Proper error handling for nbformat versions in client code.

1. Users are warned when an older nbformat notebook is converted.
2. The notebook notifies the user when a notebook that is too new
   is attempted to load.

To test these things, create a simple notebook. Then change its
nbformat field by hand to 2 (to test 1) and then 4 (to test 2).

IPython/frontend/html/notebook/static/js/kernel.js

@@ -106,8 +106,10 b' var IPython = (function (IPython) {'
             resizable: false,
             modal: true,
             title: "Websocket closed",
+            closeText: "",
+            close: function(event, ui) {$(this).dialog('destroy').remove();},
             buttons : {
-                "Okay": function () {
+                "OK": function () {
                     $(this).dialog('close');
                 }
             }

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

@@ -29,6 +29,7 b' var IPython = (function (IPython) {'
         this.notebook_id = null;
         this.notebook_name = null;
         this.notebook_name_blacklist_re = /[\/\\]/;
+        this.nbformat = 3 // Increment this when changing the nbformat
         this.style();
         this.create_elements();
         this.bind_events();
@@ -1222,7 +1223,7 b' var IPython = (function (IPython) {'
         // We may want to move the name/id/nbformat logic inside toJSON?
         var data = this.toJSON();
         data.metadata.name = this.notebook_name;
-        data.nbformat = 3;
+        data.nbformat = this.nbformat;
         // We do the call with settings so we can set cache to false.
         var settings = {
             processData : false,
@@ -1230,8 +1231,8 b' var IPython = (function (IPython) {'
             type : "PUT",
             data : JSON.stringify(data),
             headers : {'Content-Type': 'application/json'},
-            success : $.proxy(this.notebook_save_success,this),
-            error : $.proxy(this.notebook_save_failed,this)
+            success : $.proxy(this.save_notebook_success,this),
+            error : $.proxy(this.save_notebook_error,this)
         };
         $([IPython.events]).trigger('notebook_saving.Notebook');
         var url = $('body').data('baseProjectUrl') + 'notebooks/' + this.notebook_id;
@@ -1239,13 +1240,13 b' var IPython = (function (IPython) {'
     };
 
 
-    Notebook.prototype.notebook_save_success = function (data, status, xhr) {
+    Notebook.prototype.save_notebook_success = function (data, status, xhr) {
         this.dirty = false;
         $([IPython.events]).trigger('notebook_saved.Notebook');
     };
 
 
-    Notebook.prototype.notebook_save_failed = function (xhr, status, error_msg) {
+    Notebook.prototype.save_notebook_error = function (xhr, status, error_msg) {
         $([IPython.events]).trigger('notebook_save_failed.Notebook');
     };
 
@@ -1259,9 +1260,8 b' var IPython = (function (IPython) {'
             cache : false,
             type : "GET",
             dataType : "json",
-            success : function (data, status, xhr) {
-                that.load_notebook_success(data, status, xhr);
-            }
+            success : $.proxy(this.load_notebook_success,this),
+            error : $.proxy(this.load_notebook_error,this),
         };
         $([IPython.events]).trigger('notebook_loading.Notebook');
         var url = $('body').data('baseProjectUrl') + 'notebooks/' + this.notebook_id;
@@ -1280,9 +1280,59 b' var IPython = (function (IPython) {'
         }
         this.select(0);
         this.scroll_to_top();
+        if (data.orig_nbformat !== undefined && data.nbformat !== data.orig_nbformat) {
+            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 verions of IPython " +
+            "may not be able to read it. To keep the older version, close the " +
+            "notebook without saving it.";
+            var dialog = $('<div/>');
+            dialog.html(msg);
+            this.element.append(dialog);
+            dialog.dialog({
+                resizable: false,
+                modal: true,
+                title: "Notebook converted",
+                closeText: "",
+                close: function(event, ui) {$(this).dialog('destroy').remove();},
+                buttons : {
+                    "OK": function () {
+                        $(this).dialog('close');
+                    }
+                },
+                width: 400
+            });
+        }
         $([IPython.events]).trigger('notebook_loaded.Notebook');
     };
 
+
+    Notebook.prototype.load_notebook_error = function (xhr, textStatus, errorThrow) {
+        if (xhr.status === 500) {
+            msg = "An error occurred while loading this notebook. Most likely " +
+            "this notebook is in a newer format than is supported by this " +
+            "version of IPython. This version can load notebook formats " +
+            "v"+this.nbformat+" or earlier.";
+            var dialog = $('<div/>');
+            dialog.html(msg);
+            this.element.append(dialog);
+            dialog.dialog({
+                resizable: false,
+                modal: true,
+                title: "Error loading notebook",
+                closeText: "",
+                close: function(event, ui) {$(this).dialog('destroy').remove();},
+                buttons : {
+                    "OK": function () {
+                        $(this).dialog('close');
+                    }
+                },
+                width: 400
+            });
+        }
+    }
+    
     IPython.Notebook = Notebook;
 
 

IPython/nbformat/current.py

@@ -28,14 +28,14 b' from IPython.nbformat import v1'
 from IPython.nbformat.v3 import (
     NotebookNode,
     new_code_cell, new_text_cell, new_notebook, new_output, new_worksheet,
-    parse_filename, new_metadata, new_author, new_heading_cell
+    parse_filename, new_metadata, new_author, new_heading_cell, nbformat
 )
 
 #-----------------------------------------------------------------------------
 # Code
 #-----------------------------------------------------------------------------
 
-current_nbformat = 3
+current_nbformat = nbformat
 
 
 class NBFormatError(Exception):
@@ -45,8 +45,8 b' class NBFormatError(Exception):'
 def parse_json(s, **kwargs):
     """Parse a string into a (nbformat, dict) tuple."""
     d = json.loads(s, **kwargs)
-    nbformat = d.get('nbformat',1)
-    return nbformat, d
+    nbf = d.get('nbformat',1)
+    return nbf, d
 
 
 def parse_py(s, **kwargs):
@@ -54,25 +54,25 b' def parse_py(s, **kwargs):'
     pattern = r'# <nbformat>(?P<nbformat>\d+)</nbformat>'
     m = re.search(pattern,s)
     if m is not None:
-        nbformat = int(m.group('nbformat'))
+        nbf = int(m.group('nbformat'))
     else:
-        nbformat = 3
-    return nbformat, s
+        nbf = current_nbformat
+    return nbf, s
 
 
 def reads_json(s, **kwargs):
     """Read a JSON notebook from a string and return the NotebookNode object."""
-    nbformat, d = parse_json(s, **kwargs)
-    if nbformat == 1:
+    nbf, d = parse_json(s, **kwargs)
+    if nbf == 1:
         nb = v1.to_notebook_json(d, **kwargs)
         nb = v3.convert_to_this_nbformat(nb, orig_version=1)
-    elif nbformat == 2:
+    elif nbf == 2:
         nb = v2.to_notebook_json(d, **kwargs)
         nb = v3.convert_to_this_nbformat(nb, orig_version=2)
-    elif nbformat == 3:
+    elif nbf == 3:
         nb = v3.to_notebook_json(d, **kwargs)
     else:
-        raise NBFormatError('Unsupported JSON nbformat version: %i' % nbformat)
+        raise NBFormatError('Unsupported JSON nbformat version: %i' % nbf)
     return nb
 
 
@@ -82,13 +82,13 b' def writes_json(nb, **kwargs):'
 
 def reads_py(s, **kwargs):
     """Read a .py notebook from a string and return the NotebookNode object."""
-    nbformat, s = parse_py(s, **kwargs)
-    if nbformat == 2:
+    nbf, s = parse_py(s, **kwargs)
+    if nbf == 2:
         nb = v2.to_notebook_py(s, **kwargs)
-    elif nbformat == 3:
+    elif nbf == 3:
         nb = v3.to_notebook_py(s, **kwargs)
     else:
-        raise NBFormatError('Unsupported PY nbformat version: %i' % nbformat)
+        raise NBFormatError('Unsupported PY nbformat version: %i' % nbf)
     return nb
 
 

IPython/nbformat/v3/convert.py

@@ -40,6 +40,9 b' def convert_to_this_nbformat(nb, orig_version=2):'
         nb = v2.convert_to_this_nbformat(nb)
         orig_version = 2
     if orig_version == 2:
+        # Mark the original nbformat so consumers know it has been converted.
+        nb.nbformat = 3
+        nb.orig_nbformat = 2
         return nb
     elif orig_version == 3:
         return nb