[IPython-dev] A top-level metadata attribute to all messages

[Jason Grout]

Min and I have been discussing on 
https://github.com/ipython/ipython/pull/2051 a way to have metadata 
attributes on all messages.  Min has a great comment summarizing at 
least 3 ways this could be done [1]:

1. metadata in the header (possibly via subheader). I don't like this, 
because I don't think most library code should be using headers for 
anything. The content should be everything you need to know about the 
message itself, and the headers should only be used for low-level 
routing of handlers. But this is exactly what I did in the parallel code 
for message introspection without unpacking content.

2. metadata as a fourth top-level component of all messages. On some 
level, this is the cleanest, but most handlers really only need the 
'content' of the message after the Application has used the headers to 
figure out what handlers to call. So this would mean we are always 
passing the whole Message around, or we are always passing two dicts 
(content and metadata) around.

3. metadata as an optional key in all content dicts. This has the 
benefit of extending what we already have, rather than changing it. But 
we don't actually use the metadata key from displaypub for anything, so 
there's not a high cost to change.

Personally, the elegance of having a top-level metadata attribute, 
separate from the content attribute makes a lot of sense to me.

As for use-cases, in the Sage cell server project, we'd use such 
metadata to determine where to display the results of a message, for 
example (i.e., put this pyout in div 34 on the html page).

The patch on the pull request enables one to set session defaults for 
the subheader, as well as defaults for a 'metadata' attribute for stream 
messages.  But I can change the patch to instead do a session default 
for a top-level metadata attribute easily.

Thanks,

Jason

[1] https://github.com/ipython/ipython/pull/2051#issuecomment-6688471