Catch AttributeErrors stemming from ipython_genutils as ValidationErr…

…ors on read (#241)

* Catch AttributeErrors thrown by ipython_genutils during reading and conversion as ValidationErrors

* fix docstrings

* catch attribute errors as validation errors while reading json

* revert changes to converter (in another PR)

nbformat/reader.py

@@ -4,6 +4,7 @@
 # Distributed under the terms of the Modified BSD License.
 
 import json
+from .validator import ValidationError
 
 class NotJSONError(ValueError):
     pass
@@ -52,13 +53,24 @@ def reads(s, **kwargs):
     -------
     nb : NotebookNode
         The notebook that was read.
+    
+    Raises
+    ------
+    ValidationError 
+        Notebook JSON for a given version is missing an expected key and cannot be read.
+    
+    NBFormatError
+        Specified major version is invalid or unsupported.
     """
     from . import versions, NBFormatError
 
     nb_dict = parse_json(s, **kwargs)
     (major, minor) = get_version(nb_dict)
     if major in versions:
-        return versions[major].to_notebook_json(nb_dict, minor=minor)
+        try:
+            return versions[major].to_notebook_json(nb_dict, minor=minor)
+        except AttributeError as e:
+            raise ValidationError(f"The notebook is invalid and is missing an expected key: {e}")
     else:
         raise NBFormatError('Unsupported nbformat version %s' % major)
 
@@ -80,3 +92,4 @@ def read(fp, **kwargs):
         The notebook that was read.
     """
     return reads(fp.read(), **kwargs)
+

nbformat/tests/test3_no_worksheets.ipynb

@@ -0,0 +1,7 @@
+{
+ "metadata": {
+  "name": ""
+ },
+ "nbformat": 3,
+ "nbformat_minor": 0
+}

nbformat/tests/test3_worksheet_with_no_cells.ipynb

@@ -0,0 +1,12 @@
+{
+ "metadata": {
+  "name": ""
+ },
+ "nbformat": 3,
+ "nbformat_minor": 0,
+ "worksheets": [
+  {
+   "metadata": {}
+  }
+ ]
+}

nbformat/tests/test_reader.py

@@ -15,6 +15,7 @@
 from .base import TestsBase
 
 from ..reader import read, get_version
+from ..validator import ValidationError
 
 #-----------------------------------------------------------------------------
 # Classes and functions
@@ -36,3 +37,14 @@ def test_read(self):
             nb = read(f)
         (major, minor) = get_version(nb)
         self.assertEqual(major, 2)
+
+    def test_read_fails_on_missing_worksheets(self):
+        with self.fopen(u'test3_no_worksheets.ipynb', u'r') as f:
+            with self.assertRaisesRegex(ValidationError, r'worksheets'):
+                nb = read(f)
+
+    def test_read_fails_on_missing_worksheet_cells(self):
+        with self.fopen(u'test3_worksheet_with_no_cells.ipynb', u'r') as f:
+            with self.assertRaisesRegex(ValidationError, r'cells'):
+                nb = read(f)
+