notebook.insert_cell_above('code');
             notebook.select_prev();
             return false;
         } else if (event.which === 66) {
             // Insert code cell below selected = b
             notebook.insert_cell_below('code');
             notebook.select_next();
             return false;
         } else if (event.which === 89) {
             // To code = y
             notebook.to_code();
             return false;
         } else if (event.which === 77) {
             // To markdown = m
             notebook.to_markdown();
             return false;
         } else if (event.which === 84) {
             // To Raw = t
             notebook.to_raw();
             return false;
         } else if (event.which === 49) {
             // To Heading 1 = 1
             notebook.to_heading(undefined, 1);
             return false;
         } else if (event.which === 50) {
             // To Heading 2 = 2
             notebook.to_heading(undefined, 2);
             return false;
         } else if (event.which === 51) {
             // To Heading 3 = 3
             notebook.to_heading(undefined, 3);
             return false;
         } else if (event.which === 52) {
             // To Heading 4 = 4
             notebook.to_heading(undefined, 4);
             return false;
         } else if (event.which === 53) {
             // To Heading 5 = 5
             notebook.to_heading(undefined, 5);
             return false;
         } else if (event.which === 54) {
             // To Heading 6 = 6
             notebook.to_heading(undefined, 6);
             return false;
         } else if (event.which === 79) {
             // Toggle output = o
             if (event.shiftKey) {
                 notebook.toggle_output_scroll();
             } else {
                 notebook.toggle_output();
             };
             return false;
         } else if (event.which === 83) {
             // Save notebook = s
             notebook.save_checkpoint();
             return false;
         } else if (event.which === 74) {
             // Move cell down = j
             notebook.move_cell_down();
             return false;
         } else if (event.which === 75) {
             // Move cell up = k
             notebook.move_cell_up();
             return false;
         } else if (event.which === 80) {
             // Select previous = p
             notebook.select_prev();
             return false;
         } else if (event.which === 78) {
             // Select next = n
             notebook.select_next();
             return false;
         } else if (event.which === 76) {
             // Toggle line numbers = l
             notebook.cell_toggle_line_numbers();
             return false;
         } else if (event.which === 73) {
             // Interrupt kernel = i
             notebook.kernel.interrupt();
             return false;
         } else if (event.which === 190) {
             // Restart kernel = .  # matches qt console
             notebook.restart_kernel();
             return false;
         } else if (event.which === 72) {
             // Show keyboard shortcuts = h
             IPython.quick_help.show_keyboard_shortcuts();
             return false;
         } else if (event.which === 90) {
             // Undo last cell delete = z
             notebook.undelete();
             return false;
+        } else if (event.which === 189 || event.which === 173) {
+            // how fun! '-' is 189 in Chrome, but 173 in FF and Opera
+            // Split cell = -
+            notebook.split_cell();
+            return false;     
         };
         // If we havn't handled it, let someone else.
         return true;
     };
 
     KeyboardManager.prototype.edit_mode = function () {
         console.log('KeyboardManager', 'changing to edit mode');
         this.last_mode = this.mode;
         this.mode = 'edit';
     }
 
     KeyboardManager.prototype.command_mode = function () {
         console.log('KeyboardManager', 'changing to command mode');
         this.last_mode = this.mode;
         this.mode = 'command';
     }
 
     KeyboardManager.prototype.enable = function () {
         this.enable = true;
     }
 
     KeyboardManager.prototype.disable = function () {
         this.enable = false;
     }
 
 
     IPython.KeyboardManager = KeyboardManager;
 
     return IPython;
 
 }(IPython));

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

@@ -1,2152 +1,2152 b''
 //----------------------------------------------------------------------------
 //  Copyright (C) 2011  The IPython Development Team
 //
 //  Distributed under the terms of the BSD License.  The full license is in
 //  the file COPYING, distributed as part of this software.
 //----------------------------------------------------------------------------
 
 //============================================================================
 // Notebook
 //============================================================================
 
 var IPython = (function (IPython) {
     "use strict";
 
     var utils = IPython.utils;
 
     /**
      * A notebook contains and manages cells.
      * 
      * @class Notebook
      * @constructor
      * @param {String} selector A jQuery selector for the notebook's DOM element
      * @param {Object} [options] A config object
      */
     var Notebook = function (selector, options) {
         var options = options || {};
         this._baseProjectUrl = options.baseProjectUrl;
         this.notebook_path = options.notebookPath;
         this.notebook_name = options.notebookName;
         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;
         // 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;
         // single worksheet for now
         this.worksheet_metadata = {};
         this.notebook_name_blacklist_re = /[\/\\:]/;
         this.nbformat = 3 // Increment this when changing the nbformat
         this.nbformat_minor = 0 // Increment this when changing the nbformat
         this.style();
         this.create_elements();
         this.bind_events();
     };
 
     /**
      * Tweak the notebook's CSS style.
      * 
      * @method style
      */
     Notebook.prototype.style = function () {
         $('div#notebook').addClass('border-box-sizing');
     };
 
     /**
      * Get the root URL of the notebook server.
      * 
      * @method baseProjectUrl
      * @return {String} The base project URL
      */
     Notebook.prototype.baseProjectUrl = function() {
         return this._baseProjectUrl || $('body').data('baseProjectUrl');
     };
 
     Notebook.prototype.notebookName = function() {
         return $('body').data('notebookName');
     };
     
     Notebook.prototype.notebookPath = function() {
         return $('body').data('notebookPath');
     };
     
     /**
      * Create an HTML and CSS representation of the notebook.
      * 
      * @method create_elements
      */
     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.append(end_space);
     };
 
     /**
      * Bind JavaScript events: key presses and custom IPython events.
      * 
      * @method bind_events
      */
     Notebook.prototype.bind_events = function () {
         var that = this;
 
         $([IPython.events]).on('set_next_input.Notebook', function (event, data) {
             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;
         });
 
         $([IPython.events]).on('set_dirty.Notebook', function (event, data) {
             that.dirty = data.value;
         });
 
         $([IPython.events]).on('select.Cell', function (event, data) {
             var index = that.find_cell_index(data.cell);
             that.select(index);
         });
 
         $([IPython.events]).on('edit_mode.Cell', function (event, data) {
             var index = that.find_cell_index(data.cell);
             that.select(index);
             that.edit_mode();
         });
 
         $([IPython.events]).on('command_mode.Cell', function (event, data) {
             that.command_mode();
         });
 
         $([IPython.events]).on('status_autorestarting.Kernel', function () {
             IPython.dialog.modal({
                 title: "Kernel Restarting",
                 body: "The kernel appears to have died. It will restart automatically.",
                 buttons: {
                     OK : {
                         class : "btn-primary"
                     }
                 }
             });
         });
 
         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.kill_kernel();
             }
             // 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.";
                 }
             };
             // Null is the *only* return value that will make the browser not
             // pop up the "don't leave" dialog.
             return null;
         };
     };
 
     /**
      * Set the dirty flag, and trigger the set_dirty.Notebook event
      * 
      * @method set_dirty
      */
     Notebook.prototype.set_dirty = function (value) {
         if (value === undefined) {
             value = true;
         }
         if (this.dirty == value) {
             return;
         }
         $([IPython.events]).trigger('set_dirty.Notebook', {value: value});
     };
 
     /**
      * Scroll the top of the page to a given cell.
      * 
      * @method scroll_to_cell
      * @param {Number} cell_number An index of the cell to view
      * @param {Number} time Animation time in milliseconds
      * @return {Number} Pixel offset from the top of the container
      */
     Notebook.prototype.scroll_to_cell = function (cell_number, time) {
         var cells = this.get_cells();
         var time = time || 0;
         cell_number = Math.min(cells.length-1,cell_number);
         cell_number = Math.max(0             ,cell_number);
         var scroll_value = cells[cell_number].element.position().top-cells[0].element.position().top ;
         this.element.animate({scrollTop:scroll_value}, time);
         return scroll_value;
     };
 
     /**
      * Scroll to the bottom of the page.
      * 
      * @method scroll_to_bottom
      */
     Notebook.prototype.scroll_to_bottom = function () {
         this.element.animate({scrollTop:this.element.get(0).scrollHeight}, 0);
     };
 
     /**
      * Scroll to the top of the page.
      * 
      * @method scroll_to_top
      */
     Notebook.prototype.scroll_to_top = function () {
         this.element.animate({scrollTop:0}, 0);
     };
 
     // Edit Notebook metadata
 
     Notebook.prototype.edit_metadata = function () {
         var that = this;
         IPython.dialog.edit_metadata(this.metadata, function (md) {
             that.metadata = md;
         }, 'Notebook');
     };
 
     // Cell indexing, retrieval, etc.
 
     /**
      * Get all cell elements in the notebook.
      * 
      * @method get_cell_elements
      * @return {jQuery} A selector of all cell elements
      */
     Notebook.prototype.get_cell_elements = function () {
         return this.container.children("div.cell");
     };
 
     /**
      * Get a particular cell element.
      * 
      * @method get_cell_element
      * @param {Number} 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;
     };
 
     /**
      * Count the cells in this notebook.
      * 
      * @method ncells
      * @return {Number} The number of cells in this notebook
      */
     Notebook.prototype.ncells = function () {
         return this.get_cell_elements().length;
     };
 
     /**
      * Get all Cell objects in this notebook.
      * 
      * @method get_cells
      * @return {Array} This notebook's Cell objects
      */
     // TODO: we are often calling cells as cells()[i], which we should optimize
     // to cells(i) or a new method.
     Notebook.prototype.get_cells = function () {
         return this.get_cell_elements().toArray().map(function (e) {
             return $(e).data("cell");
         });
     };
 
     /**
      * Get a Cell object from this notebook.
      * 
      * @method get_cell
      * @param {Number} index An index of a cell to retrieve
      * @return {Cell} A particular cell
      */
     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.
      * 
      * @method get_next_cell
      * @param {Cell} cell The provided cell
      * @return {Cell} The next cell
      */
     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.
      * 
      * @method get_prev_cell
      * @param {Cell} cell The provided cell
      * @return {Cell} The previous cell
      */
     Notebook.prototype.get_prev_cell = function (cell) {
         // TODO: off-by-one
         // nb.get_prev_cell(nb.get_cell(1)) is null
         var result = null;
         var index = this.find_cell_index(cell);
         if (index !== null && index > 1) {
             result = this.get_cell(index-1);
         }
         return result;
     }
     
     /**
      * Get the numeric index of a given cell.
      * 
      * @method find_cell_index
      * @param {Cell} cell The provided cell
      * @return {Number} The cell's numeric index
      */
     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;
     };
 
     /**
      * Get a given index , or the selected index if none is provided.
      * 
      * @method index_or_selected
      * @param {Number} index A cell's index
      * @return {Number} The given index, or selected index if none is provided.
      */
     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.
      * @method get_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.
      * 
      * @method is_valid_cell_index
      * @param {Number} 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.
      
      * @method get_selected_index
      * @return {Number} 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.
      * 
      * @method select
      * @param {Number} 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) {
                 this.command_mode();
                 this.get_cell(sindex).unselect();
             };
             var cell = this.get_cell(index);
             cell.select();
             if (cell.cell_type === 'heading') {
                 $([IPython.events]).trigger('selected_cell_type_changed.Notebook',
                     {'cell_type':cell.cell_type,level:cell.level}
                 );
             } else {
                 $([IPython.events]).trigger('selected_cell_type_changed.Notebook',
                     {'cell_type':cell.cell_type}
                 );
             };
         };
         return this;
     };
 
     /**
      * Programmatically select the next cell.
      *
      * @method select_next
      * @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.
      *
      * @method select_prev
      * @return {Notebook} This notebook
      */
     Notebook.prototype.select_prev = function () {
         var index = this.get_selected_index();
         this.select(index-1);
         return this;
     };
 
 
     // Edit/Command mode
 
     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;
     };
 
     Notebook.prototype.command_mode = function () {
         if (this.mode !== 'command') {
             console.log('\nNotebook', 'changing to command mode');
             var index = this.get_edit_index();
             var cell = this.get_cell(index);
             if (cell) {
                 cell.command_mode();
             };
             this.mode = 'command';
             IPython.keyboard_manager.command_mode();
         };
     };
 
     Notebook.prototype.edit_mode = function () {
         if (this.mode !== 'edit') {
             console.log('\nNotebook', 'changing to edit mode');
             var cell = this.get_selected_cell();
             if (cell === null) {return;}  // No cell is selected
             // We need to set the mode to edit to prevent reentering this method
             // when cell.edit_mode() is called below.
             this.mode = 'edit';
             IPython.keyboard_manager.edit_mode();
             cell.edit_mode();
         };
     };
 
 
     // Cell movement
 
     /**
      * Move given (or selected) cell up and select it.
      * 
      * @method move_cell_up
      * @param [index] {integer} 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
      * 
      * @method move_cell_down
      * @param [index] {integer} 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.
      * 
      * @method delete_cell
      * @param [index] A 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_selected_cell();
         this.undelete_backup = cell.toJSON();
         $('#undelete_cell').removeClass('disabled');
         if (this.is_valid_cell_index(i)) {
             var ce = this.get_cell_element(i);
             ce.remove();
             if (i === (this.ncells())) {
                 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;
             };
             $([IPython.events]).trigger('delete.Cell', {'cell': cell, 'index': i});
             this.set_dirty(true);
         };
         return this;
     };
 
     /**
      * Insert a cell so that after insertion the cell is at given index.
      *
      * Similar to insert_above, but index parameter is mandatory
      *
      * Index will be brought back into the accissible range [0,n]
      *
      * @method insert_cell_at_index
      * @param type {string} in ['code','markdown','heading']
      * @param [index] {int} a valid index where to inser cell
      *
      * @return cell {cell|null} created cell or null
      **/
     Notebook.prototype.insert_cell_at_index = function(type, index){
 
         var ncells = this.ncells();
         var index = Math.min(index,ncells);
             index = Math.max(index,0);
         var cell = null;
 
         if (ncells === 0 || this.is_valid_cell_index(index) || index === ncells) {
             if (type === 'code') {
                 cell = new IPython.CodeCell(this.kernel);
                 cell.set_input_prompt();
             } else if (type === 'markdown') {
                 cell = new IPython.MarkdownCell();
             } else if (type === 'raw') {
                 cell = new IPython.RawCell();
             } else if (type === 'heading') {
                 cell = new IPython.HeadingCell();
             }
 
             if(this._insert_element_at_index(cell.element,index)) {
                 cell.render();
                 $([IPython.events]).trigger('create.Cell', {'cell': cell, 'index': index});
                 cell.refresh();
                 // TODO: should we really get rid of this?
                 //this.select(this.find_cell_index(cell));
                 this.set_dirty(true);
             }
         }
         return cell;
 
     };
 
     /**
      * Insert an element at given cell index.
      *
      * @method _insert_element_at_index
      * @param element {dom element} a cell element
      * @param [index] {int} a valid index where to inser cell
      * @private
      *
      * return true if everything whent fine.
      **/
     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.element.find('div.end_space').before(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.
      *
      * default index value is the one of currently selected cell
      *
      * @method insert_cell_above
      * @param type {string} cell type
      * @param [index] {integer}
      *
      * @return 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 thatn number of cell
      *
      * default index value is the one of currently selected cell
      *
      * @method insert_cell_below
      * @param type {string} cell type
      * @param [index] {integer}
      *
      * @return 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
      *
      * @method insert_cell_at_bottom
      * @param {String} type cell type
      *
      * @return the added 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.
      * 
      * @method to_code
      * @param {Number} [index] A cell's index
      */
     Notebook.prototype.to_code = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_element = this.get_cell_element(i);
             var source_cell = source_element.data("cell");
             if (!(source_cell instanceof IPython.CodeCell)) {
                 var target_cell = this.insert_cell_below('code',i);
                 var text = source_cell.get_text();
                 if (text === source_cell.placeholder) {
                     text = '';
                 }
                 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_element.remove();
                 this.select(i);
                 this.edit_mode();
                 this.set_dirty(true);
             };
         };
     };
 
     /**
      * Turn a cell into a Markdown cell.
      * 
      * @method to_markdown
      * @param {Number} [index] A cell's index
      */
     Notebook.prototype.to_markdown = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_element = this.get_cell_element(i);
             var source_cell = source_element.data("cell");
             if (!(source_cell instanceof IPython.MarkdownCell)) {
                 var target_cell = this.insert_cell_below('markdown',i);
                 var text = source_cell.get_text();
                 if (text === source_cell.placeholder) {
                     text = '';
                 };
                 // 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_element.remove();
                 this.select(i);
                 this.edit_mode();
                 this.set_dirty(true);
             };
         };
     };
 
     /**
      * Turn a cell into a raw text cell.
      * 
      * @method to_raw
      * @param {Number} [index] A cell's index
      */
     Notebook.prototype.to_raw = function (index) {
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_element = this.get_cell_element(i);
             var source_cell = source_element.data("cell");
             var target_cell = null;
             if (!(source_cell instanceof IPython.RawCell)) {
                 target_cell = this.insert_cell_below('raw',i);
                 var text = source_cell.get_text();
                 if (text === source_cell.placeholder) {
                     text = '';
                 };
                 // 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_element.remove();
                 this.select(i);
                 this.edit_mode();
                 this.set_dirty(true);
             };
         };
     };
 
     /**
      * Turn a cell into a heading cell.
      * 
      * @method to_heading
      * @param {Number} [index] A cell's index
      * @param {Number} [level] A heading level (e.g., 1 becomes &lt;h1&gt;)
      */
     Notebook.prototype.to_heading = function (index, level) {
         level = level || 1;
         var i = this.index_or_selected(index);
         if (this.is_valid_cell_index(i)) {
             var source_element = this.get_cell_element(i);
             var source_cell = source_element.data("cell");
             var target_cell = null;
             if (source_cell instanceof IPython.HeadingCell) {
                 source_cell.set_level(level);
             } else {
                 target_cell = this.insert_cell_below('heading',i);
                 var text = source_cell.get_text();
                 if (text === source_cell.placeholder) {
                     text = '';
                 };
                 // We must show the editor before setting its contents
                 target_cell.set_level(level);
                 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_element.remove();
                 this.select(i);
                 this.edit_mode();
                 this.set_dirty(true);
             };
             $([IPython.events]).trigger('selected_cell_type_changed.Notebook',
                 {'cell_type':'heading',level:level}
             );
         };
     };
 
 
     // Cut/Copy/Paste
 
     /**
      * Enable UI elements for pasting cells.
      * 
      * @method enable_paste
      */
     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 UI elements for pasting cells.
      * 
      * @method disable_paste
      */
     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.
      * 
      * @method cut_cell
      */
     Notebook.prototype.cut_cell = function () {
         this.copy_cell();
         this.delete_cell();
     }
 
     /**
      * Copy a cell.
      * 
      * @method copy_cell
      */
     Notebook.prototype.copy_cell = function () {
         var cell = this.get_selected_cell();
         this.clipboard = cell.toJSON();
         this.enable_paste();
     };
 
     /**
      * Replace the selected cell with a cell in the clipboard.
      * 
      * @method paste_cell_replace
      */
     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.
      * 
      * @method paste_cell_above
      */
     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);
         };
     };
 
     /**
      * Paste a cell from the clipboard below the selected cell.
      * 
      * @method paste_cell_below
      */
     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);
         };
     };
 
     // Cell undelete
 
     /**
      * Restore the most recently deleted cell.
      * 
      * @method undelete
      */
     Notebook.prototype.undelete = 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);
             this.select(current_index);
             this.undelete_backup = null;
             this.undelete_index = null;
         }
         $('#undelete_cell').addClass('disabled');
     }
 
     // Split/merge
 
     /**
      * Split the selected cell into two, at the cursor.
      * 
      * @method split_cell
      */
     Notebook.prototype.split_cell = function () {
         // Todo: implement spliting for other cell types.
         var cell = this.get_selected_cell();
         if (cell.is_splittable()) {
             var texta = cell.get_pre_cursor();
             var textb = cell.get_post_cursor();
             if (cell instanceof IPython.CodeCell) {
                 cell.set_text(textb);
                 var new_cell = this.insert_cell_above('code');
                 new_cell.set_text(texta);
                 this.select_next();
             } else if (cell instanceof IPython.MarkdownCell) {
                 cell.set_text(textb);
                 cell.render();
                 var new_cell = this.insert_cell_above('markdown');
                 new_cell.unrender(); // editor must be visible to call set_text
                 new_cell.set_text(texta);
                 new_cell.render();
                 this.select_next();
             }
         };
     };
 
     /**
      * Combine the selected cell into the cell above it.
      * 
      * @method merge_cell_above
      */
     Notebook.prototype.merge_cell_above = function () {
         var index = this.get_selected_index();
         var cell = this.get_cell(index);
         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 IPython.CodeCell) {
                 cell.set_text(upper_text+'\n'+text);
             } else if (cell instanceof IPython.MarkdownCell) {
                 cell.unrender();
-                cell.set_text(upper_text+'\n'+text);
+                cell.set_text(upper_text+'\n\n'+text);
                 cell.render();
             };
             this.delete_cell(index-1);
             this.select(this.find_cell_index(cell));
         };
     };
 
     /**
      * Combine the selected cell into the cell below it.
      * 
      * @method merge_cell_below
      */
     Notebook.prototype.merge_cell_below = function () {
         var index = this.get_selected_index();
         var cell = this.get_cell(index);
         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 IPython.CodeCell) {
                 cell.set_text(text+'\n'+lower_text);
             } else if (cell instanceof IPython.MarkdownCell) {
                 cell.unrender();
-                cell.set_text(text+'\n'+lower_text);
+                cell.set_text(text+'\n\n'+lower_text);
                 cell.render();
             };
             this.delete_cell(index+1);
             this.select(this.find_cell_index(cell));
         };
     };
 
 
     // Cell collapsing and output clearing
 
     /**
      * Hide a cell's output.
      * 
      * @method collapse
      * @param {Number} index A cell's numeric index
      */
     Notebook.prototype.collapse = function (index) {
         var i = this.index_or_selected(index);
         this.get_cell(i).collapse();
         this.set_dirty(true);
     };
 
     /**
      * Show a cell's output.
      * 
      * @method expand
      * @param {Number} index A cell's numeric index
      */
     Notebook.prototype.expand = function (index) {
         var i = this.index_or_selected(index);
         this.get_cell(i).expand();
         this.set_dirty(true);
     };
 
     /** Toggle whether a cell's output is collapsed or expanded.
      * 
      * @method toggle_output
      * @param {Number} index A cell's numeric index
      */
     Notebook.prototype.toggle_output = function (index) {
         var i = this.index_or_selected(index);
         this.get_cell(i).toggle_output();
         this.set_dirty(true);
     };
 
     /**
      * Toggle a scrollbar for long cell outputs.
      * 
      * @method toggle_output_scroll
      * @param {Number} index A cell's numeric index
      */
     Notebook.prototype.toggle_output_scroll = function (index) {
         var i = this.index_or_selected(index);
         this.get_cell(i).toggle_output_scroll();
     };
 
     /**
      * Hide each code cell's output area.
      * 
      * @method collapse_all_output
      */
     Notebook.prototype.collapse_all_output = function () {
         var ncells = this.ncells();
         var cells = this.get_cells();
         for (var i=0; i<ncells; i++) {
             if (cells[i] instanceof IPython.CodeCell) {
                 cells[i].output_area.collapse();
             }
         };
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /**
      * Expand each code cell's output area, and add a scrollbar for long output.
      * 
      * @method scroll_all_output
      */
     Notebook.prototype.scroll_all_output = function () {
         var ncells = this.ncells();
         var cells = this.get_cells();
         for (var i=0; i<ncells; i++) {
             if (cells[i] instanceof IPython.CodeCell) {
                 cells[i].output_area.expand();
                 cells[i].output_area.scroll_if_long();
             }
         };
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /**
      * Expand each code cell's output area, and remove scrollbars.
      * 
      * @method expand_all_output
      */
     Notebook.prototype.expand_all_output = function () {
         var ncells = this.ncells();
         var cells = this.get_cells();
         for (var i=0; i<ncells; i++) {
             if (cells[i] instanceof IPython.CodeCell) {
                 cells[i].output_area.expand();
                 cells[i].output_area.unscroll_area();
             }
         };
         // this should not be set if the `collapse` key is removed from nbformat
         this.set_dirty(true);
     };
 
     /**
      * Clear each code cell's output area.
      * 
      * @method clear_all_output
      */
     Notebook.prototype.clear_all_output = function () {
         var ncells = this.ncells();
         var cells = this.get_cells();
         for (var i=0; i<ncells; i++) {
             if (cells[i] instanceof IPython.CodeCell) {
                 cells[i].clear_output();
                 // Make all In[] prompts blank, as well
                 // TODO: make this configurable (via checkbox?)
                 cells[i].set_input_prompt();
             }
         };
         this.set_dirty(true);
     };
 
 
     // Other cell functions: line numbers, ...
 
     /**
      * Toggle line numbers in the selected cell's input area.
      * 
      * @method cell_toggle_line_numbers
      */
     Notebook.prototype.cell_toggle_line_numbers = function() {
         this.get_selected_cell().toggle_line_numbers();
     };
 
     // Session related things
 
     /**
      * Start a new session and set it on each code cell.
      * 
      * @method start_session
      */
     Notebook.prototype.start_session = function () {
         this.session = new IPython.Session(this.notebook_name, this.notebook_path, this);
         this.session.start($.proxy(this._session_started, this));
     };
 
 
     /**
      * Once a session is started, link the code cells to the kernel
      *
      */
     Notebook.prototype._session_started = function(){
         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 IPython.CodeCell) {
                 cell.set_kernel(this.session.kernel);
             };
         };  
     };
     
     /**
      * Prompt the user to restart the IPython kernel.
      * 
      * @method restart_kernel
      */
     Notebook.prototype.restart_kernel = function () {
         var that = this;
         IPython.dialog.modal({
             title : "Restart kernel or continue running?",
             body : $("<p/>").html(
                 '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.session.restart_kernel();
                     }
                 }
             }
         });
     };
     
     /**
      * Run the selected cell.
      * 
      * Execute or render cell outputs.
      * 
      * @method execute_selected_cell
      * @param {Object} options Customize post-execution behavior
      */
     Notebook.prototype.execute_selected_cell = function (mode) {
         // mode = shift, ctrl, alt
         mode = mode || 'shift'
         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.insert_cell_below('code');
             this.select(cell_index+1);
             this.edit_mode();
             this.scroll_to_bottom();
             this.set_dirty(true);
             return;
         }      
   
         if (mode === 'shift') {
             this.command_mode();
         } else if (mode === 'ctrl') {
             this.select(cell_index+1);
             this.get_cell(cell_index+1).focus_cell();
         } else if (mode === 'alt') {
             // Only insert a new cell, if we ended up in an already populated cell
             var next_text = this.get_cell(cell_index+1).get_text();
             if (/\S/.test(next_text) === true) {
                 this.insert_cell_below('code');
             }
             this.select(cell_index+1);
             this.edit_mode();
         }
         this.set_dirty(true);
     };
 
 
     /**
      * Execute all cells below the selected cell.
      * 
      * @method execute_cells_below
      */
     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.
      * 
      * @method execute_cells_above
      */
     Notebook.prototype.execute_cells_above = function () {
         this.execute_cell_range(0, this.get_selected_index());
     };
 
     /**
      * Execute all cells.
      * 
      * @method 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.
      * 
      * @method execute_cell_range
      * @param {Number} start Index of the first cell to execute (inclusive)
      * @param {Number} end Index of the last cell to execute (exclusive)
      */
     Notebook.prototype.execute_cell_range = function (start, end) {
         for (var i=start; i<end; i++) {
             this.select(i);
             this.execute_selected_cell({add_new:false});
         };
     };
 
     // Persistance and loading
 
     /**
      * Getter method for this notebook's name.
      * 
      * @method get_notebook_name
      * @return {String} This notebook's name
      */
     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.
      *
      * @method set_notebook_name
      * @param {String} name A new name for this notebook
      */
     Notebook.prototype.set_notebook_name = function (name) {
         this.notebook_name = name;
     };
 
     /**
      * Check that a notebook's name is valid.
      * 
      * @method test_notebook_name
      * @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 (this.notebook_name_blacklist_re.test(nbname) == false && nbname.length>0) {
             return true;
         } else {
             return false;
         };
     };
 
     /**
      * Load a notebook from JSON (.ipynb).
      * 
      * This currently handles one worksheet: others are deleted.
      * 
      * @method fromJSON
      * @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.delete_cell(0);
         };
         // Save the metadata and name.
         this.metadata = content.metadata;
         this.notebook_name = data.name;
         // Only handle 1 worksheet for now.
         var worksheet = content.worksheets[0];
         if (worksheet !== undefined) {
             if (worksheet.metadata) {
                 this.worksheet_metadata = worksheet.metadata;
             }
             var new_cells = worksheet.cells;
             ncells = new_cells.length;
             var cell_data = null;
             var new_cell = null;
             for (i=0; i<ncells; i++) {
                 cell_data = new_cells[i];
                 // VERSIONHACK: plaintext -> raw
                 // handle never-released plaintext name for raw cells
                 if (cell_data.cell_type === 'plaintext'){
                     cell_data.cell_type = 'raw';
                 }
 
                 new_cell = this.insert_cell_at_index(cell_data.cell_type, i);
                 new_cell.fromJSON(cell_data);
             };
         };
         if (content.worksheets.length > 1) {
             IPython.dialog.modal({
                 title : "Multiple worksheets",
                 body : "This notebook has " + data.worksheets.length + " worksheets, " +
                     "but this version of IPython can only handle the first.  " +
                     "If you save this notebook, worksheets after the first will be lost.",
                 buttons : {
                     OK : {
                         class : "btn-danger"
                     }
                 }
             });
         }
     };
 
     /**
      * Dump this notebook into a JSON-friendly object.
      * 
      * @method toJSON
      * @return {Object} A JSON-friendly representation of this notebook.
      */
     Notebook.prototype.toJSON = function () {
         var cells = this.get_cells();
         var ncells = cells.length;
         var cell_array = new Array(ncells);
         for (var i=0; i<ncells; i++) {
             cell_array[i] = cells[i].toJSON();
         };
         var data = {
             // Only handle 1 worksheet for now.
             worksheets : [{
                 cells: cell_array,
                 metadata: this.worksheet_metadata
             }],
             metadata : this.metadata
         };
         return data;
     };
 
     /**
      * Start an autosave timer, for periodically saving the notebook.
      * 
      * @method set_autosave_interval
      * @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);
         }
         
         this.autosave_interval = this.minimum_autosave_interval = interval;
         if (interval) {
             this.autosave_timer = setInterval(function() {
                 if (that.dirty) {
                     that.save_notebook();
                 }
             }, interval);
             $([IPython.events]).trigger("autosave_enabled.Notebook", interval);
         } else {
             this.autosave_timer = null;
             $([IPython.events]).trigger("autosave_disabled.Notebook");
         };
     };
     
     /**
      * Save this notebook on the server.
      * 
      * @method save_notebook
      */
     Notebook.prototype.save_notebook = function (extra_settings) {
         // Create a JSON model to be sent to the server.
         var model = {};
         model.name = this.notebook_name;
         model.path = this.notebook_path;
         model.content = this.toJSON();
         model.content.nbformat = this.nbformat;
         model.content.nbformat_minor = this.nbformat_minor;
         // time the ajax call for autosave tuning purposes.
         var start =  new Date().getTime();
         // We do the call with settings so we can set cache to false.
         var settings = {
             processData : false,
             cache : false,
             type : "PUT",
             data : JSON.stringify(model),
             headers : {'Content-Type': 'application/json'},
             success : $.proxy(this.save_notebook_success, this, start),
             error : $.proxy(this.save_notebook_error, this)
         };
         if (extra_settings) {
             for (var key in extra_settings) {
                 settings[key] = extra_settings[key];
             }
         }
         $([IPython.events]).trigger('notebook_saving.Notebook');
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebook_path,
             this.notebook_name
         );
         $.ajax(url, settings);
     };
     
     /**
      * Success callback for saving a notebook.
      * 
      * @method save_notebook_success
      * @param {Integer} start the time when the save request started
      * @param {Object} data JSON representation of a notebook
      * @param {String} status Description of response status
      * @param {jqXHR} xhr jQuery Ajax object
      */
     Notebook.prototype.save_notebook_success = function (start, data, status, xhr) {
         this.set_dirty(false);
         $([IPython.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 how long the last save took
      * 
      * @method _update_autosave_interval
      * @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);
             }
         }
     };
     
     /**
      * Failure callback for saving a notebook.
      * 
      * @method save_notebook_error
      * @param {jqXHR} xhr jQuery Ajax object
      * @param {String} status Description of response status
      * @param {String} error HTTP error message
      */
     Notebook.prototype.save_notebook_error = function (xhr, status, error) {
         $([IPython.events]).trigger('notebook_save_failed.Notebook', [xhr, status, error]);
     };
 
     Notebook.prototype.new_notebook = function(){
         var path = this.notebook_path;
         var base_project_url = this._baseProjectUrl;
         var settings = {
             processData : false,
             cache : false,
             type : "POST",
             dataType : "json",
             async : false,
             success : function (data, status, xhr){
                 var notebook_name = data.name;
                 window.open(
                     utils.url_join_encode(
                         base_project_url,
                         'notebooks',
                         path,
                         notebook_name
                     ),
                     '_blank'
                 );
             }
         };
         var url = utils.url_join_encode(
             base_project_url,
             'api/notebooks',
             path
         );
         $.ajax(url,settings);
     };
 
 
     Notebook.prototype.copy_notebook = function(){
         var path = this.notebook_path;
         var base_project_url = this._baseProjectUrl;
         var settings = {
             processData : false,
             cache : false,
             type : "POST",
             dataType : "json",
             data : JSON.stringify({copy_from : this.notebook_name}),
             async : false,
             success : function (data, status, xhr) {
                 window.open(utils.url_join_encode(
                     base_project_url,
                     'notebooks',
                     data.path,
                     data.name
                 ), '_blank');
             }
         };
         var url = utils.url_join_encode(
             base_project_url,
             'api/notebooks',
             path
         );
         $.ajax(url,settings);
     };
 
     Notebook.prototype.rename = function (nbname) {
         var that = this;
         var data = {name: nbname + '.ipynb'};
         var settings = {
             processData : false,
             cache : false,
             type : "PATCH",
             data : JSON.stringify(data),
             dataType: "json",
             headers : {'Content-Type': 'application/json'},
             success : $.proxy(that.rename_success, this),
             error : $.proxy(that.rename_error, this)
         };
         $([IPython.events]).trigger('rename_notebook.Notebook', data);
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebook_path,
             this.notebook_name
         );
         $.ajax(url, settings);
     };
     
     
     Notebook.prototype.rename_success = function (json, status, xhr) {
         this.notebook_name = json.name;
         var name = this.notebook_name;
         var path = json.path;
         this.session.rename_notebook(name, path);
         $([IPython.events]).trigger('notebook_renamed.Notebook', json);
     }
 
     Notebook.prototype.rename_error = function (xhr, status, error) {
         var that = this;
         var dialog = $('<div/>').append(
             $("<p/>").addClass("rename-message")
             .html('This notebook name already exists.')
         )
         $([IPython.events]).trigger('notebook_rename_failed.Notebook', [xhr, status, error]);
         IPython.dialog.modal({
             title: "Notebook Rename Error!",
             body: dialog,
             buttons : {
                 "Cancel": {},
                 "OK": {
                     class: "btn-primary",
                     click: function () {
                         IPython.save_widget.rename_notebook();
                 }}
                 },
             open : function (event, ui) {
                 var that = $(this);
                 // Upon ENTER, click the OK button.
                 that.find('input[type="text"]').keydown(function (event, ui) {
                     if (event.which === utils.keycodes.ENTER) {
                         that.find('.btn-primary').first().click();
                     }
                 });
                 that.find('input[type="text"]').focus();
             }
         });
     }
 
     /**
      * Request a notebook's data from the server.
      * 
      * @method load_notebook
      * @param {String} notebook_name and path A notebook to load
      */
     Notebook.prototype.load_notebook = function (notebook_name, notebook_path) {
         var that = this;
         this.notebook_name = notebook_name;
         this.notebook_path = notebook_path;
         // We do the call with settings so we can set cache to false.
         var settings = {
             processData : false,
             cache : false,
             type : "GET",
             dataType : "json",
             success : $.proxy(this.load_notebook_success,this),
             error : $.proxy(this.load_notebook_error,this),
         };
         $([IPython.events]).trigger('notebook_loading.Notebook');
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebook_path,
             this.notebook_name
         );
         $.ajax(url, settings);
     };
 
     /**
      * Success callback for loading a notebook from the server.
      * 
      * Load notebook data from the JSON response.
      * 
      * @method load_notebook_success
      * @param {Object} data JSON representation of a notebook
      * @param {String} status Description of response status
      * @param {jqXHR} xhr jQuery Ajax object
      */
     Notebook.prototype.load_notebook_success = function (data, status, xhr) {
         this.fromJSON(data);
         if (this.ncells() === 0) {
             this.insert_cell_below('code');
             this.select(0);
             this.edit_mode();
         } else {
             this.select(0);
             this.command_mode();
         };
         this.set_dirty(false);
         this.scroll_to_top();
         if (data.orig_nbformat !== undefined && data.nbformat !== data.orig_nbformat) {
             var msg = "This notebook has been converted from an older " +
             "notebook format (v"+data.orig_nbformat+") to the current notebook " +
             "format (v"+data.nbformat+"). The next time you save this notebook, the " +
             "newer notebook format will be used and older versions of IPython " +
             "may not be able to read it. To keep the older version, close the " +
             "notebook without saving it.";
             IPython.dialog.modal({
                 title : "Notebook converted",
                 body : msg,
                 buttons : {
                     OK : {
                         class : "btn-primary"
                     }
                 }
             });
         } else if (data.orig_nbformat_minor !== undefined && data.nbformat_minor !== data.orig_nbformat_minor) {
             var that = this;
             var orig_vs = 'v' + data.nbformat + '.' + data.orig_nbformat_minor;
             var this_vs = 'v' + data.nbformat + '.' + this.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 some features " +
             "introduced in later notebook versions may not be available."
 
             IPython.dialog.modal({
                 title : "Newer Notebook",
                 body : msg,
                 buttons : {
                     OK : {
                         class : "btn-danger"
                     }
                 }
             });
 
         }
         
         // Create the session after the notebook is completely loaded to prevent
         // code execution upon loading, which is a security risk.
         if (this.session == null) {
             this.start_session();
         }
         // load our checkpoint list
         this.list_checkpoints();
         
         // load toolbar state
         if (this.metadata.celltoolbar) {
             IPython.CellToolbar.global_show();
             IPython.CellToolbar.activate_preset(this.metadata.celltoolbar);
         }
         
         $([IPython.events]).trigger('notebook_loaded.Notebook');
     };
 
     /**
      * Failure callback for loading a notebook from the server.
      * 
      * @method load_notebook_error
      * @param {jqXHR} xhr jQuery Ajax object
      * @param {String} status Description of response status
      * @param {String} error HTTP error message
      */
     Notebook.prototype.load_notebook_error = function (xhr, status, error) {
         $([IPython.events]).trigger('notebook_load_failed.Notebook', [xhr, status, error]);
         if (xhr.status === 400) {
             var msg = error;
         } else if (xhr.status === 500) {
             var msg = "An unknown error occurred while loading this notebook. " +
             "This version can load notebook formats " +
             "v" + this.nbformat + " or earlier.";
         }
         IPython.dialog.modal({
             title: "Error loading notebook",
             body : msg,
             buttons : {
                 "OK": {}
             }
         });
     }
 
     /*********************  checkpoint-related  *********************/
     
     /**
      * Save the notebook then immediately create a checkpoint.
      * 
      * @method save_checkpoint
      */
     Notebook.prototype.save_checkpoint = function () {
         this._checkpoint_after_save = true;
         this.save_notebook();
     };
     
     /**
      * Add a checkpoint for this notebook.
      * for use as a callback from checkpoint creation.
      * 
      * @method add_checkpoint
      */
     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.
      * 
      * @method list_checkpoints
      */
     Notebook.prototype.list_checkpoints = function () {
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebook_path,
             this.notebook_name,
             'checkpoints'
         );
         $.get(url).done(
             $.proxy(this.list_checkpoints_success, this)
         ).fail(
             $.proxy(this.list_checkpoints_error, this)
         );
     };
 
     /**
      * Success callback for listing checkpoints.
      * 
      * @method list_checkpoint_success
      * @param {Object} data JSON representation of a checkpoint
      * @param {String} status Description of response status
      * @param {jqXHR} xhr jQuery Ajax object
      */
     Notebook.prototype.list_checkpoints_success = function (data, status, xhr) {
         var data = $.parseJSON(data);
         this.checkpoints = data;
         if (data.length) {
             this.last_checkpoint = data[data.length - 1];
         } else {
             this.last_checkpoint = null;
         }
         $([IPython.events]).trigger('checkpoints_listed.Notebook', [data]);
     };
 
     /**
      * Failure callback for listing a checkpoint.
      * 
      * @method list_checkpoint_error
      * @param {jqXHR} xhr jQuery Ajax object
      * @param {String} status Description of response status
      * @param {String} error_msg HTTP error message
      */
     Notebook.prototype.list_checkpoints_error = function (xhr, status, error_msg) {
         $([IPython.events]).trigger('list_checkpoints_failed.Notebook');
     };
     
     /**
      * Create a checkpoint of this notebook on the server from the most recent save.
      * 
      * @method create_checkpoint
      */
     Notebook.prototype.create_checkpoint = function () {
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebookPath(),
             this.notebook_name,
             'checkpoints'
         );
         $.post(url).done(
             $.proxy(this.create_checkpoint_success, this)
         ).fail(
             $.proxy(this.create_checkpoint_error, this)
         );
     };
 
     /**
      * Success callback for creating a checkpoint.
      * 
      * @method create_checkpoint_success
      * @param {Object} data JSON representation of a checkpoint
      * @param {String} status Description of response status
      * @param {jqXHR} xhr jQuery Ajax object
      */
     Notebook.prototype.create_checkpoint_success = function (data, status, xhr) {
         var data = $.parseJSON(data);
         this.add_checkpoint(data);
         $([IPython.events]).trigger('checkpoint_created.Notebook', data);
     };
 
     /**
      * Failure callback for creating a checkpoint.
      * 
      * @method create_checkpoint_error
      * @param {jqXHR} xhr jQuery Ajax object
      * @param {String} status Description of response status
      * @param {String} error_msg HTTP error message
      */
     Notebook.prototype.create_checkpoint_error = function (xhr, status, error_msg) {
         $([IPython.events]).trigger('checkpoint_failed.Notebook');
     };
     
     Notebook.prototype.restore_checkpoint_dialog = function (checkpoint) {
         var that = this;
         var 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(
                 Date(checkpoint.last_modified)
             ).css("text-align", "center")
         );
         
         IPython.dialog.modal({
             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.
      * 
      * @method restore_checkpoint
      * @param {String} checkpoint ID
      */
     Notebook.prototype.restore_checkpoint = function (checkpoint) {
         $([IPython.events]).trigger('notebook_restoring.Notebook', checkpoint);
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebookPath(),
             this.notebook_name,
             'checkpoints',
             checkpoint
         );
         $.post(url).done(
             $.proxy(this.restore_checkpoint_success, this)
         ).fail(
             $.proxy(this.restore_checkpoint_error, this)
         );
     };
     
     /**
      * Success callback for restoring a notebook to a checkpoint.
      * 
      * @method restore_checkpoint_success
      * @param {Object} data (ignored, should be empty)
      * @param {String} status Description of response status
      * @param {jqXHR} xhr jQuery Ajax object
      */
     Notebook.prototype.restore_checkpoint_success = function (data, status, xhr) {
         $([IPython.events]).trigger('checkpoint_restored.Notebook');
         this.load_notebook(this.notebook_name, this.notebook_path);
     };
 
     /**
      * Failure callback for restoring a notebook to a checkpoint.
      * 
      * @method restore_checkpoint_error
      * @param {jqXHR} xhr jQuery Ajax object
      * @param {String} status Description of response status
      * @param {String} error_msg HTTP error message
      */
     Notebook.prototype.restore_checkpoint_error = function (xhr, status, error_msg) {
         $([IPython.events]).trigger('checkpoint_restore_failed.Notebook');
     };
     
     /**
      * Delete a notebook checkpoint.
      * 
      * @method delete_checkpoint
      * @param {String} checkpoint ID
      */
     Notebook.prototype.delete_checkpoint = function (checkpoint) {
         $([IPython.events]).trigger('notebook_restoring.Notebook', checkpoint);
         var url = utils.url_join_encode(
             this._baseProjectUrl,
             'api/notebooks',
             this.notebookPath(),
             this.notebook_name,
             'checkpoints',
             checkpoint
         );
         $.ajax(url, {
             type: 'DELETE',
             success: $.proxy(this.delete_checkpoint_success, this),
             error: $.proxy(this.delete_notebook_error,this)
         });
     };
     
     /**
      * Success callback for deleting a notebook checkpoint
      * 
      * @method delete_checkpoint_success
      * @param {Object} data (ignored, should be empty)
      * @param {String} status Description of response status
      * @param {jqXHR} xhr jQuery Ajax object
      */
     Notebook.prototype.delete_checkpoint_success = function (data, status, xhr) {
         $([IPython.events]).trigger('checkpoint_deleted.Notebook', data);
         this.load_notebook(this.notebook_name, this.notebook_path);
     };
 
     /**
      * Failure callback for deleting a notebook checkpoint.
      * 
      * @method delete_checkpoint_error
      * @param {jqXHR} xhr jQuery Ajax object
      * @param {String} status Description of response status
      * @param {String} error_msg HTTP error message
      */
     Notebook.prototype.delete_checkpoint_error = function (xhr, status, error_msg) {
         $([IPython.events]).trigger('checkpoint_delete_failed.Notebook');
     };
 
 
     IPython.Notebook = Notebook;
 
 
     return IPython;
 
 }(IPython));