Informative repr for streamz when ipywidgets is not installed

[mrocklin]

Currently if the user does not have ipywidgets installed then running something like the following in a notebook produces no output

import streamz.dataframe
df = streamz.dataframe.Random()
df.tail()

It might be nicer to include a message encouraging users to install ipywidgets in this case.

[martindurant]

Good point. This should be easy to fix.
Do you happen to have a convenient mock/fixture for having a package in the env raise ImportError within a test?

[mrocklin]

I do not personally, no. One way to solve this would be to install ipywidgets in some CI runs but not in others or to have a minimal install run. Then one would have a test that checked that "install ipywidgets" in df.__ipywidget_repr__ (I forget the exact protocol there) if ipywidgets is not installed.

That may be too much work for this case though