Improved error reporting on comms

holoviews/plotting/bokeh/callbacks.py

@@ -79,6 +79,9 @@ class Callback(object):
           var comm = HoloViewsWidget.comms["{comms_target}"];
           var comm_state = HoloViewsWidget.comm_state["{comms_target}"];
           if (msg.msg_type == "Ready") {{
+            if (msg.content) {{
+              console.log("Python callback returned following output:", msg.content);
+            }}
             if (comm_state.event) {{
               comm.send(comm_state.event);
             }} else {{

holoviews/plotting/comms.py

@@ -1,10 +1,38 @@
 import json
 import uuid
+import sys
+import os
+import traceback
+try:
+    from StringIO import StringIO
+except:
+    from io import StringIO
 
 from ipykernel.comm import Comm as IPyComm
 from IPython import get_ipython
 
 
+class StandardOutput(list):
+    """
+    Context manager to capture standard output for any code it
+    is wrapping and make it available as a list, e.g.:
+
+    >>> with StandardOutput() as stdout:
+    ...   print('This gets captured')
+    >>> print(stdout[0])
+    This gets captured
+    """
+
+    def __enter__(self):
+        self._stdout = sys.stdout
+        sys.stdout = self._stringio = StringIO()
+        return self
+
+    def __exit__(self, *args):
+        self.extend(self._stringio.getvalue().splitlines())
+        sys.stdout = self._stdout
+
+
 class Comm(object):
     """
     Comm encompasses any uni- or bi-directional connection between
@@ -70,12 +98,26 @@ def _handle_msg(self, msg):
         if it has been defined.
         """
         try:
+            stdout = []
+            msg = self.decode(msg)
             if self._on_msg:
-                self._on_msg(self.decode(msg))
+                # Comm swallows standard output so we need to capture
+                # it and then send it to the frontend
+                with StandardOutput() as stdout:
+                    self._on_msg(msg)
         except Exception as e:
-            msg = {'msg_type': "Error", 'traceback': str(e)}
+            frame =traceback.extract_tb(sys.exc_info()[2])[-2]
+            fname,lineno,fn,text = frame
+            error_kwargs = dict(type=type(e).__name__, fn=fn, fname=fname,
+                                line=lineno, error=str(e))
+            error = '{fname} {fn} L{line}\n\t{type}: {error}'.format(**error_kwargs)
+            if stdout:
+                stdout = '\n\t'+'\n\t'.join(stdout)
+                error = '\n'.join([stdout, error])
+            msg = {'msg_type': "Error", 'traceback': error}
         else:
-            msg = {'msg_type': "Ready"}
+            stdout = '\n\t'+'\n\t'.join(stdout) if stdout else ''
+            msg = {'msg_type': "Ready", 'content': stdout}
         self.comm.send(json.dumps(msg))