%%timeit should warn of empty contents

[takluyver]

I accidentally used %%timeit when I meant to use %timeit in the notebook (tab completion makes this all too easy). This means that the expression I thought I was timing was being treated as the setup expression, and it was actually looping over an empty block. I thought those speedups were too good to be true.

For %%timeit, and perhaps for line+cell magics in general, it would be good to get some warning on an empty main block, in case the user meant to use the line version.

[rlmv]

It looks like this can't be a general line+cell feature without changing the specification for %%prun, since %prun and %%prun with an empty cell produce the same output.

That said, %%timeit and %%system are the only other line+cell magics (I'm not missing any, am I?), so it shouldn't be a problem to implement separate warnings for each.

[jankatins]

I can have a go at the timeit case as part of #2855

[rlmv]

Oh, sorry @JanSchulz - already stuck it in there.

[rlmv]

I don't have tests for these since there seems to be a problem with running cell magics in docstring tests. Opening an issue for that...

[takluyver]

Closed by PR #2874.