[IPython-dev] messaging protocol

Brian Granger ellisonbg at gmail.com
Sun Apr 10 02:06:32 EDT 2011

On Fri, Apr 8, 2011 at 2:05 PM, Robert Kern <robert.kern at gmail.com> wrote:
> On 4/8/11 3:32 PM, Fernando Perez wrote:
>> On Fri, Apr 8, 2011 at 1:21 PM, Thomas Kluyver<takowl at gmail.com>  wrote:
>>> In fact, now that we're using AST instead of code blocks, we could actually
>>> do what you suggest. We could check the last node, and only run it
>>> interactively if it was a single expression. Whether or not that's what we
>>> want to do, I don't know: any views?
>> That's a very good point.  The fragile heuristics we had were
>> precisely because we lacked this information.  But I really do like
>> this suggestion, because I think it provides the most intuitive
>> semantics.  Things like:
>> for i in range(10):
>>    plot(foo[i])
>> won't produce 10 different Out[] outputs, and yet any last-block
>> expression, even if it contains some multline string or other complex
>> formatting that makes it be more than one *line of text* will still be
>> executed interactively, yielding just one result.
>> I'm very much +1 on this idea.  Big benefit of your recent tackling
>> inputsplitter!! Awesome.
> I don't think we need to solve this in the splitter. You can do everything you
> need to do in the display trap. The logic is very simple. The display trap sets
> the displayhook before the code In[N] is executed. Every time the displayhook
> gets called during this execution, the display trap records the object and its
> formatted representation, overwriting whatever was there. Once the execution is
> done, *then* the formatted representation is given to the reply message (or
> printed if in the terminal frontend) and the history DB (if it is storing
> outputs), and the object is shoved into Out[N]. Then the display trap is cleared
> before In[N+1] is executed.

This is a good point that is worth investigating.  The only hitch is
that the formatted representation is not sent in the reply to the
original code execution request.  It is published over the PUB socket,
which makes sure that gets sent to all frontends.  This means that the
display hook data can be sent out to all frontends *before* the
execute reply message goes back.  But we might still be able to get
this to work.  It is attractive because it would be this logic out of
the splitter, where it seems a bit out of place.



> This allows code like "x; y = x+1" to put x into Out[N] even though the last AST
> node is not an expression. It's not *entirely* clear to me that this is a real
> use case, but I think it would be less surprising behavior.
> --
> Robert Kern
> "I have come to believe that the whole world is an enigma, a harmless enigma
>  that is made terrible by our own mad attempt to interpret it as though it had
>  an underlying truth."
>   -- Umberto Eco
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Brian E. Granger
Cal Poly State University, San Luis Obispo
bgranger at calpoly.edu and ellisonbg at gmail.com

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