Which configuration does Vim finally apply after setting values in .vimrc, ftplugin, after/ftplugin etc.?

Question

I am editing a python file. My current settings for tabstop are as follows(just experimenting things):

• ~/.vimrc: set tabstop=4
• ~/.vim/ftplugin/python.vim: set tabstop=2
• There is no ~/.vim/after/plugin/python.vim file

set tabstop? shows tabstop=8

verbose set tabstop? shows tabstop=8. Last set from /usr/share/vim/vim80/ftplugin/python.vim

My questions:

• Why is tabstop settings at 8 even though i have explicitly set it to 4 in .vimrc file or 2 in ~/.vim/ftplugin/python.vim file
• When is each of the files i.e. ~/.vimrc, ~/.vim/ftplugin/python.vim, ~/.vim/after/plugin/python.vim, /usr/share/vim/vim80/ftplugin/python.vim loaded?
• Which file takes priority and where should I define my settings to override the others?
• How is set different from set local?

It would be great if someone answers all these questions. It will surely benefit people especially Vim beginner and intermediate users as all of these concepts are inter-related

Answer 1

tabstop is the width of space represented by a tab character in the file, but the width of space inserted when you press Tab on your keyboard is controlled by softtabstop (if it's set to a non-zero value). Nothing weird is going on with order of evaluation, or setlocal, or any of the other things you're asking about.

See also What is softtabstop used for? on the Vim StackExchange.

Answer 2

1. tabstop is a buffer-local setting. That means that every buffer has its own value. So by setting it from your vimrc you only set the global default which will probably be overwritten by ftplugin and such.

2. The plugin's code resides in $VIMRUNTIME/ftplugin.vim. Roughly speaking, what it does is runtime! ftplugin/python.vim. runtime! differs from source in two aspects: (a) it searches along :h 'runtimepath', and not in the current directory; (b) it sources all files found (that's what the "bang" stands for), not only the first one.

So, because of (b), ftplugin first sources your ~/.vim/ftplugin/python.vim, and then $VIMRUNTIME/ftplugin/python.vim. For this reason your setting gets finally overwritten.

This is why usually we want ~/.vim/after/ftplugin/python.vim instead. Also note that standard "ftplugin"-scripts have the guard against multiple sourcing:

if exists("b:did_ftplugin")
    finish
endif
let b:did_ftplugin = 1
....

So if you really want it, you can put this on top of your ~/.vim/ftplugin/python.vim and exclude all the stuff from $VIMRUNTIME/ftplugin/python.vim. Rarely wanted though.

3. The difference between set, setglobal and setlocal is actually the difference between option types: global, local-to-buffer, local-to-window, global-local. To keep the things simple, setglobal sets a global value for an option, setlocal sets a local one, and set does something "reasonable" (tm). In particular, set tabstop=x sets both global and local (current buffer's one) values.