How do I prevent Vim from unindenting empty lines?

Question

Vim keeps unindenting the lines I want to keep indented if I don't write anything on them... I'll give an example, where the | is the caret.

1. Caret is indented and I press enter

   if expression:
       print("hello world")
       |
   

2. Caret is indented on the next line, and now I press the up arrow

   if expression:
       print("hello world")
   
       |
   

3. Now the caret isn't indented anymore, what happened?

   if expression:
       print("hello world")
   |
   

So how do I keep the indent?

Answer 1

What you're seeing is the normal behavior of 'autoindent' which the Python indent plugin uses. See :help 'ai'. You can watch what happens to the indent as you type by executing

:set list

There is no way that I know of to make Vim automatically preserve your indent in the case you describe. Setting the I flag of 'cpoptions' doesn't help.

You can work around this problem by using the following mapping.

:inoremap <cr> <space><bs><cr>

That will cause Vim to leave an indentation of spaces when you type Enter on an empty line. I don't recommend doing that as it will leave your file cluttered with unnecessary spaces until you remove them with something like

:%s/\s\+$//

before saving the file, but you may find that less annoying than the normal indent behavior.

Answer 2

To keep the indent ... write something on that line, before moving to other lines! Otherwise, since you are a good Vim user and stay mostly in normal mode (right?), when you have to write something on that line, just go there and press S - you'll find yourself at the correct indent level and ready to type. Vim does this to avoid useless ('trailing') tabs/spaces on empty lines.