add nbformat.output_from_msg

for creating the appropriate NotebookNode from
an output message on the IOPub channel.

IPython/nbconvert/preprocessors/execute.py

@@ -13,7 +13,7 b' except ImportError:'
 
 from IPython.utils.traitlets import List, Unicode
 
-from IPython.nbformat.current import reads, writes, new_output
+from IPython.nbformat.current import reads, writes, output_from_msg
 from .base import Preprocessor
 from IPython.utils.traitlets import Integer
 
@@ -96,39 +96,19 b' class ExecutePreprocessor(Preprocessor):'
                     break
                 else:
                     continue
-            elif msg_type in {'execute_input'}:
+            elif msg_type == 'execute_input':
                 continue
             elif msg_type == 'clear_output':
                 outs = []
                 continue
-
-            # set the prompt number for the input and the output
-            if msg_type == 'execute_result':
+            elif msg_type == 'execute_result':
                 cell['prompt_number'] = content['execution_count']
-                out = new_output(output_type=msg_type,
-                    metadata=content['metadata'],
-                    mime_bundle=content['data'],
-                    prompt_number=content['execution_count'],
-                )
 
-            elif msg_type == 'stream':
-                out = new_output(output_type=msg_type,
-                    name=content['name'],
-                    data=content['data'],
-                )
-            elif msg_type == 'display_data':
-                out = new_output(output_type=msg_type,
-                    metadata=content['metadata'],
-                    mime_bundle=content['data'],
-                )
-            elif msg_type == 'error':
-                out = new_output(output_type=msg_type,
-                    ename=content['ename'],
-                    evalue=content['evalue'],
-                    traceback=content['traceback'],
-                )
-            else:
+            try:
+                out = output_from_msg(msg)
+            except ValueError:
                 self.log.error("unhandled iopub msg: " + msg_type)
+            else:
+                outs.append(out)
 
-            outs.append(out)
         return outs

IPython/nbformat/v4/__init__.py

@@ -7,7 +7,7 b' from .nbbase import ('
     NotebookNode, from_dict,
     nbformat, nbformat_minor, nbformat_schema,
     new_code_cell, new_heading_cell, new_markdown_cell, new_notebook,
-    new_output,
+    new_output, output_from_msg,
 )
 
 from .nbjson import reads as reads_json, writes as writes_json

IPython/nbformat/v4/nbbase.py

@@ -16,11 +16,13 b' nbformat = 4'
 nbformat_minor = 0
 nbformat_schema = 'nbformat.v4.schema.json'
 
+
 def validate(node, ref=None):
     """validate a v4 node"""
     from ..current import validate
     return validate(node, ref=ref, version=nbformat)
 
+
 class NotebookNode(Struct):
     pass
 
@@ -50,6 +52,51 b' def new_output(output_type, mime_bundle=None, **kwargs):'
     validate(output, output_type)
     return output
 
+
+def output_from_msg(msg):
+    """Create a NotebookNode for an output from a kernel's IOPub message.
+
+    Returns
+    -------
+
+    NotebookNode: the output as a notebook node.
+
+    Raises
+    ------
+
+    ValueError: if the message is not an output message.
+
+    """
+    msg_type = msg['header']['msg_type']
+    content = msg['content']
+
+    if msg_type == 'execute_result':
+        return new_output(output_type=msg_type,
+            metadata=content['metadata'],
+            mime_bundle=content['data'],
+            prompt_number=content['execution_count'],
+        )
+
+    elif msg_type == 'stream':
+        return new_output(output_type=msg_type,
+            name=content['name'],
+            data=content['data'],
+        )
+    elif msg_type == 'display_data':
+        return new_output(output_type=msg_type,
+            metadata=content['metadata'],
+            mime_bundle=content['data'],
+        )
+    elif msg_type == 'error':
+        return new_output(output_type=msg_type,
+            ename=content['ename'],
+            evalue=content['evalue'],
+            traceback=content['traceback'],
+        )
+    else:
+        raise ValueError("Unrecognized output msg type: %r" % msg_type)
+
+
 def new_code_cell(source='', **kwargs):
     """Create a new code cell"""
     cell = NotebookNode(cell_type='code', source=source)