Initial draft of repr_data message type.

docs/source/development/messaging.txt

@@ -700,6 +700,49 b" socket with the names 'stdin' and 'stdin_reply'. This will allow other clients"
 to monitor/display kernel interactions and possibly replay them to their user
 or otherwise expose them.
 
+Representation Data
+-------------------
+
+This type of message is used to bring back representations (text, html, svg,
+etc.) of Python objects to the frontend. Each message can have multiple
+representations of the object; it is up to the frontend to decide which to use
+and how. A single message should contain the different representations of a
+single Python object. Each representation should be a JSON'able data structure,
+and should be a valid MIME type.
+
+Some questions remain about this design:
+
+* Do we use this message type for pyout/displayhook?
+* What is the best way to organize the content dict of the message?
+
+Message type: ``repr_data``::
+
+    # Option 1: if we only allow a single source.
+    content = {
+        'source' : str  # Who create the data
+        'data' : dict  # {'mimetype1' : data1, 'mimetype2' : data2}
+        'metadata' : dict  # Any metadata that describes the data
+    }
+
+    # Option 2: allowing for a different source for each representation,
+    but not keyed by anything.
+    content = {
+        'data' = [(source, type, data), (source, type, data)]
+        'metadata' = dict
+    }
+
+    # Option 3: like option 2, but keyed by the MIME types.
+    content = {
+        'data' = {'mimetype1' : (source, data), 'mimetype2' : (source, data)}
+        'metadata' = dict
+    }
+
+    # Option 4: like option 2, but keyed by the source.
+    content = {
+        'data' = {'source' : (mimetype, data), 'source' : (mimetype, data)}
+        'metadata' = dict
+    }
+
 Python inputs
 -------------
 
@@ -708,8 +751,7 b' These messages are the re-broadcast of the ``execute_request``.'
 Message type: ``pyin``::
 
     content = {
-        # Source code to be executed, one or more lines
-    'code' : str
+        'code' : str  # Source code to be executed, one or more lines
     }
 
 Python outputs