pretty print list too slow

[rwst]

It seems ipython's behaviour is very different if I say:

print range(10**6)       # case 1, fraction of a second, output in ONE line

range(10**6)             # case 2, ten seconds, 10^6 lines

and the difference is not due to the actual output, but presumably the formatting of the one-value-per-line output.

As this is a factor of about 10x (not counting the actual output), and growing with list size, there appears to be a problem because this is unexpected by the user. It was only through debugging, then web search, and a stackoverflow comment that I found the %pprint magic which is nowhere documented. So even if you dismiss this as bug or enhancement, this ticket should add helpful documentation.

[rwst]

Blocks http://trac.sagemath.org/ticket/15912

[minrk]

I think we could cap the size for pretty printing of lists, dicts, etc. easily enough.

[jankatins]

See also here: #2747 (also "one value per line", one print per loop)

[minrk]

While its symptoms are similar, #2747 is actually unrelated. Here, the issue is in IPython.lib.pretty formatting the output on the kernel (see #5376 for a proposed fix). #2747 is a performance issue in the notebook UI's javascript, when it is handling the many messages produced by print (or more precisely stdout.flush).