What is the difference between `au BufNewFile,BufRead *.py` and `au Filetype python` in this specific case?

Question

The issue

Consider the following two Vimrc configuration lines for the YouCompleteMe plugin:

au BufRead, BufNewFile *.py,*.pyw match BadWhitespace /\s\+$/

au FileType python match BadWhitespace /\s\+$/

When opening an existing file (vim somefile.py), the first one loads and works correctly (wrong whitespace gets highlighted), while the second one results in the following error message but the functionality still works correctly (wrong whitespace gets highlighted):

Error detected while processing BufRead Autocommands for "*.py"..FileType Autocommands for "python":
E28: No such highlight group name: BadWhitespace

Questions

This issue is confusing on so many aspects...

1. Based on the Q&A linked below, I understood that the two main differences between BufRead+BufNewFile vs FileType are the following:

• BufRead+BufNewFile does not affect writing to an existing file
• FileType is not aware of all file formats

However, neither of these explains the reported behavior, because FileType is, in fact, aware of Python files, and in this case we are not writing a file (but reading one).

2. Why does the functionality still work in spite of the error message?

3. The error message is slightly confusing, because it talks about BufRead Autocommands, whereas the used keyword was FileType (and there is no error message when BufRead is used for real). So is FileType just a wrapper around BufRead?

Remark

As stated, I am aware of this Question and the related Answers (1, 2), but as I mentioned, they don't explain the case above. So I believe this is some kind of an edge case and hence a new question is appropriate.

Answer 1

The BufRead event is triggered after the FileType event if the corresponding autocommand is after the filetype plugin on command (or after the plug#end() call if you are using plug)

I suppose the BadWhitespace is defined in between the two.

Which make the the second trigger a warning where the second does not.

Remark: the FileType event is not equivalent to the BufRead event. But practically it come close for a lot of files. If the file is recognized by Vim there is a BufRead BufNew event autocmd that set the filetype (e.g.: in $VIMRUNTIME/filetype.vim) triggering the FileType event.

Associating your command to FileType makes that you associate it indirectly to one of the first BufRead. Associating your command to BufRead makes that you add it to the list of commands that is already associated to it. It depends where you have it in you vimrc But if it is after filetype plugin on it is after FileType.