How can I make Vimscript check if the current line has a comment?

Question

I want to make a Python ftplugin (but other filetypes will also have this option) where, if the previous line has a comment that starts with #, the next line can be commented via <Shift><CR>, an insert-mode mapping (inoremap). How can I code Vimscript so that it checks whether the current line is commented, and subsequently allows the inoremap to make a new line commented?

And please don't recommend a third-party plugin like NERD Commenter (which, btw, I respect very much), I want something as close to vanilla-Vim as possible for this.

Answer 1

To check if the current line is commented, you can use

:echo synIDattr(synIDtrans(synID(line("."), col("$")-1, 1)), "name")

This checks, if the current line ends with a comment syntax item.

You can then do something like this:

inoremap <expr><silent> <f6> synIDattr(synID(line('.')-1, strlen(getline(line('.')-1)), 1),'name')=~?'comment'?'#':' '

this maps the F6 key to check if the syntax group from the end of the previous line matches comment and if so returns the comment character #, else it will simply return a Space.