repr_html exception warning on qtconsole with pandas #4745

[ghost]

#4745 - warn on failed formatter calls

since qtconsole html output is limited, pandas choses to provide an
html repr on ipnb only and raise an exception from _repr_html_ when in qtconsole to fallback
to a text repr. This used to work fine, but #4745 now results in a warning message constantly
being emitted about the exception being raised.

@minrk, how about supressing a NotImplementedError as a form of contract?

[minrk]

There is already an official way to communicate no output - returning None. We can add support for NotImplementedError (and/or NotImplemented), but it would be redundant.

[minrk]

#4832 should prevent warning on NotImplementedError

[ghost]

Thanks.

I was not aware of None being a signal, tested it now and it works just fine.
I'll modify pandas to return None, but honoring NotImplementedError as well
makes sense to me.

Until now we raised ValueError which normally should cause a warning, so now
all previous versions of pandas will emit one at every step.

IPython.utils.warn does not actually delegate to the python warning module, so it's
not possible to supress these in a standard way. Is there an IPython way we can
suggest to users?

[minrk]

utils.warn is little more than a shortcut for sys.stderr, so there's no good way to suppress it. I've switched it to a real warning in #4832, so you should be able to suppress IPython.core.formatters.FormatterWarning.

[ghost]

#4832 tested and resolves the issue, I'll add a mention in the pandas docs. Thanks for the quick fix.