Quick fix for failing tests. This will be cleaned up soon.

This is only a quick fix to make the tests pass. Note that some tests
which access the 'prompt_number' attribute were not affected by the
additional unexecuted cell in tests/test.ipynb which was added in
commit ec19b407dee30fb7e692ce753af9fc5bd3808f8f. So this probably
indicates that not all corner cases are covered.

converters/python.py

@@ -33,16 +33,21 b' class ConverterPy(Converter):'
         return ['#{0} {1}'.format('#'*cell.level, cell.source), '']
 
     def render_code(self, cell):
+        try:
+            prompt_number = cell.prompt_number
+        except AttributeError:
+            prompt_number = " "
+
         if not cell.input:
             return []
         lines = []
         if self.show_prompts:
-            lines.extend(['# In[%s]:' % cell.prompt_number])
+            lines.extend(['# In[%s]:' % prompt_number])
         src = cell.input
         lines.extend([src, ''])
         if self.show_output:
             if cell.outputs :
-                lines.extend(['# Out[%s]:' % cell.prompt_number])
+                lines.extend(['# Out[%s]:' % prompt_number])
             for output in cell.outputs:
                 conv_fn = self.dispatch(output.output_type)
                 lines.extend(conv_fn(output))

converters/rst.py

@@ -11,10 +11,16 b' class ConverterRST(Converter):'
         return ['{0}\n{1}\n'.format(cell.source, marker * len(cell.source))]
 
     def render_code(self, cell):
+        # Note: cell has type 'IPython.nbformat.v3.nbbase.NotebookNode'
         if not cell.input:
             return []
 
-        lines = ['In[%s]:' % cell.prompt_number, '']
+        try:
+            prompt_number = cell.prompt_number
+        except AttributeError:
+            prompt_number = " "
+
+        lines = ['In[%s]:' % prompt_number, '']
         lines.extend(rst_directive('.. code:: python', cell.input))
 
         for output in cell.outputs: