Table of Contents

Introduction
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I have been using Vim-airline for a while to customize my statusline. We can change the theme used for statusline with the many themes available in vim-airline-themes. Not all these themes looks good since it depends on the background color you use for Neovim and certainly your aesthetics.

I only use a handful of themes myself and I change my themes when I feel bored with one theme. It occurs to me that I can write a simple script to randomly pick a theme from my favorite list¹. It turns out writing such a script is not easy for Neovim newbies like me.

In this post, I would like to share how I end up achieving what I want by mixing Vim script and Python script.

Prerequisite
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This post is specifically targeted at Neovim users so that some of the settings may not apply if you use Vim.

First, you need to install Python and Python 3 is preferable since Python 2 support will stop in 2020. Make sure that you can call python from the command line. When you start Neovim and :echo has('python3') will print 1 in the command line.

You should also install pynvim, which is the Python client and plugin host for Neovim. You can install it with pip:

pip install pynvim

Finally, you should set the variable g:python3_host_prog to the location of Python executable. If your Python executable path is ~/anaconda3/bin/python, you should add the following setting in your init.vim:

let g:python3_host_prog=expand('~/anaconda3/bin/python')

After that, open Neovim and run :checkhealth provider and make sure there is no error in the part about Python 3. A sample output is shown below:

## Python 3 provider (optional)
  - INFO: Using: g:python3_host_prog = "D:/Anaconda/python"
  - INFO: Executable: D:\Anaconda\python
  - INFO: Python version: 3.6.5
  - INFO: pynvim version: 0.3.2
  - OK: Latest pynvim is installed.

Pick a random airline theme
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Generate a random number in the given range
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To pick a random theme from my favorite themes, I need to create a function which return a random index of the theme given the lowest and highest index. First, I will show my final function and then I will explain it bit by bit.

Click to see the function.

" generate a random integer from range [Low, High] using Python
function! RandInt(Low, High) abort
" if you use Python 3, the python block should start with `python3` instead of
" `python`, see https://github.com/neovim/neovim/issues/9927
python3 << EOF
import vim
import random

# using vim.eval to import variable outside Python script to python
idx = random.randint(int(vim.eval('a:Low')), int(vim.eval('a:High')))

# using vim.command to export variable inside Python script to vim script so
# we can return its value in vim script
vim.command("let index = {}".format(idx))
EOF

return index
endfunction

In the above function RandInt(), we define a function which accept two parameter Low and High. We generate a random integer between in the range $[Low, High]$ using the Python package random and return the index.

The format for Python script is as follows:

python[3] << MARKER
# python code here
MARKER

If you use Python 2, you should use python keyword, otherwise use python3, which is explained here. To manipulate Neovim inside Python script, we need to import package vim.

To get the value of Vim variable Low and High inside Python script, we need to import them. vim.eval()² is used to get their value, which is used to evaluate the expression using built-in expression evaluator. According to documentation, the return value is:

a string if the Vim expression evaluates to a string or number

So we need to use int() to convert the return value to integer.

After we get the value of idx, how do we use it outside the Python script? We can use vim.command() to achieve that:

vim.command("let index = {}".format(idx))

vim.command(str) will execute str as Vim command, thus assigning the value of idx to Vim variable index.

Pick a airline theme
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Now we can define a list of themes to pick and generate a random index and set the airline theme using the theme at the index. An example code is shown below:

let s:candidate_airlinetheme = ['alduin', 'ayu_mirage', 'base16_flat',
    \ 'monochrome', 'base16_grayscale', 'lucius', 'base16_tomorrow',
    \ 'base16color', 'biogoo', 'distinguished', 'gruvbox', 'jellybeans',
    \ 'luna', 'raven', 'seagull', 'solarized_flood', 'term', 'vice', 'zenburn']

let s:idx = RandInt(0, len(s:candidate_airlinetheme)-1)
let s:theme = s:candidate_airlinetheme[s:idx]
let g:airline_theme=s:theme

In the above script, len() is used to get the length of a list.

Conclusion
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In this post, I introduced how to mix Python script with Vim script. Specifically, I write a simple function to generate a random index and then pick a airline theme based on the index. I have just scratched the surface of the powerful ability of Python programming inside Vim. For more information, see :h python.

References
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Note that vim-airline provides a command AirlineTheme random to pick a random theme, but you cannot set the list of color themes you want to use. ↩︎
See :h python-eval for detailed documentation. ↩︎