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Pylab banner shows up with first cell to execute #1022

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takluyver opened this Issue November 20, 2011 · 8 comments

3 participants

Thomas Kluyver Fernando Perez Min RK
Thomas Kluyver
Collaborator

If the notebook server is started with --pylab=inline, the first cell to be run shows the pylab banner ("Welcome to pylab...").

Fernando Perez
Owner

yes, that's been bugging me lately. It's ugly, but we need to come up with a clean solution in general. The problem is that it's not really in the banner, it's a print statement made by the pylab code. Since the first time stdout gets flushed is after the first execution, then we get it out at that point, which looks weird. For example, if you open a qtconsole with --existing before executing anything in the nb, then the console gets the printout and it doesn't show up on the nb.

So the first problem is: where pylab shows its backend info. Since that code gets executed after the banner has been built and the shell initialized, it can't be part of the banner itself (which is built at construction time). In fact, %pylab can do that at any point later in the life of the kernel. I don't see any sensible solution other than leaving it be as a print statement, since if it's done post-init, we do want to give feedback to the user.

The question then becomes, where do we show output that may have come from the kernel at startup? I'm starting to think we need a little 'log area' that's very unobtrusive (perhaps even off by default) where this output is flushed. Most of the time nobody needs to see that, and it's annoying to pollute the notebook area with initialization information all the time. In my view, the notebook should only contain output from actions typed by the user in the notebook itself.

Perhaps for now we could just run an initial flush of the streams and store that output in a variable. We can then offer a button (later menu entry or better UI element) for the user to see that output in a popup. Most people will never even bother looking for it.

@minrk, do you know the init logic of the output streams well enough to run that flush at startup and grab any output that might be in stdout/err to get it out of the way?

ps - I'll make this high priority for 0.12 (but not blocker) in case we manage to get to it; it's really an ugly wart that takes away from the experience and I'd like to come out with a really clean and polished notebook out of the gate.

Min RK
Owner

Fix is one line: add sys.stdout.flush() after the print statement in pylabtools.pylab_activate, and it won't show up in the notebook.

Min RK
Owner

Or, to be a bit more general, a sys.stdout.flush() should be added to the very end of the startup process in IPKernelApp, so that any print statements made during startup have been sent prior to the first execution.

An alternative would be to add stderr/stdout.flush to the beginning of the execute block, to prevent any lingering messages in the buffer from being associated with a later execution. Under normal circumstances, this would be entirely superfluous after the very first call, which is the only point actually likely to have anything printed but not yet flushed.

Fernando Perez
Owner

@minrk: I actually agree with both approaches: things like pylab_activate should probably flush right away just to be safe, and we should also have a flush right at the end of the IPKernelApp startup, to hand off things in a clean state. I can look into the code tomorrow, but if you have a minute just push those small changes up right away. I'd rather not add flushing on every block execute at the start; since we do flush at the end, it's superfluous on all but the first.

Min RK
Owner

It's actually a tiny bit more subtle than that, because it turns out the qtconsole receiving the pylab message depended on this bug. I'll do a PR with a first crack. It should still be tiny, but warrants a quick second opinion since there isn't one unambiguous right answer.

Fernando Perez
Owner
Min RK minrk referenced this issue from a commit in minrk/ipython November 21, 2011
Min RK flush messages printed during startup
Prevents print statements during init methods from being associated with the first execution.

closes #1022
f967531
Min RK
Owner

PR #1023 issued, fixing this and clarifying the difference in behavior between Qt and notebook wrt init messages.

Thomas Kluyver takluyver closed this November 24, 2011
Thomas Kluyver
Collaborator

Closed by PR #1023.

Brian E. Granger ellisonbg referenced this issue from a commit January 10, 2012
Commit has since been removed from the repository and is no longer available.
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