`au BufWritePost` equivalent that matches on FileType?

Question

There are a few cases where the file extension isn't necessarily indicative of the file contents, and filetype needs to be set with a modeline. For example, Salt *.sls files can contain many different types of data, including YAML, Jinja2, or Python code, and I include a modeline at the bottom of those to set the correct filetype for syntax highlighting.

# vim:ft=python

I have a BufWritePost autocommand in my .vimrc that runs a style linter on my python code any time I save a buffer:

    au BufWritePost *.py call Flake8()

I would like to do something similar whenever a buffer of FileType python is written, but haven't been able to find an autocommand that matches on filetype. Is there one? Or is there some other approach that will get the effect I want?

Answer 1

You don't "find" autocommands, you write them.

First, choose the most appropriate event.

Either of BufWritePost or BufWritePre is the event you need. Which one to choose depends on what that Flake8() function does: if it operates on the buffer, use BufWritePre, if it operates on a file, use BufWritePost.

So, assuming Flake8() operates on the buffer:

autocmd BufWritePre *.py call Flake8()

Second, decide on a strategy.

Here you must choose between two approaches: keep the autocommand in your vimrc or move it to a proper filetype plugin.

If you want to keep the autocommand in your vimrc, you will need to change the pattern to * and add logic to the right-hand side because BufWritePre and BufWritePost don't know anything about filetypes:

autocmd BufWritePre * if &filetype == "python" | call Flake8() | endif

This gives you an autocommand that will decide whether or not to call Flake8() whenever you write a buffer, any buffer. That works but what a mess! You have some logic that will be executed very often and for nothing.

The better strategy is to move your autocommand to a proper filetype plugin:

after/ftplugin/python.vim

This gives you:

• a cleaner vimrc,

• access to the special pattern <buffer>, see :help autocmd-buflocal,

• and a much simpler autocommand:

    autocmd BufWritePre <buffer> call Flake8()