Return <motion> character from Python function in <expr> mapping

Question

I am trying to return this :GrepperAck ''<Left> from a Python function and use it in the following mapping

def invoke_search_command():
  return ":GrepperAck ''<Left>"

:nnoremap <expr> <leader>f py3eval('invoke_search_command()')

But the <Left> is printed literally like :GrepperGit ''<Left>.

But if I use :nnoremap <expr> <leader>f py3eval('invoke_search_command()') . '<Left>', it works. I tried escaping the returned string like ":GrepperAck ''\<Left>", but it is still getting printed literally. What am I doing wrong?

Answer 1

You definitely need the backslash version \<Left>. Is use of py3eval necessary? Unless you need to translate data structures from Python to Vim or something it would be better to return a straight string. You need to make sure the string :GrepperAck ''\<Left> is passed literally to the command line. Then it will be processed as an expression due to <expr>.

I don't have a Python installation but I can demonstrate the fundamental workings without it. Given...

func! Foo()
    return ":GrepperAck ''\<Left>"
endfunc

And this mapping...

nnoremap <expr> <leader>f Foo()

Then when I enter leader key and f this is what I'll see on the command line..

:GrepperAck ''

...and the cursor will be inside the quotes.

If you must use py3eval then one thing you might try is to wrap that in a string() function call.

nnoremap <expr> <leader>f string(py3eval(..))

---

Update: In retrospect the reason \<Left> doesn't work is almost certainly because the parsing of the mapping occurs in a certain order, i.e. the phase that substitutes actual keys for <xxx> tokens occurs before any external programs are executed. I haven't confirmed this by looking at the Vim source code or anything but as a veteran software developer I can tell you that it's a solid theory. It wouldn't make much sense for the parser to have multiple passes with <xxx> resolution nor, generally speaking, to make such a pass over external (non-Vim originated) runtime data.

Can anything be done? Well, I thought that substituting the actual key code would do the trick....

def invoke_search_command():
    return ":GrepperAck ''<C-V><Left>"

That means rather than type \<Left> you actually enter Ctrl+V followed by ← (left arrow key). (For me the combo comes out looking like ^[OD.) Unfortunately, I'm getting an error: Not an editor command: GrepperAck ''. But I know I'm on the right track because if I replace ← with Ctrl+H (commonly the same function as the backspace key) then the cursor is actually moved on the resulting GrepperAck command line (erasing the last ' as expected).

So why isn't the left arrow key code working? Dunno. It might be that we can substitute something else in the terminal codes but that's a stretch and getting into possibly non-portable behavior. I'll play around with that for a bit but unfortunately, unless your system behaves differently than mine (try it out) I believe the ultimate answer to your question is "this is not possible".