"norun" cell metadata

[stevengj]

It occurred to me that it would be useful to have a norun code-cell metadata flag, that used to:

• Prevent the cell from being executed in the notebook interface when Run All is chosen — you have to run the cell explicitly with shift-enter etc.
• Prevent the cell from being executed when it the notebook is included in another notebook via the %run magic in IPython.
• Prevent the cell from being executed when the notebook is included in other contexts, e.g. in my NBInclude package for including IJulia notebooks in Julia programs.

I have been thinking about literate programming styles based on the notebook format, which is why I made NBInclude so that notebooks can be first-class collections of Julia code (as an alternative/supplement to .jl files). If I'm developing code in a notebook, I might want runnable code examples and tests mixed with the "production" code, but I wouldn't want those snippets to be executed along with the rest of the file when I include the file elsewhere.

The UI in the notebook could be a checkable "Skip Cell in Run All" menu item in the Cell menu, and perhaps the norun cells could be displayed slightly differently, e.g. with a darker-gray background.

[minrk]

I think this is a reasonable candidate for a tag. We've been talking about using a list of tags for some simple metadata markers, and this seems like a good one. cc @ellisonbg

[stevengj]

Yes, tags are potentially very useful. e.g. I'd also like to use a test tag for marking unit tests, and a doc tag to label markdown cells as docstrings for Julia functions and methods.

[stevengj]

(There's also the question of whether the cell metadata tags should be sent to the kernel as part of the execute_request message. I don't have any immediate use for this, but I can imagine it could be useful.)

[rgbkrk]

I think this is a reasonable candidate for a tag. We've been talking about using a list of tags for some simple metadata markers, and this seems like a good one. cc @ellisonbg

I've closed my PR that attempted to standardize tags and description: #134

If anyone wants to have at it, feel free.