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I want to format only a part of file and keep the rest unchanged using ALE. I imagine that its kinda possible by providing a fragment of code for example for black formatter and replacing the selected lines with the result retrieved from black.

Applying formatter for a large file could cause too many changes. Its way easier to track what have changed if only a part of the file is changed at once.

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  • How would ALE know which part of the file to operate on? Setting formatprg is probably a better option, I think? But you need to invoke that manually. Feb 26, 2021 at 4:02
  • @MartinTournoij I imagine that visual-line mode could be used to select the block for the formatting.
    – niekas
    Feb 26, 2021 at 7:40
  • Yeah, you could use formatprg for that, assuming that black can operate on partial files: set formatprg=black (may need flags?), select what you want to be formatted, and then gq. Many people like to map gq to q btw, which is a bit easier to type. Feb 26, 2021 at 9:24
  • @MartinTournoij i use Q, perhaps that’s what you meant? Black does have some issues when the partial file is not syntactically correct, and it’s also noisy (needs some cleanup). And i think a stdin or - flag is required
    – D. Ben Knoble
    Feb 26, 2021 at 14:01

2 Answers 2

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Range formatting is not supported by black because range of lines are in general not valid Python code.

You may have a look at black-macchiato.

Install black-macchiato by running the shell command:

pip install black-macchiato

To format a range of select lines run the vim command:

:'<,'>!python -m macchiato

You can also set the corresponding formatprg command.

:setlocal formatprg=python -m macchiato

This will allow you to format using the gq command.

You can also install the pluging smbl64/vim-black-macchiato: vim-black-macchiato

Remark: Black-Macchiato doesn't work out of the box on Windows but there are two forks that solve the problem on Windows, e.g., black-macchiato.

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I'm doing this now with vim9script in ~/.vim/after/ftplugin/python.vim:

autocmd! BufWritePre <buffer> {
  var view = winsaveview()
  silent! execute ':%!black --quiet -'
  winrestview(view)
}

To automatically format python filetype buffers just before saving.

To use the gq operator, you can set also do setlocal formatprg=black\ --quiet\ - but beware this will not always work, for example if you select a line like def add(a b) black can't parse that and you'll get the black error put in your buffer, however, if you select the entire function, let's say it's:

def add(a               b):
    answer       = a     + b
    return answer

and you do gqap on it, this will work, or if you did vip then gq, that works as well.

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