the goal is to be able to write functions names that follow PEP8 as easily and naturally as writing this sentence.

there are many types of situations that need handling

  • insert _ (underscores) for lines that
    1. start with <optional indentation>def
    2. end with .<optional_start_of_method_name>

only when the cursor is not

  1. directly after a comma (,)
  2. in an inline comment
  3. after the closing parenthesis ) of a function definition or call

is there a plugin that already does that? if not, how can I test for the aforementioned situations in vimscript?

  • Try the match function and expr mappings for a start
    – D. Ben Knoble
    Commented May 11, 2019 at 19:42

2 Answers 2


This remap <space> to _ if the line starts with def, which is the key to declare a function in python.

inoremap <expr> <space> getline('.')[0:3] == 'def ' ? "_" : "<space>"

Try also the autocmd way:

autocmd FileType python inoremap <expr> <space> getline('.')[0:3] == 'def ' ? "_" : "<space>"


Perhaps it's better to avoid using '_' between function arguments:

inoremap <expr> <space> getline('.')[0:3] == 'def ' && getline('.') !~ '(' ? "_" : "<space>"


I modified the approach detecting if the line ends with a dot as you ask:

function! IsPythonFunction()
    return getline('.')[0:3] == 'def ' || getline('.')[-1:] == '.'

:inoremap <expr> <space> (IsPythonFunction() && getline('.') !~ '(' ) ? "_" : "<space>" 

Again, I suggest to use the autocmd:

autocmd FileType python inoremap <expr> <space> (IsPythonFunction() && getline('.') !~ '(' ) ? "_" : "<space>" 
  • awesome, is there a way to add an or to that condition? i'm getting 0 or 1 as output with this: autocmd FileType python inoremap <expr> <space> getline('.') =~? '\.\(.\+\)\?' || getline('.')[0:3] == 'def ' ? "_" : "<space>" and parsing error when using parenthesis for the condition.
    – Wis
    Commented May 11, 2019 at 21:35
  • I modified the post to handle the 'def ' and the '.' at the beginning of the line. It also detect if you start declaring arguments opening a parenthesis.
    – xyx
    Commented May 12, 2019 at 10:10
  • I think you misunderstood what this regex check getline('.') =~? '\.\(.\+\)\?' does. it checks if the line ends with a dot or a dot+an unfinished function/var name. all the slashes are confusing, but that's just vim regex. I just tested your last edit, edit2, it doesn't insert _ instead of space on lines ending with .<optional_unfinished_method_or_var_name>.
    – Wis
    Commented May 12, 2019 at 15:10
  • My given answer satisfies what you asked in your main question: remap <space> to _ if the line starts with def or . . The second edit inserts _.
    – xyx
    Commented May 13, 2019 at 14:29
  • you're right, I didn't think of indentation and all the situations you can write a method name, to be able list them all, however the question title is clear and not specific. :) I'll edit the list in the question right now.
    – Wis
    Commented May 13, 2019 at 14:50

I improved upon @simo-zz (currently deleted/hidden) answer to

  1. insert a space directly after a comma and underscores after that
  2. insert underscores after a dot or after a dot + start of a method name
  3. insert underscores after def no matter it's indentation level
function! IsPythonFunction()
    let line = getline('.')
    let coln = col('.')
    let lineUptoCursor = line[0:coln-2]
    return (lineUptoCursor[-1:] != ',') && ((line =~? '\(.\+ \)\?def ') || (line =~? '.\+\.\(.\+\)\?'))
autocmd FileType python inoremap <expr> <space> (IsPythonFunction()) ? "_" : "<space>"

what's left for improvement:

  1. insert spaces not underscores in inline comments
    • harder that it initially seems, needs to exclude cases where the # is inside a string, which probably requires an ast parser
  2. insert spaces not underscores after the colon : which comes after the parenthesis ()

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