Character level buffer modification

Question

It is possible to modify buffer at the line granularity. E.g. :py3 vim.current.buffer[0] = "abc". But it deletes the attached marks even if a single character is replaced. Is it possible to modify the buffer characterwise? Something like :py3 vim.current.buffer[0][0] = "a".

Answer 1

The vim.current.buffer list is a list of strings:

:pyx import vim; print(type(vim.current.buffer[0]))
<class 'str'>

and strings are immutable in Python; something like a_str[2] = 'b' doesn't work either. This is just a property of the Python programming language (and many other languages, including VimScript).

The general way to do this is to get the line and then modify it:

:pyx line = vim.current.buffer[0]            # hello
:pyx vim.current.buffer[0] = 'w' + line[1:]  # wello

Or shorter:

:pyx vim.current.buffer[0] = 'w' + vim.current.buffer[0][1:]

Or in VimScript with getline() and setline().

Using things like substitute() or str.replace() in Python is also useful. With substitute() you can use \%c to replace a specific column:

:call setline('.', substitute(getline('.'), '\%2c.', 'a', ''))

\%2c matches the next character (. for any character in this case) at this column, turning hello in to hallo. You can also match multiple columns; see :help \%c for details. I'm not sure how to call this from Python though but it's probably possible 😅