I have two dicts:

:let defaults = {'hello': 'world', 'bye': 'jupiter'}

:let override = {'hello': 'mars'}

How can I merge the keys from override so that I end up with a new dict like so:

{'hello': 'mars', 'bye': 'jupiter'}

1 Answer 1


You can use extend():

:let defaults = {'hello': 'world', 'bye': 'jupiter'}
:let override = {'hello': 'mars'}

:echo extend(defaults, override)
{'hello': 'mars', 'bye': 'jupiter'}

Keys from the second argument override any existing ones in the first. The defaults dict will be modified in place, which may not be wanted. Use copy() to prevent that:

:call extend(copy(defaults), override)
:echo defaults
{'hello': 'world', 'bye': 'jupiter'}

This is especially something to be careful of when you're passing a dict to a function, since it's passed by reference (so it'll be modified outside of the function, too).

Note that it's won't merge nested dicts, which may or may not be a good thing, depending on what you're looking for:

:echo extend({'nest': {'a': 'b'}}, {'nest': {'b': 'XXXX'}})
{'nest': {'b': 'XXXX'}}

You'll need a small helper function to recursively merge nested dicts:

" Merge two dictionaries, also recursively merging nested keys.
" Use extend() if you don't need to merge nested keys.
fun! s:merge(defaults, override) abort
  let l:new = copy(a:defaults)
  for [l:k, l:v] in items(a:override)
    let l:new[l:k] = (type(l:v) is v:t_dict && type(get(l:new, l:k)) is v:t_dict)
          \ ? s:merge(l:new[l:k], l:v)
          \ : l:v
  return l:new

You can remove the copy() if you want to modify it in-place (bit faster, but possibly unexpected).

  • Instead of extend(copy(defaults), override) you can use extendnew(defaults, override). Details.
    – Shamaoke
    Jun 9, 2023 at 10:52
  • That didn't exist yet when I wrote this @Shamaoke; I prefer to keep it as-is for now, as many people are running old Vim versions from their distro and extendnew() is "only" 2 years old, and an extra copy() an extra copy() is just a few more extra characters. Jun 9, 2023 at 14:36

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