Add cell error instruction configurable (voila-dashboards#471)

* add cell error instruction configurable in VoilaExecutePreprocessor

voila/execute.py

@@ -13,27 +13,9 @@
 from nbconvert.preprocessors.execute import CellExecutionError, ExecutePreprocessor
 from nbformat.v4 import output_from_msg
 
-from ipykernel.jsonutil import json_clean
-
-
-def strip_code_cell_errors(cell):
-    """Strip any error outputs and traceback from a code cell."""
-    # There is no 'outputs' key for markdown cells
-    if 'outputs' not in cell:
-        return cell
-
-    outputs = cell['outputs']
+from traitlets import Unicode
 
-    error_outputs = [output for output in outputs if output['output_type'] == 'error']
-
-    error_message = 'There was an error when executing cell [{}]. Please run Voila with --debug to see the error message.'.format(cell['execution_count'])
-
-    for output in error_outputs:
-        output['ename'] = 'ExecutionError'
-        output['evalue'] = 'Execution error'
-        output['traceback'] = [error_message]
-
-    return cell
+from ipykernel.jsonutil import json_clean
 
 
 def strip_code_cell_warnings(cell):
@@ -52,19 +34,6 @@ def strip_code_cell_warnings(cell):
     return cell
 
 
-def strip_notebook_errors(nb):
-    """Strip error messages and traceback from a Notebook."""
-    cells = nb['cells']
-
-    code_cells = [cell for cell in cells if cell['cell_type'] == 'code']
-
-    for cell in code_cells:
-        strip_code_cell_warnings(cell)
-        strip_code_cell_errors(cell)
-
-    return nb
-
-
 def should_strip_error(config):
     """Return True if errors should be stripped from the Notebook, False otherwise, depending on the current config."""
     return 'Voila' not in config or 'log_level' not in config['Voila'] or config['Voila']['log_level'] != logging.DEBUG
@@ -146,6 +115,14 @@ def set_state(self, state):
 
 class VoilaExecutePreprocessor(ExecutePreprocessor):
     """Execute, but respect the output widget behaviour"""
+    cell_error_instruction = Unicode(
+        'Please run Voila with --debug to see the error message.',
+        config=True,
+        help=(
+            'instruction given to user to debug cell errors'
+        )
+    )
+
     def __init__(self, **kwargs):
         super(VoilaExecutePreprocessor, self).__init__(**kwargs)
         self.output_hook_stack = collections.defaultdict(list)  # maps to list of hooks, where the last is used
@@ -160,7 +137,7 @@ def preprocess(self, nb, resources, km=None):
 
         # Strip errors and traceback if not in debug mode
         if should_strip_error(self.config):
-            strip_notebook_errors(nb)
+            self.strip_notebook_errors(nb)
 
         return result
 
@@ -176,7 +153,7 @@ def preprocess_cell(self, cell, resources, cell_index, store_history=True):
         # Strip errors and traceback if not in debug mode
         if should_strip_error(self.config):
             strip_code_cell_warnings(cell)
-            strip_code_cell_errors(cell)
+            self.strip_code_cell_errors(cell)
 
         return result
 
@@ -226,6 +203,36 @@ def clear_output(self, outs, msg, cell_index):
             return
         super(VoilaExecutePreprocessor, self).clear_output(outs, msg, cell_index)
 
+    def strip_notebook_errors(self, nb):
+        """Strip error messages and traceback from a Notebook."""
+        cells = nb['cells']
+
+        code_cells = [cell for cell in cells if cell['cell_type'] == 'code']
+
+        for cell in code_cells:
+            strip_code_cell_warnings(cell)
+            self.strip_code_cell_errors(cell)
+
+        return nb
+
+    def strip_code_cell_errors(self, cell):
+        """Strip any error outputs and traceback from a code cell."""
+        # There is no 'outputs' key for markdown cells
+        if 'outputs' not in cell:
+            return cell
+
+        outputs = cell['outputs']
+
+        error_outputs = [output for output in outputs if output['output_type'] == 'error']
+
+        error_message = 'There was an error when executing cell [{}]. {}'.format(cell['execution_count'], self.cell_error_instruction)
+
+        for output in error_outputs:
+            output['ename'] = 'ExecutionError'
+            output['evalue'] = 'Execution error'
+            output['traceback'] = [error_message]
+
+        return cell
 
 def executenb(nb, cwd=None, km=None, **kwargs):
     resources = {}